5. water quality impact ASSESSMENT

5.1 Introduction

5.1.1 This Section describes the baseline conditions and potential impacts on marine water quality from the construction and operation of the Project at Sha Tau Kok (STK). Mathematical modelling was used to predict potential impacts to water quality, and the predictions were assessed with reference to the relevant environmental legislation, standards and tolerance criteria.

5.2 Legislation Requirement & Guidelines

5.2.1 The following relevant legislation and associated guidance are applicable to the evaluation of water quality impacts associated with the Project:

a) Water Pollution Control Ordinance (WPCO);

b) Technical Memorandum for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM- ICW);

c) Environmental Impact Assessment Ordinance (Cap. 499. S.16) and the Technical Memorandum on EIA Process (EIAO-TM), Annexes 6 and 14; and

d) Practice Note for Professional Persons, Construction Site Drainage (ProPECC PN1/94).

Water Pollution Control Ordinance

5.2.2 The Water Pollution Control Ordinance (WPCO) is the primary legislation for the control of water pollution and water quality in Hong Kong. Under the WPCO, Hong Kong waters are divided into 10 Water Control Zones (WCZs). Each WCZ has a designated set of statutory Water Quality Objectives (WQOs).

5.2.3 The Study Area will cover Mirs Bay WCZs. The applicable WQOs for the Mirs Bay WCZ are presented in Table 5.1 and used in the following water quality impact assessment.

Table 5.1 Water Quality Objectives Applicable to the Mirs Bay WCZ

	Water Quality Objective	Applicable Area
A	AESTHETIC APPEARANCE
a)	Waste discharges shall cause no objectionable odours or discolouration of the water.	Whole zone
b)	Tarry residues, floating wood, articles made of glass, plastic, rubber or of any other substances should be absent.	Whole zone
c)	Mineral oil should not be visible on the surface. Surfactants should not give rise to lasting foam.	Whole zone
d)	There should be no recognisable sewage-derived debris	Whole zone
e)	Floating, submerged and semi-submerged objects of a size likely to interfere with the free movement of vessels, or cause damage to vessels, should be absent.	Whole zone
f)	Waste discharges shall not cause the water to contain substances which settle to form objectionable deposits.	Whole zone
B	BACTERIA
a)	The level of Escherichia coli should not exceed 610 per 100 milligrams per litre, calculated as the geometric mean of all samples collected in one calendar year.	Secondary Contact Recreation Subzone & Fish Culture Zones
b)	The level of Escherichia coli should be zero per 100 ml, calculated as the running median of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days.	Water Gathering Ground Subzones
(c)	The level of Escherichia coli should not exceed 1000 per 100 ml, calculated as the running median of the most recent 5 consecutive samples taken at intervals of between 7 and 21 days.	Other inland waters of the Zone
C	COLOUR
a)	Waste discharges shall not cause the colour of water to exceed 30 Hazen units.	Water Gathering Ground Subzones
b)	Waste discharges shall not cause the colour of water to exceed 50 Hazen units.	Other inland waters of the Zone
D	DISSOLVED OXYGEN
a)	Waste discharges shall not cause the level of dissolved oxygen to fall below 4 mg per litre for 90% of the sampling occasions during the year; values should be calculated as water column average. In addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year.	Marine waters excepting Fish Culture Subzones
b)	The dissolved oxygen level should not be less than 5 milligrams per litre for 90% of the sampling occasions during the year; values should be calculated as water column average (arithmetic mean of at least 3 measurements at 1 metre below surface, mid-depth and 1 metre above seabed). In addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year.	Fish Culture Subzones
c)	Waste discharges shall not cause the level of dissolved oxygen to be less than 4 milligrams per litre.	Inland waters of the Zone
E	pH
a)	The pH of the water should be within the range of 6.5 - 8.5 units. In addition, waste discharges shall not cause the natural pH range to be extended by more than 0.2 units.	Marine waters
b)	Waste discharges shall not cause the pH of the water to exceed the range of 6.5-8.5 units.	Water Gathering Ground Subzones
c)	The pH of the water should be within the range of 6.0 - 9.0 units.	Other inland waters of the Zone
F	TEMPERATURE
	Waste discharges shall not cause the natural daily temperature range to change by more than 2.0 °C.	Whole zone
G	SALINITY
	Waste discharges shall not cause the natural ambient salinity level to change by more than 10%.	Whole zone
H	SUSPENDED SOLIDS
a)	Waste discharges shall neither cause the natural ambient level to be raised by 30% nor give rise to accumulation of suspended solids which may adversely affect aquatic communities.	Marine waters
b)	Waste discharges shall not cause the annual median of suspended solids to exceed 20 milligrams per litre.	Water Gathering Ground Subzones and Other inland waters of the Zone
I	AMMONIA
	The un-ionized ammoniacal nitrogen level should not be more than 0.021 milligram per litre, calculated as the annual average (arithmetic mean).	Whole zone
J	NUTRIENTS
a)	Nutrients shall not be present in quantities sufficient to cause excessive or nuisance growth of algae or other aquatic plants.	Whole zone
b)	Without limiting the generality of objective (a) above, the level of inorganic nitrogen should not exceed 0.3 milligram per litre, expressed as annual water column average (arithmetic mean of at least 3 measurements at 1m below surface, mid-depth and 1m above seabed).
K	5-DAY BIOCHEMICAL OXYGEN DEMAND
a)	Waste discharges shall not cause the 5-day biochemical oxygen demand to exceed 3 milligrams per litre.	Water Gathering Ground Subzones
b)	Waste discharges shall not cause the 5-day biochemical oxygen demand to exceed 5 milligrams per litre.	Other inland waters of the Zone
L	CHEMICAL OXYGEN DEMAND
a)	Waste discharges shall not cause the chemical oxygen demand to exceed 15 milligrams per litre.	Water Gathering Ground Subzones
b)	Waste discharges shall not cause the chemical oxygen demand to exceed 30 milligrams per litre.	Other inland waters of the Zone
M	TOXINS
a)	Waste discharges shall not cause the toxins in water to attain such levels as to produce significant toxic, carcinogenic, mutagenic or teratogenic effects in humans or fish or any other aquatic organisms, with due regard to biologically cumulative effects in food chains and to toxicant interactions with each other.	Whole zone
b)	Waste discharges shall not cause a risk to Whole Zone any beneficial uses of the aquatic environment.	Whole zone
Note: (a) Reference: Statement of Water Quality Objectives (Mirs Bay Water Control Zone)

Technical Memorandum for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-ICW)

5.2.4 All discharges during both the construction and operation phases of the Project are required to comply with the TM-ICW issued under Section 21 of the WPCO.

5.2.5 The TM-ICW defines acceptable discharge limits to different types of receiving waters. Under the TM-ICW, effluents discharged into the drainage and sewerage systems, inshore and coastal waters of the WCZs are subject to pollutant concentration standards for specified discharge volumes. These are defined by the Environmental Protection Department (EPD) and are specified in licence conditions for any new discharge within a WCZ.

5.2.6 The TM-ICW also prohibits new effluent (1) within 100m of the boundaries of a gazetted beach in any direction, including rivers, streams and storm water drains, (2) within 200m of the seaward boundaries of a marine fish culture zone or a site of special scientific interest, and within 100m of the landward boundaries (3) in any typhoon shelter, (4) in any marina and (5) within 100m of a seawater intake point. As shown in Figure 5.1, there is no gazetted beach, site of special scientific interest, typhoon shelter, marina and existing seawater intake within 100m from the proposed submarine outfall. The nearest fish culture zone at Sha Tau Kok is more than 500 m away. However, the outfall of the existing STKSTW, which would also be used by the temporary sewage treatment plant (TSTP), is already located within 500 m from the Sha Tau Kok Fish Culture Zone.

Environmental Impact Assessment Ordinance (Cap. 499. S.16) and the Technical Memorandum on EIA Process (EIAO-TM), Annexes 6 and 14

5.2.7 Annexes 6 and 14 of the EIAO-TM provide general guidelines and criteria to be used in assessing water quality impacts.

5.2.8 The EIAO-TM recognizes that, in the application of the above water quality criteria, it may not be possible to achieve the WQO at the point of discharge as there are areas which are subjected to greater impacts (which are termed by the EPD as the mixing zones) where the initial dilution of the discharge takes place. The definition of this area is determined on a case-by-case basis. In general, the criteria for acceptance of the mixing zone are that it must not impair the integrity of the water body as a whole and must not damage the ecosystems.

Practice Note for Professional Persons, Construction Site Drainage (ProPECC PN 1/94)

5.2.9 Apart from the above statutory requirements, the ProPECC PN 1/94, issued by EPD in 1994, also provide useful guidelines on water pollution associated with construction activities.

Criterion for Fish Culture Zones

5.2.10 The water quality objective for SS elevation (less than 30% increase ambient) would be applied for the protection of water quality at Fish Culture Zones (FCZs) within the Study area.

5.2.11 For FCZs, in accordance with the WQO, the DO criterion is set at > 5 mg/L measured as water column average. The maximum allowable SS elevation and DO depletion is provided in Table 5.7 and Table 5.8 below. As shown in Table 5.8, the 10th-percentile ambient DO levels at FCZs are both below the proposed criterion of 5 mg/L in wet season. Yet such exceedance is not observed at FCZs near MM1 if the 10^th-percentile ambient DO levels are considered annually. Further elaboration would be provided under Section 5.4.2.

5.3 Baseline Condition

Overview

5.3.1 The Study Area for water quality consists of marine waters of Mirs Bay within 7 km from the boundary of the proposed expansion of the STKSTW and the submarine outfall. The Study Area is located at the Starling Inlet (also known as Sha Tau Kok Hoi / Sha Tau Kok Sea) and the Northern Mirs Bay. The extent of the Study Area for water quality is shown in Figure 5.1 for reference. The terrestrial and marine boundaries of the Mainland China are close to the Project site and the proposed submarine outfall is less than 300 metres away from the marine boundary. As shown in Figure 5.1, the Project would not encroach into any water courses, natural streams, ponds and wetlands. Since the Project is constructed within developed area, no change of stormwater catchment would be expected.

5.3.2 Starling Inlet is landlocked and is away from the Pearl River and other major rivers in HK or Mainland China. Waters within Starling Inlet are quite shallow in general. Water depth varies widely over the Study Area, from shallow waters off Pak Hok Lam and Wu Shek Kok (about -1 mPD) to the deeper northern Mirs Bay near Yantian Harbour (over 10 mPD).

Marine Water Quality

5.3.3 Baseline marine water quality of the Study Area has been determined through a review of EPD routine monthly water quality monitoring data collected in 2005 to 2014. Four EPD monitoring stations are identified within the Study Area and the locations of these stations are presented in Figure 5.1.

5.3.4 Water quality monitoring data from stations MM1, MM2, MM3 and MM7 were used to provide the baseline water quality condition in the vicinity of the Project Site. The monitoring results in 2005 to 2014 at the selected monitoring stations are summarized in Table 5.2. According to the EPD’s Marine Water Quality in Hong Kong in 2014, the overall WQO compliance rate of the Mirs Bay WCZ was 98%. The water quality of the Mirs Bay WCZ was good in 2014 with high DO and low TIN levels. Moreover, the Mirs Bay WCZ also complied with the bacteriological WQO of ≤ 610 E. coli cfu / 100 mL (annual geometric mean) for secondary contact recreation.

Sediment Quality

5.3.5 Baseline marine sediment quality in the Study Area has been determined through a review of EPD routine biannual sediment quality monitoring data collected between 2010 and 2014. Locations of EPD sediment monitoring stations within the Study Area are presented in Figure 5.1.

5.3.6 Sediment monitoring data from stations MS1, MS2, MS3 and MS7 were collected to represent the sediment quality adjacent to the Project Site (Table 5.3). The levels for metals, Polycyclic Aromatic Hydrocarbons (PAHs) and Polychlorinated Biphenyls (PCBs) were compared to the relevant sediment quality criteria specified in ETWB TC(W) No. 34/2002 Management of Dredged/Excavated Sediment.

5.3.7 The EPD routine monitoring data indicate that level of sediment contamination level is low in the vicinity of the Project Site. The maximum level of silver in sediment at station MS1 is found to be higher than the corresponding Lower Chemical Exceedance Level (LCEL). Other chemical parameters comply with the corresponding LCEL.

5.3.8 Sediment sampling and testing was conducted under this Study to identify the level of sediment contamination within the marine construction works area. Sediment sampling locations are shown in Figure 5.2. The testing results are compared against the LCEL and UCEL below in Table 5.4. As shown, there are slight exceedances of the arsenic LCEL identified for sediment samples at GB8, 2.9 m to 3.9 m subsample at SD1, 5.9 m to 6.9 m subsample at SD2 and 5.9 m to 6.9 m subsample at SD3. No exceedance in LCEL for all other chemical parameters is identified at all stations. Furthermore, sediment elutriate test was conducted using sediment samples from these sampling stations to identify the potential of dissolution of sediment-bounded nutrients, heavy metals and trace organic pollutants due to disturbance from marine works under this Project. The sediment elutriate test results are shown in Table 5.5 below.

Table 5.2 Summary of EPD Routine Water Quality Monitoring Data from Selected Stations of the Mirs Bay Water Control Zones in 2005 - 2014

Parameter	MM1	MM2	MM3	MM7
Temperature (°C)	23.7	23.3	22.9	23.2
	(15.2-30.3)	(14.9-30.0)	(14.4-29.6)	(14.5-29.8)
Salinity (psu)	31.6	32.1	32.4	32.1
	(27.1-33.7)	(28.7-33.6)	(29.5-33.7)	(28.8-33.5)
Dissolved Oxygen (mg/L) - Depth Averaged	7.0	6.7	6.5	6.6
	(4.2-12.9)	(3.7-12.1)	(2.9-10.3)	(2.4-12.1)
Dissolved Oxygen (mg/L) - Bottom	6.6	6.1	5.8	6.0
	(1.5-12.7)	(1.2-11.5)	(1.3-10.7)	(0.2-11.6)
Suspended Solids (mg/L)	3.9	2.5	3.9	1.9
	(1.0-10.6)	(<0.5-16.9)	(0.7-18.2)	(0.6-8.4)
5-day Biochemical Oxygen Demand (mg/L)	1.3	0.9	0.7	0.9
	(0.2-4.0)	(<0.1-2.6)	(<0.1-3.2)	(0.2-2.2)
Unionised Ammonia (mg/L)	0.003	0.002	0.002	0.002
	(<0.001-0.019)	(<0.001-0.008)	(<0.001-0.005)	(<0.001-0.009)
Total Inorganic Nitrogen (mg/L)	0.1	0.1	0.1	0.1
	(0.0-0.4)	(0.0-0.2)	(0.0-0.2)	(0.0-0.2)
Orthophosphate Phosphorus (mg/L)	0.008	0.007	0.007	0.007
	(<0.002-0.024)	(<0.002-0.023)	(<0.002-0.022)	(<0.002-0.027)
Total Phosphorus (mg/L)	0.03	0.02	0.02	0.02
	(<0.02-0.06)	(<0.02-0.04)	(<0.02-0.05)	(<0.02-0.05)
Chlorophyll-a (mg/L)	7.2	4.0	2.7	3.6
	(0.5-30.0)	(0.4-20.6)	(0.4-14.3)	(0.6-14.4)
Escherichia coli (cfu/100ml)	31	2	2	1
	(<1-7683)	(<1-424)	(<1-174)	(<1-161)

Notes:

(a) Data presented are depth-averaged values calculated by taking the means of three depths, i.e. surface (S), mid-depth (M) and bottom (B), except as specified.

(b) Data presented are annual arithmetic means except for E. coli, which are geometric means. Data enclosed in brackets indicate the ranges of the corresponding depths.

(c) With the exception E.coli, data below the corresponding reporting limits (RL) are calculated as 0.5×RL. For E.coli, data below the corresponding RL are calculated as 1.

(d) WQOs for TIN in Mirs Bay = 0.3 mg/L.

Table 5.3 Summary of EPD Routine Marine Sediment Quality Monitoring Data from Selected Station of for the Mirs Bay Water Control Zone (2010-2014)

Parameter	ETWB TC(W) No. 34/2002 Guideline		Starling Inlet	Crooked Island		Mirs Bay (North)
	LCEL	UCEL	MS1	MS2	MS7	MS3
Arsenic (mg/kg)	12	42	8.3	7.3	6.7	5.7
			(7.1-9.5)	(6.0-8.6)	(5.8-7.8)	(3.8-7.3)
Cadmium (mg/kg)	1.5	4	0.2	0.3	0.3	<0.1
			(0.1-0.3)	(0.1-0.4)	(<0.1-0.5)	(<0.1-<0.1)
Chromium (mg/kg)	80	160	27	33	33	26
			(19-32)	(27-37)	(27-38)	(21-35)
Copper (mg/kg)	65	110	24	21	20	11
			(17-32)	(19-24)	(13-26)	(7-17)
Lead (mg/kg)	75	110	45	45	41	29
			(34-54)	(34-50)	(27-47)	(20-42)
Mercury (mg/kg)	0.5	1	0.06	0.06	0.07	<0.05
			(<0.05-0.07)	(<0.05-0.07)	(0.05-0.10)	(<0.05-<0.05)
Nickel (mg/kg)	40	40	17	22	23	18
			(12-20)	(18-25)	(18-25)	(13-23)
Silver (mg/kg)	1	2	0.6	0.3	0.2	<0.2
			(0.3-1.1)	(0.2-0.4)	(<0.2-0.3)	(<0.2-<0.2)
Zinc (mg/kg)	200	270	94	100	96	67
			(69-110)	(87-110)	(82-110)	(52-89)
Total Polychlorinated Biphenyls (PCBs) (mg / kg)	23	180	18	18	18	18
			(18-18)	(18-18)	(18-18)	(18-18)
Low Molecular Weight Polycyclic Aromatic Hydrocarbons (PAHs) (μg/kg)	550	3,160	110	120	130	100
			(90-200)	(90-210)	(90-220)	(90-130)
High Molecular Weight Polycyclic Aromatic Hydrocarbons (PAHs) (μg/kg)	1,700	9,600	48	52	77	31
			(29-90)	(32-81)	(31-170)	(18-65)
Chemical Oxygen Demand (mg/kg)	--	--	14100	14500	15700	12000
			(11000-17000)	(11000-18000)	(13000-21000)	(9900-14000)
Total Kjeldahl Nitrogen (mg/kg)	--	--	530	610	670	500
			(350-620)	(460-770)	(530-800)	(250-610)
Ammonia Nitrogen (mg/kg)	--	--	5.85	6.96	9.94	8.09
			(0.13-10.00)	(0.07-14.00)	(7.30-13.00)	(1.40-25.00)
Total Phosphorus (mg/kg)	--	--	180	180	190	180
			(140-200)	(150-210)	(170-210)	(100-220)

Table 5.4 Summary of Marine Sediment Quality at Geophysical Survey Stations conducted under this Study

Parameters	Ag	As	Cd	Cr	Cu	Ni	Pb	Zn	Hg	Total PCB	Low M.W. PAHs	High M.W. PAHs	TBT	Classification
Unit	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	µg/kg	µg/kg	µg/kg	µg TBT/L
Reporting Limits	0.1	1	0.2	1	1	1	1	1	0.05	18	550	1700	0.015
(LCEL)	1	12	1.5	80	65	40	75	200	0.5	23	550	1700	0.15
(UCEL)	2	42	4	160	110	40	110	270	1	180	3160	9600	0.15
10 × (LCEL)	10	120	15	800	650	400	750	2000	5	230	5500	17000	1.5
GB1	0.8	8	0.2	36	38	22	49	154	0.05	<18	<550	<1700	<0.015	L
GB2	1	8	0.3	39	41	24	55	164	<0.05	<18	<550	<1700	<0.015	L
GB3	0.9	10	0.3	38	37	24	54	157	0.06	<18	<550	<1700	<0.015	L
GB4	0.9	8	0.2	37	39	23	54	167	<0.05	<18	<550	<1700	<0.015	L
GB5	0.9	9	0.3	39	40	23	53	167	0.07	<18	<550	<1700	<0.015	L
GB6	0.4	3	<0.2	11	18	5	16	64	<0.05	<18	<550	<1700	--	L
GB7	0.3	3	<0.2	8	13	4	12	47	<0.05	<18	<550	<1700	--	L
GB8	0.7	14	0.2	17	38	10	47	134	0.07	<18	<550	<1700	<0.015	M
SD1 0M-0.9M	0.2	11	<0.2	28	21	16	49	102	0.08	<18	<550	<1700	<0.015	L
SD1 0.9M-1.9M	0.1	8	<0.2	29	10	18	36	92	<0.05	<18	<550	<1700	<0.015	L
SD1 1.9M-2.9M	0.1	8	<0.2	23	8	14	30	69	<0.05	<18	<550	<1700	<0.015	L
SD1 2.9M-3.9M	0.1	16	<0.2	23	10	16	26	63	<0.05	<18	<550	<1700	<0.015	M
SD2 0M-0.9M	0.8	9	0.3	42	39	23	62	166	0.06	<18	<550	<1700	<0.015	L
SD2 0.9M-1.9M	0.1	12	<0.2	27	12	16	43	89	<0.05	<18	<550	<1700	<0.015	L
SD2 1.9M-2.9M	0.1	6	<0.2	33	9	20	35	90	<0.05	<18	<550	<1700	<0.015	L
SD2 2.9M-3.9M	0.1	6	<0.2	31	10	18	37	86	<0.05	<18	<550	<1700	<0.015	L
SD2 5.9M-6.9M	<0.1	16	<0.2	6	5	3	16	23	<0.05	<18	<550	<1700	<0.015	M
SD3 0M-0.9M	0.2	12	<0.2	27	15	16	53	97	0.10	<18	<550	<1700	<0.015	L
SD3 0.9M-1.9M	0.1	7	<0.2	29	9	17	33	90	<0.05	<18	<550	<1700	<0.015	L
SD3 1.9M-2.9M	0.1	10	<0.2	28	8	16	34	86	<0.05	<18	<550	<1700	<0.015	L
SD3 2.9M-3.9M	0.1	7	<0.2	33	10	19	36	99	<0.05	<18	<550	<1700	<0.015	L
SD3 5.9M-6.9M	0.2	12	<0.2	25	13	16	43	80	<0.05	<18	<550	<1700	<0.015	L
SD3 8.9M-9.9M	0.2	12	<0.2	30	12	20	37	75	<0.05	<18	<550	<1700	<0.015	L
SD4 0M-0.9M	0.1	10	<0.2	26	10	15	39	77	<0.05	<18	<550	<1700	<0.015	L
SD4 0.9M-1.9M	0.1	10	<0.2	31	11	16	39	88	<0.05	<18	<550	<1700	<0.015	L
SD4 1.9M-2.9M	0.1	11	<0.2	35	10	20	36	98	<0.05	<18	<550	<1700	<0.015	L
SD4 2.9M-3.9M	0.1	9	<0.2	28	12	17	44	85	<0.05	<18	<550	<1700	<0.015	L
SD5 0M-0.9M	0.2	10	<0.2	27	13	15	46	95	0.08	<18	<550	<1700	<0.015	L
SD5 0.9M-1.9M	<0.1	7	<0.2	24	8	15	31	75	<0.05	<18	<550	<1700	--	L
SD5 1.9M-2.9M	0.1	9	<0.2	30	10	18	37	84	<0.05	<18	<550	<1700	--	L
SD5 2.9M-3.9M	0.2	10	<0.2	23	12	14	46	80	<0.05	<18	<550	<1700	--	L

Notes:

LCEL: Lower Chemical Exceedance Level

UCEL: Upper Chemical Exceedance Level

PCB: Polychlorinated Biphenyl

Low / High M.W. PAHs: Low / High Molecular Weight Polycyclic Aromatic Hydrocarbons

TBT: Tributyltin

Table 5.5 Summary of Sediment Elutriate Test Results at Geophysical Survey Stations conducted under this Study

Parameter (Unit)	WQC	LOR	SD3	SD3	SD3	SD3	SD3	SD3	SD3	SD5	SD5	SD5	SD5	SD5	GB1	GB2	GB4	GB5	GB1	GB2	GB4	GB5
			0M-0.9M	0.9M-1.9M	1.9M-2.9M	2.9M-3.9M	5.9M-6.9M	8.9M-9.9M	Water Blank	0M-0.9M	0.9M-1.9M	1.9M-2.9M	2.9M-3.9M	Water Blank					Water Blank	Water Blank	Water Blank	Water Blank
Mercury (µg/L)	0.3	0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5	<0.5
Arsenic (µg/L)	25	10	20	<10	<10	<10	<10	<10	<10	30	20	<10	20	<10	<10	<10	<10	10	<10	<10	<10	<10
Cadmium (µg/L)	2.5	0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2	<0.2
Chromium (µg/L)	15	1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1
Copper (µg/L)	5	1	<1	<1	1	<1	1	1	2	1	1	2	2	2	1	<1	1	1	2	2	2	2
Lead (µg/L)	25	1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	1	<1	<1	<1	<1	<1	<1	<1	<1	<1
Nickel (µg/L)	30	1	<1	1	<1	<1	1	1	1	1	<1	<1	1	1	1	1	1	1	1	1	<1	<1
Silver (µg/L)	1.9	1	1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1	<1
Zinc (µg/L)	40	10	<10	<10	<10	22	<10	<10	13	<10	<10	<10	<10	16	<10	<10	<10	<10	<10	<10	<10	<10
Total PCBs (µg/L)	0.03	0.18	<0.18	<0.18	<0.18	<0.18	<0.18	<0.18	<0.18	<0.18	<0.18	<0.18	<0.18	<0.18	<0.18	<0.18	<0.18	<0.18	<0.18	<0.18	<0.18	<0.18
Total PAHs (µg/L)	3	9	<9	<9	<9	<9	<9	<9	<9	<9	<9	<9	<9	<9	<9	<9	<9	<9	<9	<9	<9	<9
TBT (µg/L)	0.1	0.015	<0.015	<0.015	<0.015	<0.015	<0.015	<0.015	<0.015	<0.015	<0.015	<0.015	<0.015	<0.015	<0.015	<0.015	<0.015	<0.015	<0.015	<0.015	<0.015	<0.015
Ammonia (mg/L)	-	0.01	0.89	0.47	0.51	0.52	0.71	0.31	0.02	0.96	0.58	0.47	0.45	0.04	0.64	0.65	0.72	1.15	0.02	0.02	0.03	<0.01
Reactive Phosphorous (µg/L)	-	10	170	150	110	100	40	40	<10	220	200	140	140	<10	20	20	20	20	<10	<10	<10	<10
Total Kjeldahl Nitrogen (mg/L)	-	0.1	1.3	1.1	0.9	1	1.5	0.7	0.3	1.4	1.0	0.9	1.0	0.3	1.1	1.1	1.3	3.4	0.3	0.3	0.3	0.2
Total Phosphorous (mg/L)	-	0.1	0.2	0.18	0.15	0.1	0.07	0.08	<0.01	0.25	0.22	0.16	0.19	<0.01	0.06	0.05	0.05	0.09	<0.01	<0.01	0.01	<0.01
Nitrate (mg/L)	-	0.01	0.04	0.01	<0.01	<0.01	0.01	0.03	<0.01	0.07	0.05	<0.01	0.06	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Nitrite (mg/L)	-	0.01	0.07	0.05	0.08	0.1	0.06	0.07	0.05	0.06	0.09	0.14	0.09	0.09	0.04	0.05	0.07	0.06	0.05	0.06	0.09	0.07

Notes:

(a) WQC: Water quality assessment criteria (stipulated under Section 5.5.11); LOR: Limit of Reporting

(b) Contaminant concentration values above the corresponding WQC are underlined, including those with LOR > WQC.

5.4 Water Quality Sensitive Receivers

5.4.1 The water quality sensitive receivers (WSRs) have been identified in accordance with Annex 14 of the Technical Memorandum on EIA Process (EIAO, Cap.499, S.16) and Environmental Impact Assessment Study Brief for Expansion of Sha Tau Kok Sewage Treatment Works (No. ESB-253/2012). These WSRs are illustrated in Figure 5.1 and listed in Table 5.6.

Table 5.6 Water Quality Sensitive Receivers (WSRs) in the Vicinity of the Project Site

Description	Location	Model Output Location
Fisheries Sensitive Receivers
Fish Culture Zones	Sha Tau Kok Ap Chau Kat O O Pui Tong Sai Lau Kong Wong Wan Temporary Relocation Zone of Fish Rafts for the Sha Tau Kok Fish Culture Zone 1 and 2	FCZ1 FCZ2 FCZ3 FCZ4 FCZ5 FCZ6 FCZ7, FCZ8
Spawning and Nursery Grounds of Commercial Fisheries Resources	North Mirs Bay	FCZ2-FCZ6, M8-M14, MP1, MP2 *
Ecological Sensitive Receivers
Seagrass bed	-	SG
Horseshoe crab	Off STKSTW	H1
	Off Pak Hok Lam	H2
	Off Nga Yiu Tau	H3
	A Chau	H4
	Off Luk Keng	H5
Mangrove stand	Off Nga Yiu Tau	M1
	Off Wu Shek Kok	M2
	Off Tai Wan	M3
	Off Luk Keng	M4
	Off Kuk Po	M5
	Kei Shan Tsui	M6
	Tai Sham Chung	M7
	So Lo Pun	M8
	Pak Kok Wan	M9
	Yan Chau Tong Marine Park	M10, M11, M13, M14
	Ngau Shi Wu Wan	M12
Marine Park	Yan Chau Tong	MP1, MP2
Coral sites identified under this EIA	Off Ah Kung Au	T1, T2, T3
EPD Water Quality Monitoring Station
Water Quality Monitoring Station	Mirs Bay Water Control Zone (WCZ)	MM1, MM2, MM3, MM7
*Note: The spawning and nursery grounds of commercial fisheries resources covers a wide range in the Study Area and included about half of the model output locations identified under this Study. The model output location FCZ2 which is closest to the proposed and existing outfall would be adopted to represent the worst case impact to this WSR.

5.4.2 A potential concurrent dredging project by CEDD is proposed to be conducted at STKFCZ, Sha Tau Kok boat shelter, approach channel and dredging area between the shore and the island. When such dredging project proceeds, the fish rafts of STKFCZ may be relocated to temporary relocation sites (FCZ7 and FCZ8) ⁽[1]⁾ shown in Figure 5.1. The temporary relocation sites for fish rafts of the STKFCZ are also considered as WSR in the construction phase sediment dispersion modelling as well as operation phase water quality modelling. Further details on consideration of concurrent project are provided under Section 5.11.

5.5 Assessment Criteria

Water Quality Objectives

Suspended Solids

5.5.1 For fish culture zones (FCZs), seagrass habitat and the Yan Chau Tong Marine Park, elevation in SS concentrations resulting from the Project’s construction and operational activities were assessed against the WQO. The WQO for SS is defined as not to raise the natural ambient level by 30%, nor cause the accumulation of SS which may adversely affect aquatic communities. The assessment criterion is hence defined as the WQO allowable increase in SS concentrations within the corresponding WCZs.

5.5.2 SS data from EPD’s routine water quality monitoring programme in 2005 to 2014 have been analyzed to determine the WQO allowable increase at the WSRs during construction phase. This is calculated as 30% of the ambient level (90^th-percentile value) from the 2005 to 2014 baseline marine water quality data. For each WSR, ambient level was derived from the nearest EPD water quality monitoring station. The assessment criterion for construction phase SS at each WSR is summarized in Table 5.7. For project operation, the potential change in SS level is calculated based on the difference between the predicted level during the baseline and the operation of the TSTP / expanded STKSTW and the values in Table 5.7 are not considered.

5.5.3 For mangrove and horseshoe crab WSRs, organisms living in these habitats are generally adapted to muddy or sandy substrate and used to turbid water. No SS criterion is recommended for these WSRs.

Sediment Deposition Rate and Suspended Solids Criteria for Corals

5.5.4 Impacts to coral communities, if any, have been assessed with regard to sediment deposition. The assessment criterion of 100 g/m²/day, which represents an indicative level above which sustained deposition could harm sediment sensitive hermatypic corals, has been used in approved EIA Reports ^{([2]) ([3]) ([4])} and was adopted here.

5.5.5 Limited isolated colonies of coral have been identified along the coastline of Ah Kung Au. There are no established legislative criteria for water quality at coral communities; however, information on hard coral tolerances to SS indicates that a 20% reduction in annual growth rate corresponds to a 30% increase in average long-term background SS levels ^([5]). WQOs of SS (30% increase) at the identified coral communities are presented in Table 5.7 and have been utilized for determining the acceptability of impacts on corals ^([6]) in this EIA.

Dissolved Oxygen

5.5.6 Oxygen depletion resulting from the Project’s construction and operational activities were assessed against the WQO. The assessment criterion is defined as the WQO allowable depletion in DO levels at the WSRs.

5.5.7 DO data from EPD’s routine water quality monitoring programme from 2005 to 2014 have been analyzed to determine WQO allowable depletion at the WSRs. Allowable DO depletion is calculated as the ambient DO level minus the WQO, i.e. 5 mg/L for fish culture zone, 4 mg/L for depth-averaged values at other WSRs. Ambient level is calculated as the 10^th-percentile value from the 2005 to 2014 marine water quality data for construction phase. For each WSR, ambient level was derived from the nearest EPD water quality monitoring station. The assessment criterion for construction phase DO at each WSR is summarized in Table 5.8. For project operation, the potential change in DO level is calculated based on the difference between the predicted level during the baseline and the operation of the TSTP / expanded STKSTW and the values in Table 5.8 are not considered.

5.5.8 It should be highlighted that DO requirement for WQO requires DO at FCZs to be equal to or above 5 mg/L for 90% of sampling incidents throughout the year. By dividing the baseline data into two seasons, the assessment of DO level would be more conservative for construction phase because the 10^th-percentile DO in wet season and the DO depletion due to SS release from the marine construction are both worse case in wet season, thus impose a more stringent limit on the marine construction works. It should be noted that by dividing the baseline data into two seasons, the 10^th-percentile DO level in wet season drops below 5 mg/L at all EPD monitoring stations in the Study area, thus creating apparent WQO exceedance at all FCZs in wet season. Indeed there is no baseline DO exceedance at MM1 (for STKFCZ) based on consideration of the annual baseline as shown in Table 5.8. This would be further elaborated in the construction phase impact assessment in section 5.8. The same rationale also applies to other WQO parameters including TIN, UIA and E. coli, for operation phase assessment as well.

Table 5.7 Water Quality Assessment Criteria for Suspended Solids (mg/L) at WSRs

Sensitive Receivers	Name	WSR ID	EPD Station	Depth	Wet Season		Dry Season
					Ambient Level ^(a)	WQO Allowable Change	Ambient Level ^(a)	WQO Allowable Change
Fisheries Sensitive Receivers
Fish Culture Zones	Sha Tau Kok	FCZ1	MM1	Depth-averaged	6.12	1.84	6.67	2.00
	Ap Chau	FCZ2	MM1	Depth-averaged	6.12	1.84	6.67	2.00
	Kat O	FCZ3	MM2	Depth-averaged	3.25	0.98	3.91	1.17
	O Pui Tong	FCZ4	MM2	Depth-averaged	3.25	0.98	3.91	1.17
	Sai Lau Kong	FCZ5	MM7	Depth-averaged	2.64	0.79	3.57	1.07
	Wong Wan	FCZ6	MM7	Depth-averaged	2.64	0.79	3.57	1.07
	Temporary Relocation Zone of Fish Rafts for the STKFCZ	FCZ7,FCZ8	MM1	Depth-averaged	6.12	1.84	6.67	2.00
Spawning and Nursery Grounds of Commercial Fisheries Resources	North Mirs Bay	FCZ2	MM1	Depth-averaged	6.12	1.84	6.67	2.00
Ecological Sensitive Receivers
Seagrass bed	Seagrass bed	SG	MM1	Depth-averaged	6.12	1.84	6.67	2.00
Mangrove stand	Off Nga Yiu Tau	M1	MM1	Depth-averaged	6.12	-	6.67	-
	Off Wu Shek Kok	M2	MM1	Depth-averaged	6.12	-	6.67	-
	Off Tai Wan	M3	MM1	Depth-averaged	6.12	-	6.67	-
	Off Luk Keng	M4	MM1	Depth-averaged	6.12	-	6.67	-
	Off Kuk Po	M5	MM1	Depth-averaged	6.12	-	6.67	-
	Kei Shan Tsui	M6	MM1	Depth-averaged	6.12	-	6.67	-
	Tai Sham Chung	M7	MM2	Depth-averaged	3.25	-	3.91	-
	So Lo Pun	M8	MM2	Depth-averaged	3.25	-	3.91	-
	Pak Kok Wan	M9	MM2	Depth-averaged	3.25	-	3.91	-
	Yan Chau Tong Marine Park	M10	MM2	Depth-averaged	3.25	-	3.91	-
	Yan Chau Tong Marine Park	M11	MM2	Depth-averaged	3.25	-	3.91	-
	Yan Chau Tong Marine Park	M12	MM7	Depth-averaged	2.64	-	3.57	-
	Yan Chau Tong Marine Park	M13	MM7	Depth-averaged	2.64	-	3.57	-
	Ngau Shi Wu Wan	M14	MM7	Depth-averaged	2.64	-	3.57	-
Horseshoe crab	Off STKSTW	H1	MM1	Depth-averaged	6.12	-	6.67	-
	Off Nga Yiu Tau	H2	MM1	Depth-averaged	6.12	-	6.67	-
	Off Pak Hok Lam	H3	MM1	Depth-averaged	6.12	-	6.67	-
	A Chau	H4	MM1	Depth-averaged	6.12	-	6.67	-
	Off Luk Keng	H5	MM1	Depth-averaged	6.12	-	6.67	-
Marine Park	Yan Chau Tong	MP1, MP2	MM7	Depth-averaged	2.64	0.79	3.57	1.07
Coral sites identified under this EIA	Off Ah Kung Au	T1, T2, T3	MM1	Bottom	8.22	2.47	8.99	2.70

Notes:

(a) Ambient level is calculated as 90^th percentile of the EPD routine monitoring data of 2005-2014 at respective EPD station close to the WSRs;

(b) This table is applicable for those WSRs which were assessed against the WQO. No assessment criterion is recommended for mangrove and horseshoe crab WSR.

Table 5.8 Water Quality Assessment Criteria for Dissolved Oxygen (mg/L) at WSRs

Sensitive Receivers	Name	WSR ID	EPD Station	Depth	Wet Season		Dry Season		Annual
					Ambient Level ^(a)	WQO Allowable DO Depletion	Ambient Level ^(a)	WQO Allowable DO Depletion	Ambient Level ^(a)	WQO Allowable DO Depletion
Fisheries Sensitive Receivers
Fish Culture Zones	Sha Tau Kok	FCZ1	MM1	Depth-averaged	4.56	-	6.23	1.23	5.06	0.06
	Ap Chau	FCZ2	MM1	Depth-averaged	4.56	-	6.23	1.23	5.06	0.06
	Kat O	FCZ3	MM2	Depth-averaged	4.66	-	6.72	1.72	4.92	-
	O Pui Tong	FCZ4	MM2	Depth-averaged	4.66	-	6.72	1.72	4.92	-
	Sai Lau Kong	FCZ5	MM7	Depth-averaged	4.22	-	6.62	1.62	4.48	-
	Wong Wan	FCZ6	MM7	Depth-averaged	4.22	-	6.62	1.62	4.48	-
	Temporary Relocation Zone of Fish Rafts for the STKFCZ	FCZ7,FCZ8	MM1	Depth-averaged	4.56	-	6.23	1.23	5.06	0.06
Spawning and Nursery Grounds of Commercial Fisheries Resources	North Mirs Bay	FCZ2	MM1	Depth-averaged	4.56	-	6.23	1.23	5.06	0.06
Ecological Sensitive Receivers
Seagrass bed	Seagrass bed	SG	MM1	Depth-averaged	4.56	0.56	6.23	2.23	5.06	1.06
Mangrove stand	Off STKSTW	M1	MM1	Depth-averaged	4.56	0.56	6.23	2.23	5.06	1.06
	Off Wu Shek Kok	M2	MM1	Depth-averaged	4.56	0.56	6.23	2.23	5.06	1.06
	Off Tai Wan	M3	MM1	Depth-averaged	4.56	0.56	6.23	2.23	5.06	1.06
	Off Luk Keng	M4	MM1	Depth-averaged	4.56	0.56	6.23	2.23	5.06	1.06
	Off Kuk Po	M5	MM1	Depth-averaged	4.56	0.56	6.23	2.23	5.06	1.06
	Kei Shan Tsui	M6	MM1	Depth-averaged	4.56	0.56	6.23	2.23	5.06	1.06
	Tai Sham Chung	M7	MM2	Depth-averaged	4.66	0.66	6.72	2.72	4.92	0.92
	So Lo Pun	M8	MM2	Depth-averaged	4.66	0.66	6.72	2.72	4.92	0.92
	Pak Kok Wan	M9	MM2	Depth-averaged	4.66	0.66	6.72	2.72	4.92	0.92
	Yan Chau Tong Marine Park	M10	MM2	Depth-averaged	4.66	0.66	6.72	2.72	4.92	0.92
	Yan Chau Tong Marine Park	M11	MM2	Depth-averaged	4.66	0.66	6.72	2.72	4.92	0.92
	Yan Chau Tong Marine Park	M12	MM7	Depth-averaged	4.22	0.22	6.62	2.62	4.48	0.48
	Yan Chau Tong Marine Park	M13	MM7	Depth-averaged	4.22	0.22	6.62	2.62	4.48	0.48
	Ngau Shi Wu Wan	M14	MM7	Depth-averaged	4.22	0.22	6.62	2.62	4.48	0.48
Horseshoe crab	Off Muk Min Tau	H1	MM1	Depth-averaged	4.56	0.56	6.23	2.23	5.06	1.06
	Off Nga Yiu Tau	H2	MM1	Depth-averaged	4.56	0.56	6.23	2.23	5.06	1.06
	Off Pak Hok Lam	H3	MM1	Depth-averaged	4.56	0.56	6.23	2.23	5.06	1.06
	A Chau	H4	MM1	Depth-averaged	4.56	0.56	6.23	2.23	5.06	1.06
	Off Luk Keng	H5	MM1	Depth-averaged	4.56	0.56	6.23	2.23	5.06	1.06
Marine Park	Yan Chau Tong	MP1, MP2	MM7	Depth-averaged	4.22	0.22	6.62	2.62	4.48	0.48
Coral sites identified under this EIA	Off Ah Kung Au	T1, T2, T3	MM1	Bottom	4.20	2.20	6.20	4.20	4.50	2.50

Notes:

(a) Ambient level is calculated as 10^th-percentile of the EPD routine monitoring data of 2005 to 2014 at respective EPD station close to WSRs

(b) The 10^th-percentile DO levels from 2005 – 2014 are all below 5 mg/L at all selected EPD monitoring stations in wet season. The potential DO depletion at FCZs would be considered based on annual 10^th-percentile DO level. Further elaboration would be provided in the construction phase impact assessment in section 5.8.

(c) Reference has been made to EIAO Guidance Note No. 7/2010 (section 4.2) regarding dry (November to March) and wet season (April to October).

Criteria for Nutrients & Bacteria

5.5.9 Elevation in the levels of nutrients and bacteria, if any, as a result of the Project’s construction and operational activities were assessed in accordance with the respective WQO summarized in Table 5.1.

Criteria for Fish Culture Zones

5.5.10 For FCZs, in accordance with the WQO, the DO criterion is set at > 5 mg/L measured as water column average. The maximum allowable SS elevation and DO depletion is provided in Table 5.7 and Table 5.8 above. Level of E.coli at FCZs should also be below 610 cfu/100 mL per WQO requirement of the Mirs Bay WCZ.

Criteria for Dissolved Metals and Organic Compounds

5.5.11 There are no existing regulatory standards or guidelines for dissolved metals and organic contaminants in the marine waters of Hong Kong. It is thus proposed to make reference to relevant international standards and this approach has been adopted in previous approved EIAs, i.e., EIA for Decommissioning of Cheoy Lee Shipyard at Penny’s Bay ⁽[7]⁾, EIA for Disposal of Contaminated Mud in the East Sha Chau Marine Borrow Pit ⁽[8]⁾, EIA for Wanchai Development Phase II ⁽[9]⁾, EIA for Liquefied Natural Gas (LNG) Receiving Terminal and Associated Facilities ⁽[10]⁾, and EIA for Hong Kong Offshore Wind Farm in Southeastern Waters ⁽[11]⁾. Table 5.9 shows the assessment criteria for dissolved metals and organic compounds for this Study.

Table 5.9 Summary of Assessment Criteria for Dissolved Metals and Organic Compounds

Parameter	Unit	Assessment Criteria for this Study
Metals
Cadmium (Cd)	mg/L	2.5 ^(a) (b)
Chromium (Cr)	mg/L	15 ^(a) (b)
Copper (Cu)	mg/L	5 ^(a) (b)
Nickel (Ni)	mg/L	30 ^(a) (b)
Lead (Pb)	mg/L	25 ^(a) (b)
Zinc (Zn)	mg/L	40 ^(a) (c)
Mercury (Hg)	mg/L	0.3 ^(b)
Arsenic (As)	mg/L	25 ^(a) (b)
Silver (Ag)	mg/L	1.9 ^(d)
Total PAHs	mg/L	3.0 ^(f)
PCBs
Total PCBs	mg/L	0.03 ^(d)
Organotins
Tributyltin (TBT)	mg/L	0.1 ^(e)

Notes:

(a) UK Environment Agency, Environmental Quality Standards (EQS) for List 1 & 2 dangerous substances, EC Dangerous Substances Directive (76/464/EEC) (http://www.ukmarinesac.org.uk/activities/water-quality/wq4_1.htm).

(b) Annual average dissolved concentration (i.e. usually involving filtration a 0.45-um membrane filter before analysis).

(c) Annual average total concentration (i.e. without filtration).

(d) U.S. Environmental Protection Agency, National Recommended Water Quality Criteria, 2009. (http://www.epa.gov/waterscience/criteria/wqctable). The Criteria Maximum Concentration (CMC) is an estimate of the highest concentration of a material in surface water (i.e. saltwater) to which an aquatic community can be exposed briefly without resulting in an unacceptable effect. CMC is used as the criterion of the respective compounds in this study.

(e) Salazar MH, Salazar SM (1996) Mussels as Bioindicators: Effects of TBT on Survival, Bioaccumulation, and Growth under Natural Conditions. In Organotin, edited by M.A. Champ and P.F. Seligman. Chapman & Hall, London.

(f) Australian and New Zealand Environment and Conservation Council (ANZECC), Australian and New Zealand Guidelines for Fresh and Marine Water Quality (1992)

5.5.12 There are no existing regulatory standards or guidelines for total PCBs, total PAHs and tributyltin (TBT) in water and hence reference has been made to the United States Environmental Protection Agency (USEPA) water quality criteria, Australian water quality guidelines, and international literature, respectively. The assessment criteria for total PCBs, total PAHs and TBT are 0.03 μg/L, 3.0 μg/L and 0.1 μg/L respectively.

Standard for Reuse of Treated Effluent

5.5.13 Reuse of treated effluent for non-potable purposes is proposed under this Project to minimize usage of potable water of the expanded STKSTW. To ensure the quality of retreated effluent is good for reuse for non-potable purposes, the following standard from Ngong Ping STW (for toilet flushing and controlled irrigation), North District (for toilet flushing, unrestricted irrigation & water features), Lo Wu Correction Institution (for toilet flushing) and WSD’s water quality objectives for toilet flushing would be adopted under this Study:

Table 5.10 Proposed Reclaimed Water Quality for non-potable uses within STKSTW

Determinand	Unit	Proposed Reclaimed Water Quality Criteria for STKSTW *
pH	n/a	6-9 ^(d)
Turbidity	NTU	≤ 2 ^(b-d)
Total Suspended Solids	mg/L	≤ 10 ^(a)
Biochemical Oxygen Demand (BOD5)	mg/L	≤10 ^(a)
Colour	Hazen Unit	≤ 20 ^(a)
Ammonia Nitrogen	mg/L	≤ 1 ^(a)
Threshold Odour Number	T.O.N	≤ 100 ^(a)
Synthetic Detergents	mg/L	≤ 5 ^(a)
Escherichia coli	cfu/100ml	Not Detectable ^(b-d)
Dissolved Oxygen	mg/L	≥ 2 ^(a)
Residual Chlorine	mg/L	≥ 1 ^(b)(e)(For cleansing and toilet flushing)
	mg/L	≤ 1^(d) (For landscape irrigation)
*Note: The effluent standards are adopted from: (a) Agreement. No. CE 29/2001 – Outlying Islands Sewerage Stage 1 Phase 1 – Ngong Ping Sewage Treatment Works and Sewerage Investigation, Design and Construction. (b) Agreement No. CE 16/2004 (DS) – Demonstration Scheme on Reclaimed Water Uses in the North District – Investigation (c) Contract No. SS P320 – Design and Construction of the Redevelopment of Lo Wu Correctional Institution (d) Guideline for Water Reuse, (2004), USEPA

5.6 ASSESSMENT METHODOLOGY

General Methodology

5.6.1 The methodology employed to assess potential water quality impacts associated with the construction and operation of the Project is presented in the Water Quality Method Statement (Annex 5A) and has been based on the information presented in the Project Description (Section 2). Full details of the scenarios examined in the modelling works are provided in Annex 5A. As discussed previously, the WSRs in the vicinity of the Project are presented in Figure 5.1.

Construction Phase

5.6.2 Impacts due to the dispersion of fine sediment in suspension and the subsequent sedimentation during the construction of the proposed submarine outfall have been simulated using Delft3D PART module. The adoption of cofferdam construction by sheetpiling is considered in the modelling. Since the sediment removal at outfall structure would be conducted in dry condition after cofferdam construction is completed and the inside of the cofferdam be drained, no sediment release into the water column would be expected from the sediment removal under this Project. Assessment for sediment release from sheetpiling for the installation and removal of cofferdam is therefore considered the only source of sediment release and would be assessed under this Study. Assessment of cumulative impact from concurrent project would be conducted based on the dredging rate from the preliminary assessment of corresponding project proponent. The adoption of mitigation measures in concurrent projects, such as silt curtain, would be considered where possible.

5.6.3 The depletion of DO and the elevation in nutrient levels associated with the release of SS are calculated using the modelled maximum SS concentrations.

Dissolved Oxygen Depletion

5.6.4 The degree of DO depletion exerted by a sediment plume is a function of the sediment oxygen demand of the sediment, its concentration in the water column and the rate of oxygen replenishment. The impact of the sediment oxygen demand on DO concentrations has been calculated based on the following equation ⁽[12]⁾:

5.6.5 The assumption behind this equation is that all the released organic matter is eventually re-mineralized within the water column. This leads to an estimated depletion with respect to the background DO concentrations. This DO depletion depends on the quality of the released sediments, i.e. on the percentage of organic matter in the sediment. The fraction of organic matter in sediment (Chemical oxygen demand in Table 5.3) was taken as 17,000 mg/kg based on maximum data from EPD Sediment Monitoring Stations MS1 located near the Project Site from 2009-2013.

5.6.6 This is a conservative prediction of DO depletion since oxygen depletion is not instantaneous and will depend on tidally averaged suspended sediment concentrations. It is worth noting that the above equation does not account for re-aeration which would tend to reduce impacts of the SS on DO concentrations in the water column. The proposed analysis, which is on the conservative side, will not, therefore, underestimate the DO depletion. Further, it should be noted that, for sediment in suspension to exert any oxygen demand in the water column will take time and, in the meantime, the sediment will be transported and mixed or dispersed with oxygenated water. As a result, the oxygen demand and the impact on DO concentrations will diminish as the suspended sediment concentrations decrease.

5.6.7 As discussed in section 5.3, baseline 10^th-percentile DO level at all EPD marine water quality monitoring station is below 5 mg/L in wet season. For DO depletion assessment at FCZ WSRs, further assessment based on annual 10^th-percentile baseline data is conducted.

Nutrients

5.6.8 An assessment of nutrient release during marine dredging for submarine outfall has been carried out based on the predicted SS elevation and the testing results of EPD sediment monitoring station. In the calculation it is assumed that all total kjeldahl nitrogen (TKN) concentrations in the sediments are released to the water. This is a highly conservative assumption and will result in the overestimation of the potential impacts.

5.6.9 The maximum predicted SS concentration at each WSR is multiplied by the maximum concentration of TKN in sediment (mg/kg) at the EPD sediment quality monitoring station MS1 from 2009-2014 to give the maximum elevation in TIN (mg/L). While nitrate and nitrite may also be constituent of TIN in marine water, they are generally in negligible concentration in view of low electrochemical potential of marine sediment. The calculations of maximum elevation in TIN (from TKN) at WSRs are shown below:

5.6.10 Ammonia nitrogen is the sum of ionized ammonia and unionized ammonia (UIA). Under normal conditions of Hong Kong waters, more than 90% of the ammonia nitrogen would be in the ionized form. EPD marine water quality monitoring data at MM1 from 1986 to 2014 indicated that on average 7.8% of ammonia nitrogen exists as UIA. For the purpose of assessment, this average value would be adopted for estimation of UIA from disturbance of marine sediment due to marine construction. In view that the mineralization of the organic nitrogen will also contribute to ammonia, the calculations of NH₃-N are based on maximum total TKN concentrations from the nearby EPD sediment quality monitoring station MS1 from 2009-2014. TKN is the total of ammonia nitrogen and organic nitrogen. Note that it is a highly conservative approach since it is assumed that 100% of organic nitrogen will be mineralized to ammonia but this is unlikely to occur in reality.

5.6.11 The maximum SS concentration at each WSR is multiplied by the following factors to predict the maximum UIA elevations:

Heavy Metals and Micro-Organic Pollutants

5.6.12 Sediment elutriate test was conducted under this Study to determine the level of potential leaching of sediment-bounded pollutants during the marine construction for cofferdam. The level of pollutant presence in elutriate would be compared against the water quality assessment criteria stipulated under Section 5.5.11 and 5.5.12. In case the level pollutants presence in the elutriate exceedance the proposed water quality assessment criteria, the dispersion of pollutants would be modelled using Delft3D WAQ (inert, non-settling tracer) based on the maximum level of pollutants identified in the geophysical survey conducted under this Study, assuming 100% release of sediment-bounded pollutants.

Effluent Discharge from Temporary Sewage Treatment Plant

5.6.13 Based on the latest design information, a TSTP would be constructed to provide sewage treatment service during the expansion works at the existing STKSTW. Tentatively the TSTP would be commenced in by end 2018 and its operation would be phased out gradually and be replaced by the expanded STKSTW from 2021 to 2022. Based on the TSTP design, there would not be a net increase in pollution loading from the operation of the TSTP as compared with the operation of the STKSTW at present. Therefore, modelling assessment of the normal operation of the TSTP is not considered necessary. Detailed calculation is shown in Appendix 5L.

Operation Phase

5.6.14 As discussed in Section 2 Project Description above, the treatment capacity of the STKSTW would be increased from 1,660 m³/day (existing capacity) of average dry weather flow (ADWF) to 5,000 m³/day ADWF upon the completion of the stage 1 expansion in 2021. The treatment capacity would be further increased to 10,000 m³/day by the completion of the stage 2 expansion in 2030. For the purpose of this EIA, the assessment year of operation phase water quality assessment would be 2030. The WAQ module of the Delft3D suite of modelling would be used to predict the potential impact from the operation of the expanded STKSTW.

5.6.15 Based on the latest design information, a number of design features have been taken into account in to design of the expanded STKSTW to ensure no discharge of untreated sewage would be allowed on any occasion. Further elaboration would be provided in section 5.8.

Modelling Scenarios

5.6.16 The operation phase water quality modelling scenarios are summarized below in Table 5.11.

Table 5.11 Modelling Scenarios for Delft3d Water Quality Modelling Scenarios

Scenarios	Simulation Year	Simulated Flow from STKSTW (m^3/day)	Description
Far field water quality - baseline	2011	1,660 at existing outfall	Baseline scenario of 2011 at actual flow rate and actual effluent concentration of the existing STKSTW for model verification
Far field water quality - baseline	2030	1,660 at existing outfall	Baseline scenario of 2030 at maximum ADWF of the existing STKSTW for project operation
Far field water quality - operation	2030	10,000 at new submarine outfall	Operation scenario of 2030 at maximum ADWF of the expanded STKSTW

5.6.17 The near field behavior of effluent plume from the proposed submarine outfall is simulated using CORMIX. Detailed methodology and assumptions are provided in Annex 5A. The prediction of near field modelling of effluent plume is provided in Annex 5B. The predicted vertical profile of the effluent plume (i.e. plume height and plume thickness) is taken into account in the far field Delft3D WAQ simulation to provide a more detail account of the effluent plume. For conservative assessment, all pollution loading would be discharged within the same horizontal grid cell where the outfall is located.

Compilation of Background Loading

5.6.18 The projected population data of 2031 are provided by the Planning Department. The Territorial Population and Employment Data Matrices (TPEDM) of 2011, 2016, 2026 and 2031 are used for compiling storm runoff in the modelled area. Linear interpolation between 2026-based and 2031-based TPEDM is conducted to generate the population data of 2030. References have been made to the methodologies adopted in the EPD’s Update on Cumulative Water Quality and Hydrological Effect of Coastal Developments and Upgrading of Assessment Tool (also known as “the Update Study”, the approved EIA of HATS Stage 2A in compiling the pollution loading from population data in HK. Detailed methodology on the compilation of background pollution loading is provided in Annex 5A and would not be discussed further in this document. Reference has also been made to the approved EIA of the Tai Po Sewage Treatment Works Stage 5 on the background pollution loading from the Mainland side of the Mirs Bay.^([13])

Sewage Effluent from STKSTW

5.6.19 Based on the latest design information, the effluent flow and load of the STKSTW under the baseline, interim and operation phase scenarios are provided below in Table 5.12. The maximum allowed concentration would be adopted for simulation for all scenarios (except model verification scenarios) under this Study. For scenarios for model verification, the mean effluent discharge concentration is adopted.

Table 5.12 Flow and Load of Effluent Discharged from the STKSTW in Modelled Scenarios

Parameters	Unit	Baseline	Operation
Flow	m^3/s	0.0192	0.1157
BOD5	mg/L	10/20/40	10/20/40
SS	mg/L	15/30/60	15/30/60
TN	mg/L	22/43/86	6/12/24
Total Phosphate	mg/L	4/5/10	3/4/8
E. Coli	count/100mL	100/1500	100/1500

Note: For all parameters except E.coli, the first, second and third numbers indicate the mean, 95th percentile and maximum concentration respectively. For E.coli, the first number indicates the monthly geometric mean value while the second number indicates the 95%ile concentration.

5.6.20 As shown in Table 5.12, the loading assumed (maximum allowed concentration), is 2.5 (ortho-phosphate phosphorus) to 15 (E.coli) times of the mean effluent concentration. For comparison, the peaking factor of the expanded STKSTW is expected to be 3 and the peak wet weather flow (PWWF) would be 30,000 m³/day. This means the modelled scenarios ADWF with maximum effluent concentration would give rise to pollution load greater than the PWWF with mean effluent concentration for most of the pollutant parameters. It is therefore considered the adoption of maximum concentration would be sufficiently conservative and modelling of PWWF discharge (which would only last for very short period of time) is not necessary.

5.6.21 Not used.

Uncertainties in Assessment Methodology

Uncertainties in Sediment Transport Assessment

5.6.22 Uncertainties in the assessment of the impacts from sediment plumes will be considered when drawing conclusions from the assessment. In carrying out the assessment, the worst case assumptions have been made in order to provide a conservative assessment of environmental impacts. These assumptions are as follows:

· The calculations of loss rates of sediment to suspension are based on conservative estimates for the methods of working;

· The assessment is based on peak rate for sheetpile installation and removal for cofferdam construction. In reality these will only occur for a short period of time

Uncertainties arising from Operations

5.6.23 The uncertainties in operation phase water quality modelling assessment include the followings:

· Potential change in effluent discharge on the Mainland side of the Mirs Bay; and

· Potential change in capacity of mariculture activities in Starling Inlet and the rest of the Mirs Bay.

5.6.24 It should be highlighted that the pollution loading from the Mainland side of the Mirs Bay was adopted from the approved EIA of Tai Po Sewage Treatment Works Stage 5 and there could be some changes on the number of pollution sources as well as pollution loading intensity for each sources. Yet most WSRs under this Study are located within Starling Inlet, where pollution loadings are captured more accurately with the pollution loading inventory compiled using the latest population data (discussed in details in Annex 5A). Other WSRs beyond Starling Inlet are all located on or near the shoreline of the Shuen Wan Country Park and is away from the Mainland coastline where pollution sources are located. As such, the limitation on the accuracy of pollution loading from the Mainland side of the Mirs Bay would not significantly affect the reliability of the prediction of the water quality modelling exercise.

5.6.25 The pollution loading of mariculture activities in Starling Inlet and the rest of the Mirs Bay are adopted from the approved EIA of HATS Stage 2A and the Update Study. In the Update Study, pollution loading of mariculture activities was estimated based on the followings:

· Wastage and leaching of fish feed;

· Excretion and faecal production from fish; and

· Disposal of dead fish

5.6.26 The scale of mariculture may vary from year to year depending on mariculturists’ expectation of market demand and other commercial considerations. However, the AFCD Departmental Annual Reports (http://www.afcd.gov.hk/english/publications/publications_dep/publications_dep.html) indicate there is a trend for decrease in an annual production of mariculture fish through year 2001 to 2013. It is expected that the decrease in production scale would result in less fish feed wasted, less fish excreta and faeces and less dead fish, thus a lower pollution loading. It is therefore considered the pollution loading calculated based on 1997 data in the Update Study a conservative estimation to be used under this Study.

Limitation in Near Field Models

5.6.27 CORMIX has two key limitations. It assumes steady-state conditions and unidirectional, uniform flow in the receiving water body. Secondly, CORMIX has simplified geometric capabilities.

5.6.28 It should be highlighted that the tidal current within the Startling Inlet is unidirectional in each of the ebb and flood tide condition. During flood tide, the tidal current near the proposed new submarine outfall flows into Starling Inlet at around 210° of compass angle. During ebb tide, the tidal flows at around 30° of compass angle. The direction of the tidal current is quite stable throughout the (ebb and flood) tides with exception of a short period when the tide changes its direction. Also, the water depth near the proposed new submarine outfall is quite shallow (around 7.3 m) and the profile of water temperature, salinity and flow velocity is quite homogeneous throughout the water column. Therefore, the actual conditions are not expected to deviate significantly from the assumed steady-state conditions and unidirectional, uniform flow in the receiving water body.

5.6.29 CORMIX also assumes an idealized water body with straight sides and a uniform bottom along the flow direction. To ensure conservative prediction using CORMIX and limits the potential deviation from the actual conditions due to uneven seabed, only the predicted vertical profile of effluent plume from the CORMIX near field simulations would be implemented in the far field Delft3D water quality modelling. Pollutants from outfall discharge are assumed to discharge within the same horizontally grid cell where the outfall diffuser of the proposed submarine outfall will be located.

5.7 Potential Sources of Impact

5.7.1 Potential sources of impacts to water quality arising from the Project may occur during both the construction and operation phases. Each is discussed in turn below.

Construction Phases

5.7.2 Marine-based and land-based construction activities of the Project have the potential to affect water quality through:

· Changes in water quality, including suspended sediment dispersion, sediment deposition, DO depletion, and elevated concentrations of nutrients, heavy metals and micro-organic pollutants, due to marine construction at submarine outfall;

· Decommissioning of existing submarine outfall;

· Vessel discharges;

· Land-based site runoff from construction workforce; and

· Spillage of chemicals.

Operation Phase

5.7.3 The potential impacts to water quality arising from the expanded STKSTW would be the effluent discharge from the proposed submarine outfall. The Project is a sewage treatment works and therefore no thermal discharge is expected. The Project would be constructed within footprint of existing developed area and would not encroach into any river or water course. Also the treatment of the Project would be based on MBR and therefore chlorine or biocide use and discharge would not be expected. The use, storage and handling of chemical in the TSTP and the expanded STKSTW is expected to be similar to the existing plant and significant impact of water quality due to chemical spillage is not expected.

5.7.4 While the proposed reuse of treated effluent is not expected to result in undesirable water impacts, caution and other potential environmental issues associated would be addressed qualitatively as well.

5.8 Impact Assessment

Construction Phases

5.8.1 A brief tentative programme for the construction and operation of this Project is provided below in Pane 5.1.

Pane 5.1 Tentative Programme for Project Construction and Operation

Note: The pane shows an overlapping period of the TSTP and the expanded STKSTW during the initial commissioning of the expanded STKSTW. Please note that the TSTP and the expanded STKSW would not operate concurrently. TSTP would use the existing outfall while the expanded STKSTW would use the proposed outfall. There is no incident when discharge from both outfalls occurs at the same time.

Elevation of Suspended Solids and Sedimentation

5.8.2 Based on the latest design information, it is understood that marine sediment at submarine outfall would be excavated within the fully-drained watertight cofferdam. Schematics showing the side view and plan view of the proposed cofferdam at the proposed outfall are provided below. Therefore, release of sediment to the water column (from ordinary marine dredging) is not anticipated. Instead, the potential for sediment release from installation of sheetpiles (and removal after the completion) for cofferdam installation, which is the only marine construction process that may disturb bottom sediment and result in resuspension, would be assessed. Other temporary works for the construction of the submarine outfall would also be conducted within the cofferdam so water quality impact on the nearby marine waters is not expected.

Schematics (Side View and Plan View) for Cofferdam Construction at Proposed Submarine Outfall

5.8.3 Sheetpiles that are used for cofferdam construction come with varies dimension. A schematic drawing showing the typical cross-section of sheetpile is shown below in Pane 5.2. Based on the preliminary design information, the length (measured in the horizontal direction, w in Pane 5.2) would be about 300 – 500 mm and the breadth (measured in the horizontal direction, h in Pane 5.2) would be about 100 – 150 mm. The longitudinal length (which goes vertically down into the sediment when installed) varies and the sheetpiles are generally cut at appropriate length according to installation depth.

t

Pane 5.2 Typical Cross-section for Cofferdam Sheetpiles

5.8.4 During installation of cofferdam, sheetpiles are driven down into the sediment by vibration. Vibratory installation of sheetpiles for marine works is being adopted in major marine construction projects including HKBCF and TM-CLKL and is known to have limited impact on bottom sediment ^{([14]) ([15]) ([16])}. During installation, a vibratory hammer would be used to press sheetpiles downward. Vibration transmitted from the vibration hammer would reduce friction experienced by the sheetpiles and allow the sheetpiles to go down more easily. Limited level localized resuspension of bottom sediment could be resulted from the vertical vibrational motion from the sheetpile.

5.8.5 Similarly, the sheetpiles would be pulled out from the sediment using vibratory hammer. Since the removal of sheetpiles involves more significant upward motion, the sediment release due to removal of sheetpiles would be modelled for the worst case assessment.

5.8.6 In the transport of excavated materials, sediment may be lost through leakage from barges. However, dumping permits in Hong Kong include requirements that barges used for the transport of dredging materials have bottom-doors that are properly maintained and have tight-fitting seals in order to prevent leakage. Given this requirement, sediment release during transport is not proposed for modelling and its impact on water quality will not be addressed under this Study.

5.8.7 Sediment is also lost to the water column when discharging material at disposal sites. It is considered that potential water quality issues associated with disposal at the intended government disposal site(s) have already been assessed by Civil Engineering and Development Department (CEDD) and permitted by EPD, hence and the environmental acceptability of such disposal operations is demonstrated. Therefore modelling of impacts at disposal sites does not need to be addressed and reference to relevant studies will be provided in the EIA for this Project where appropriate.

5.8.8 Review of sediment density has been conducted based on previously accepted EIAs and DIR in Hong Kong which includes the followings:

· Shatin to Central Link - Hung Hom to Admiralty Section (AEIAR-166/2012). EP granted in Feb 2012 (EP-064/2000).

· ShaTin to Central Link Protection Works at Causeway Bay Typhoon Shelter (AEIAR-159/2011). EP granted in Feb 2011 (EP-416/2011).

· Wan Chai Development Phase II and Central-Wan Chai Bypass (AEIAR-125/2008). EP granted in Dec 2008 (EP-356/2009).

· Liquefied Natural Gas (LNG) Receiving Terminal and Associated Facilities (AEIAR-106/2007). EP granted on 3 April 2007 (EP-257/2007).

· The Proposed Submarine Gas Pipelines from Cheng Tou Jiao Liquefied Natural Gas Receiving Terminal, Shenzhen to Tai Po Gas Production Plant, Hong Kong – EIA Study (AEIAR-071/2003). EP granted on 23 April 2003 (EP-167/2003).

· 132kV Submarine Cable Installation for Wong Chuk Hang - Chung Hom Kok 132kV Circuits (AEP-126/2002). EP granted on 2 April 2002 (EP-126/2002).

· FLAG North Asian Loop (AEP-099/2001). EP granted on 18 June 2001 (EP-099/2001).

· East Asian Crossing (EAC) Cable System (TKO), Asia Global Crossing (AEP-081/2000). EP granted on 4 October 2000 (EP-081/2000).

· East Asian Crossing (EAC) Cable System, Asia Global Crossing(AEP-079/2000). EP granted on 6 September 2000 (EP-079/2000).

· Submarine Cable Landing Installation in Tong Fuk Lantau for Asia Pacific Cable Network 2 (APCN 2) Fibre Optic Submarine Cable System, EGS. EP granted on 26 July 2000 (EP-069/2000).

· Telecommunication Installation at Lot 591SA in DD 328, Tong Fuk, South Lantau Coast and the Associated Cable Landing Work in Tong Fuk, South Lantau for the North Asia Cable (NAC) Fibre Optic Submarine Cable System (AEP-064/2000). EP granted in June 2000 (EP-064/2000).

5.8.9 Dry density of marine sediment varies from 600 kg/m³ to 1600 kg/m³ based on the above EIAs. The upper limit of 1600 kg/m³ would be taken for assessment in this Study. It is conservatively assumed that the first 1 meter of sediment at the surface of the seabed that is enclosed in three sides (i.e. w × h in Pane 5.2) would be disturbed by the removal of the sheetpiles and be release to the water column continuously throughout the process. Sediment below 1 m of the existing seabed level is expected to be suppressed by the weight of sediment above and would unlikely be brought up to the surface by the action of sheetpiles. Typically, it takes around 3 hours for a piece of sheetpile to pass through 5 – 10 m of sediment by vibratory hammer. In this modelling exercise, it is assumed that the 500 mm (length) × 150 mm (breadth) sheetpile would be used and installed / removed at a rate of 3 hours per piece to maximize the potential sediment loss rate. The location and extent of the sediment that is considered above is illustrated in Pane 5.3 below. It is expected that only part of the disturbed sediment would be entrained during the installation / removal of sheetpiles. An entrainment percentage of 20% is taken from jetting EIAs / DIR projects reviewed above. It is considered conservative since the disturbance of marine sediment by installation / removal sheetpiles is expected to be far less significant than the jetting process assessed in the relevant EIAs / DIR projects above. It is also assumed that sheetpiles would be installed / removed at both end of the cofferdam simultaneously (so the modelled rate of sediment release would be doubled).

5.8.10 Based on the above assumptions, the maximum volume of sediment that would be disturbed = (60 + 60 + 30 + 30) m (perimeter of excavation area) × 0.15 m (width) × 1 m (depth) = 27 m³ (in-situ volume).^([17]) Based on the above assumptions, the maximum (horizontal) length of sheetpiles installed / removed per day would be 500 mm (horizontal length per piece of sheetpile) ÷ 3 hr (time required per piece) × 12 hr (working hours per day) × 2 (installation at both ends of cofferdam) = 4 m.

Pane 5.3 Location and Extent of Sediment Released by the Movement of Sheetpiles

Note: The above schematic is prepared to illustrate the location and extent of sediment that is expected to be disturbed by the movement of sheetpiles. It is not prepared in scale and certain engineering details may not be presented.

5.8.11 The estimated volume and mass of disturbed sediment is 27 m³ and 43,200 kg respectively. Working hours are assumed to be 12 hours per day with a maximum rate of sheetpile installation / removal of 4 m³/day (0.333 m/hr), giving a rate of release (in kg/s) of sediment as follows:

Loss Rate (kg/s)

= Total mass of Sediment Disturbed (kg) ÷ Total Time Required (s) × Entrainment Percentage

= 43200 kg ÷ (180 m ÷ 0.333 m/hr) × 20%

= 43200 kg ÷ 540hr × 20%

= 16 kg/hr

= 0.0044 kg/s

5.8.12 In view of the very small sediment loss rate, it is assumed no silt curtain would be installed to contain the sediment loss from the installation and removal of sheetpiles under unmitigated scenario. Given the small extent of marine construction works area, one stationary source at the outfall discharge point is assumed in the model to represent the installation / removal of sheetpiles at both ends.

Suspended Solids Dispersion

5.8.13 Simulation results of SS elevation arising from the removal of sheetpile at submarine outfall in both the dry and wet seasons are presented below in Table 5.13. It should be noted that the values presented in Table 5.13 are the instantaneous maximum levels predicted at the specific WSRs during the specified season (dry / wet season) throughout the simulation period. The predicted SS elevation is compared to the proposed assessment criteria stipulated under Section 5.5.2 and Table 5.7. The predicted percentage time of compliance to assessment criterion for SS elevation is also shown in Table 5.13 for each WSR. Contour plots of tidal period average and maximum depth-averaged SS elevation are also provided in Annex 5D-1 and Annex 5D-2 for both seasons.

5.8.14 Annex 5D indicated that the sediment plume tends to spread further away from the sediment source in wet season, resulting in higher SS elevation in the WSRs within Starling Inlet. As shown in Table 5.13, low level of SS elevation is predicted at the Sha Tau Kok Fish Culture Zone (STKFCZ), the proposed temporary relocation zones 1 and 2 of fish rafts as well as the location of seagrass. The predicted maximum level of SS elevation is 0.0541 mg/L at the temporary relocation zone 1 of fish rafts for the STKFCZ (FCZ7) in wet season. The predicted maximum SS elevation is 0.0267 mg/L at the STKFCZ, which is further away. The predicted maximum SS elevation is 0.0255 mg/L at the temporary relocation zone 2 (FCZ8). The predicted SS elevation is far below the recommended assessment criterion of 2.63 mg/L (in dry season) and 1.83 (in wet season). No observable elevation of SS would be observed at other WSRs identified under this Study. It should be highlighted that the predicted values are the instantaneous maximum that once occurs throughout the whole simulation period of dry and wet season. The level of SS elevation experienced at these WSRs would generally be below these predicted values. As such, no adverse impact on the identified WSRs from SS elevation would be expected from the proposed sheetpile removal operation for the submarine outfall.

Table 5.13 Predicted Maximum Elevation in Suspended Solid and Sediment Deposition at WSRs from Marine Dredging at Submarine Outfall

WSR (ID)	SS Elevation (mg/L)						Sediment Deposition (g/m^2/day)
	Wet Season			Dry Season			Criteria	Wet Season	Dry Season
	Criteria	Max	Compliance Time %	Criteria	Max	Compliance Time %		Max	Max
Fish Culture Zone (Depth-averaged)
Sha Tau Kok	1.83	0.0267	100.00%	2.63	0.0001	100.00%	-	-	-
Ap Chau	1.83	0.0000	100.00%	2.63	0.0000	100.00%	-	-	-
Kat O	1.21	0.0000	100.00%	2.66	0.0000	100.00%	-	-	-
O Pui Tong	1.21	0.0000	100.00%	2.66	0.0000	100.00%	-	-	-
Sai Lau Kong	1.01	0.0000	100.00%	2.55	0.0000	100.00%	-	-	-
Wong Wan	1.01	0.0000	100.00%	2.55	0.0000	100.00%	-	-	-
Temporary Relocation Zone of Fish Rafts for the Sha Tau Kok Fish Culture Zone (FCZ7)	1.83	0.0541	100.00%	2.63	0.0080	100.00%	-	-	-
Temporary Relocation Zone of Fish Rafts for the Sha Tau Kok Fish Culture Zone (FCZ8)	1.83	0.0255	100.00%	2.63	0.0004	100.00%	-	-	-
Spawning and Nursery Grounds of Commercial Fisheries Resources	1.83	0.0000	100.00%	2.63	0.0000	100.00%	-	-	-
Seagrass (Depth-averaged)
Seagrass bed	1.17	0.0003	100.00%	2.33	0.0000	100.00%	-	0.0008	0.000
Mangrove Stand (Depth-averaged)
Off STKFCZ	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
Off Wu Shek Kok	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
Off Tai Wan	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
Off Luk Keng	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
Off Kuk Po	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
Kei Shan Tsui	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
Tai Sham Chung	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
So Lo Pun	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
Pak Kok Wan	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
Yan Chau Tong Marine Park	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
Yan Chau Tong Marine Park	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
Yan Chau Tong Marine Park	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
Yan Chau Tong Marine Park	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
Ngau Shi Wu Wan	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
Horseshoe Crab (Depth-averaged)
Off Muk Min Tau	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
Off Nga Yiu Tau	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
Off Pak Hok Lam	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
A Chau	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
Off Luk Keng	-	0.0000	100.00%	-	0.0000	100.00%	-	-	-
Marine Park (Depth-averaged)
Yan Chau Tong (MP1)	0.58	0.0000	100.00%	1.15	0.0000	100.00%	-	-	-
Yan Chau Tong (MP2)	0.58	0.0000	100.00%	1.15	0.0000	100.00%	-	-	-
Coral sites identified under this EIA (Bottom)
Off Ah Kung Au (T1)	2.47	0.0091	100.00%	2.70	0.0009	100.00%	100	0.0444	0.0881
Off Ah Kung Au (T2)	2.47	0.0012	100.00%	2.70	0.0001	100.00%	100	0.0069	0.0460
Off Ah Kung Au (T3)	2.47	0.0092	100.00%	2.70	0.0008	100.00%	100	0.0544	0.0659

5.8.15 As discussed under Section 5.10.4 below, the CEDD’s “Sediment Removal at Sha Tau Kok Fish Culture Zone, Boat Shelter and Approach Channel” Project (referred as the sediment removal project hereafter) would be tentatively conducted from the first half of 2017 to the first half of 2018 to remove sediment from the STKFCZ, STK boat shelter and approach channel, as well as the dredging area between the island and the shore. Since the marine construction of cofferdam of this Project would also be conducted in 2017, marine construction of these projects may be conducted concurrently. As such, the sediment removal project will be assessed as a concurrent project under this Study. The extent of dredging under the sediment removal project is shown in Figure 5.3. The tentative dredging rates for different area of the sediment removal project would be 2600 (for STKFCZ), 800 (for boat shelter and approach channel) and 300 (for dredging area between the shore and the island) m³/day respectively. The selected location of sediment sources under this Project and the sediment removal project are also shown in Figure 5.3. Based on the latest information provided by CEDD, silt curtains will be adopted for marine dredging at all dredging locations.

5.8.16 The predicted level of SS elevation from the Project alone, concurrent project alone and cumulative impact are tabulated below in Table 5.14. It should be highlighted that the predicted maximum cumulative SS elevation shown is a simple addition of predicted maximum of this Project and the concurrent sediment removal project. This is a conservative approach since the maximum SS level predicted at a specific receiver contributed from this Project does not necessarily occur at the same time as the concurrent project, thus simple addition would result in overestimation of the cumulative SS level at WSRs. It should be highlighted that only WSRs with observable level of SS elevation predicted under the Project alone scenario is presented below. Also, since the fish rafts in STKFCZ (FCZ1) would be relocated to the proposed relocation sites (FCZ7 and FCZ8) during the proposed sediment removal works project by CEDD, STKFCZ would not be considered as a WSR under the cumulative water quality impact assessment. Contour plots showing the maximum and mean tidal average of depth average cumulative SS elevation are provided in Annex 5D.

Table 5.14 Predicted Cumulative Maximum Elevation in Suspended Solid at WSRs from all Concurrent Marine Works

WSR (ID)	SS Elevation (mg/L)
	Wet Season				Dry Season
	Criteria	Project alone	Concurrent Project alone	Cumulative	Criteria	Project alone	Concurrent Project alone	Cumulative
Fish Culture Zone (Depth-averaged)
Temporary Relocation Zone of Fish Rafts for the Sha Tau Kok Fish Culture Zone (FCZ7)	1.83	0.0541	0.2268	0.2809	2.63	0.0080	0.0190	0.0270
Temporary Relocation Zone of Fish Rafts for the Sha Tau Kok Fish Culture Zone (FCZ8)	1.83	0.0255	0.5226	0.5481	2.63	0.0004	0.0207	0.0211
Seagrass (Depth-averaged)
Seagrass bed	1.17	0.0003	0.7914	0.7917	2.33	0.0000	0.0109	0.0109
Coral sites identified under this EIA (Bottom)
Off Ah Kung Au (T1)	2.47	0.0091	0.0159	0.0250	2.70	0.0009	0.0000	0.0009
Off Ah Kung Au (T2)	2.47	0.0012	0.0289	0.0301	2.70	0.0001	0.0000	0.0001
Off Ah Kung Au (T3)	2.47	0.0092	0.0604	0.0696	2.70	0.0008	0.0000	0.0008

5.8.17 As shown in Table 5.14, the predicted cumulative maximum SS elevation from the concurrent construction of submarine outfall under this Project as well as the CEDD’s sediment removal project would be in compliance with the corresponding SS criterion of WQO in both seasons. The predicted SS elevation would be the highest at the seagrass WSR, which maximum contribution of 0.0003 mg/L and 0.7914 mg/L respectively from this Project and the sediment removal project. Since no non-compliance of the SS criterion is predicted, no adverse cumulative SS impact from concurrent construction is expected.

5.8.18 In the dry season, the predicted potential contribution from the CEDD’s sediment removal project of sedimentation flux at coral site T1, T2 and T3 is below 0.0001 g/m²/day. The cumulative sedimentation flux at coral site T1, T2 and T3 is predicted to remain at 0.0881, 0.0460 and 0.0659 g/m²/day, which indicate full compliance with the corresponding assessment criteria. In the wet season, the predicted potential contribution from the CEDD’s sediment removal project of sedimentation flux at coral site T1, T2 and T3 is respectively 0.4763, 0.4981 and 0.7144 g/m²/day. The cumulative sedimentation flux at coral site T1, T2 and T3 would be 0.5207, 0.5050 and 0.7688 g/m²/day respectively, which also indicate full compliance with the corresponding assessment criteria.

DO Depletion

5.8.19 Dispersion of SS may release sediment-bounded pollutant into the water column. Readily-biodegradable organic compounds could be taken up by micro-organism and result in depletion of dissolved oxygen (DO). An assessment of DO depletion during sheetpile removal is made in relation to the results of the sediment plume modelling of sheetpile removal activities and the sediment quality data near the Study Area. Only WSRs with predicted SS elevation is considered in the below assessment for DO depletion based on the methodology stipulated under Section 5.6.4 above. The allowed level of DO depletion is provided in Table 5.8 above. The predicted maximum DO depletion from sheetpile removal under this Project is presented in Table 5.15 below.

Table 5.15 Predicted Maximum Depletion in Dissolved Oxygen at Selected WSRs from Sheetpile Removal at Submarine Outfall

WSR (ID)		Wet Season						Dry Season
		Max SS Elevation (mg/L)		Max DO Depletion (mg/L)		Allowed DO Depletion (mg/L)		Max SS Elevation (mg/L)		Max DO Depletion (mg/L)		Allowed DO Depletion (mg/L)
Fish Culture Zone (Depth-averaged)
Sha Tau Kok	0.0267		0.0005		-		0.0001		>0.0001		1.29
Temporary Relocation Zone of Fish Rafts for the Sha Tau Kok Fish Culture Zone (FCZ7)	0.0541		0.0009		-		0.0080		0.0001		1.29
Temporary Relocation Zone of Fish Rafts for the Sha Tau Kok Fish Culture Zone (FCZ8)	0.0255		0.0004		-		0.0004		>0.0001		1.29
Seagrass (Depth-averaged)
Seagrass bed	0.0003			>0.0001		0.84		>0.0001		>0.0001		2.29
Coral sites identified under this EIA (Bottom)
Off Ah Kung Au (T1)	0.0091		0.0002		2.20		0.0009		>0.0001		4.20
Off Ah Kung Au (T2)	0.0012		>0.0001		2.20		0.0001		>0.0001		4.20
Off Ah Kung Au (T3)	0.0092		0.0002		2.20		0.0008		>0.0001		4.20

5.8.20 DO level for Sha Tau Kok Fish Culture Zone (STKFCZ, FCZ1) as well as the temporary relocation zones of fish rafts for the STKFCZ (FCZ7 and FCZ8) in wet season under baseline condition is observed to be below 5 mg/L in wet season based on the adopted 2005 to 2014 marine water quality monitoring data from the nearby EPD monitoring station MM1. Marine water quality monitoring data indicates that about 89.027% of the time the DO level at MM1 would be above 5 mg/L in wet season. Based on the estimation above, the potential depletion of dissolved oxygen would be well below 0.001 mg/L, which is less than 0.025% of the ambient DO level in wet season. The predicted level of DO depletion is expected to be well below any daily variation due to temperature change and photosynthetic activities of algae ^([18]). With the additional DO depletion by less than 0.001 mg/L, the percentage time with DO level above 5 mg/L would be slightly decreased to 89.021%. As shown, the predicted level of DO depletion is very small in terms of both absolute level as well as the induced reduction of time with DO level above 5 mg/L. It is therefore considered the potential change in DO level would exert limited effect on the fish culture zone receivers.

5.8.21 It should be highlighted that the WQO requires level of DO be above 5 mg/L at FCZs for 90% of the sampling occasions. This means dividing the baseline DO data into two seasons would be over-conservative for the worse part of the year (in terms of DO). Without considering the seasons, the 10^th-percentile DO level calculated from the 2004-2014 EPD monitoring data is 5.3 mg/L at MM1. This means the allowed maximum DO depletion would be 0.3 mg/L if the baseline DO data of the whole year is considered without dividing baseline data into two seasons. Since the maximum DO depletion predicted at the most impacted WSR (FCZ7) is only 0.0009 mg/L in wet season and 0.0001 mg/L in dry season, no exceedance is DO level would be expected in baseline and construction phase. In view of the above, no unacceptable water quality impact from DO depletion on identified WSRs due to sheetpile removal at submarine outfall is anticipated. No additional mitigation measure is required for the potential DO depletion.

Elevation of Nutrients

5.8.22 Other than readily-biodegradable organic compounds (which are assessed in the previous section), sediment-bounded nitrogenous compounds could also be released into the water column and result in an increase of total inorganic nitrogen (TIN) and UIA. Sediment quality data from 2009 - 2013 (provided in Table 5.3 above) indicated that maximum level of TKN is 620 mg/kg near the outfall location (represented by EPD sediment monitoring station MS1). Nevertheless, sediment elutriate test results (provided in Table 5.5 above) indicated that there will be dissolution of ammonia nitrogen as well as organic nitrogen from sediment disturbance, which could result in a change in water quality. Based on the conservative prediction of TIN and UIA using the maximum predicted SS elevation at FCZ7 (0.0541 mg/L in wet season) and the maximum TKN level in EPD sediment quality data from 2009-2013 (620 mg/kg), the predicted maximum elevation in TIN and UIA is calculated as follow:

5.8.23 As shown in the calculation above, only minimal elevation of TIN and UIA is expected at the FCZ7, which is the most adversely impacted by the proposed sheetpile removal. The predicted maximum elevation of TIN and UIA is below 1% of the allowed level of elevation (i.e. WQO criterion minus background level) as shown in Table 5.16 below. Potential elevation of TIN and UIA at other WSRs are expected to be even lower. No exceedance of WQO TIN and UIA criteria is expected at FCZ7 and any other WSRs from the proposed sheetpile removal operation. No adverse water quality impact from the release of nutrient due to marine construction for cofferdam is anticipated.

Table 5.16 Predicted Worst Case Elevation of TIN and UIA and its comparison with the corresponding WQO Criteria

Unit: mg/L	TIN			UIA
	WQO	Background Level	Allowed Elevation	WQO	Background Level	Allowed Elevation
MM1	0.3	0.10	0.20	0.021	0.002	0.019
MM2	0.3	0.07	0.23	0.021	0.001	0.020
MM3	0.3	0.07	0.23	0.021	0.001	0.020
MM7	0.3	0.06	0.24	0.021	0.001	0.020
Predicted Elevation at Most Adversely Impacted Receivers
Temporary Relocation Zone of Fish Rafts for the Sha Tau Kok Fish Culture Zone (FCZ7)	0.0000335			0.00000262
Note: Annual mean of TIN and UIA given in Table 5.2 were taken as the background level at each EPD monitoring stations.

Elevation of Heavy Metals and Micro-organic Pollutants

5.8.24 The proposed sheetpile removal operation may disturb bottom sediment and results in release of sediment-bounded pollutants, namely heavy metals, metalloid (arsenic) and trace organic compounds (PAHs, PCBs and TBT). Results of sediment sampling and testing are shown in Table 5.4 and results of sediment elutriate test are shown in Table 5.5. As shown in Table 5.4, the level of heavy metal and trace organic pollutants are generally low at the survey area with a few exceptions of elevated arsenic level in sediment samples. Table 5.5 also indicated the level of dissolution of contaminants from sediment would be low and in compliance to the proposed assessment criteria, except for arsenic in one of the sediment sample (SD5 0 m - 0.9 m). Also, the limit of reporting for elutriate analysis of total PCBs and total PAHs is higher than that of the corresponding proposed water quality criteria. Therefore, it is not possible to deduce whether there will be exceedance in total PCBs and total PAHs from elutriate testing results. For the purpose of water quality impact assessment, the potential impact from the release of contaminants from sediment would be conducted for arsenic, total PCBs and total PAHs. Conservative assessment of total PCBs and total PAHs would be conducted assuming the level of total PCBs and total PAHs as the same as the limit of reporting from the sediment testing results. Tracer modelling using Delft3D WAQ has been conducted to determine the potential loss of contaminants (arsenic, total PCBs and total PAHs) to the water column.

5.8.25 The predicted maximum elevation at the worst impacted WSRs (FCZ7) are presented in Table 5.17 below. The maximum elevations of pollutants are predicted to occur at the surface layer of the water column as a result of relatively higher current velocity at the surface. Thus only the predicted elevation at surface are presented in below. As shown, the predicted maximum elevation would be at least 3 orders of magnitude (i.e. one thousandth of) below the corresponding assessment criteria and no exceedance is predicted. No adverse impact from release of contaminants is expected from the proposed sheetpile removal operation. Contour plots showing the predicted maximum level of arsenic, total PCBs and PAHs are provided in Annex 5F.

Table 5.17 Predicted Maximum Elevation of Pollutants at FCZ7 and its comparison with the corresponding Assessment Criteria

Parameter	Assessment Criteria (mg/L)	Maximum Contaminant Level (mg/kg)	Predicted Maximum Pollutant Elevation in Wet Season (mg/L)	Predicted Maximum Pollutant Elevation in Dry Season (mg/L)
Arsenic	25	16	1.88 × 10^-2	2.12 × 10^-2
Total PCBs	0.03	0.018	2.12× 10^-^5	2.38 × 10^-^5
Total PAHs	3	2.25	2.64 × 10^-3	2.98 × 10^-3

Other Marine Construction

5.8.26 As discussed in the previous section, the construction of outfall pipeline from the expanded STKSTW to the proposed outfall diffuser location would be conducted using trenchless HDD method, which would proceed from the landside and would not involve any drilling from seaside. All drilling fluid will be handled at the landside work site and there will be no risk of marine spillage of drilling fluid.

5.8.27 The construction of the diffuser would be conducted after the dry excavation of marine sediment in the cofferdam. It is expected that the construction of diffuser would be conducted similar to other land-based construction works and no adverse water quality impact would be expected with the implementation of appropriate standard site practices and mitigation measures described in Section 5.9.4.

5.8.28 Demolition of the existing submarine outfall is considered not necessary. The opening of the abandoned submarine outfall will be sealed and no other action would be required. No adverse water quality impact would be expected from such action.

General Construction Activities

5.8.29 Discharges and runoff from the site during the construction phase, particularly during site formation, excavation and backfilling works, will contain SS which could be a source of water pollution. Drill cuttings (rock debris from drill hole) would be separated from used HDD drilling fluid and be disposed of as fill material onsite while used drilling fluid would be reconditioned (if required) and be reused as far as practicable. Spent drilling fluid which is no longer fit for reuse would be dewatered and disposed at landfill. Uncontrolled disposal of debris and rubbish such as packaging, construction materials and refuse and spillages of chemicals stored on-site, such as drilling fluid, oil, diesel and solvents would also result in contamination of construction site runoff. In view of the relatively small works area and also limited building construction, there will only be limited effluents generated from dewatering associated with piling activities, grouting and concrete washing. However, it is anticipated that no unacceptable water quality impacts would arise from the land-based works if standard site practices and mitigation measures outlined in ProPECC PN 1/94 “Construction Site Drainage”, described in Section 5.9.4, are in place and properly implemented.

Temporary or Accidental Discharge of Sewage Effluent from the STKSTW / TSTP

5.8.30 Emergency discharge is a major concern for sewage treatment works project. In case of emergency discharge, raw of partially treated sewage carrying pollution loading which is way higher than the normal level in treated effluent would be discharged into the receiving water body. Such incident could result in elevation in pollution level and turbidity, as well as reduction in dissolved oxygen or other secondary effects on water quality. Such incidents should be avoided as far as practicable through various design measures. As shown in the Project programme in Pane 5.1 above, the TSTP would be in concurrent operation with the existing STKSTW during the initial period of Project construction. This allows the use of the existing STKSTW as the backup option during the initial commissioning of the TSTP, when the operation of TSTP is more prone to error. In case of temporary malfunctioning of the TSTP in the initial commissioning, the diversion of the sewage influent to TSTP would be terminated and the sewage could be treated by the existing STKSTW.

5.8.31 The following design measures are also provided in the TSTP to avoid the risk of emergency discharge:

· Routine/ regular checking of the equipment

· Provision of dual power supply and backup generator to eliminate the risk of failure of dual power supply;

· Provision of standby equipment for all treatment units;

· 24-hour monitoring on the operation of TSTP;

· Install a remote control and monitoring system (SCADA) to allow off-site monitoring; and

· Provision of onsite storage of raw sewage up to 6 hours ^([19]).

5.8.32 The hierarchy of the above design measures for preventing emergency discharge from the TSTP is illustrated in Pane 5.4 below. As shown, there is a series of design measures to avoid any emergency discharge from the TSTP. Continuous failure of these design measures is highly unlikely. A remote control and monitoring system (SCADA) for sewage treatment facilities in TSTP would be provided, allowing inspection and any necessary repair works by DSD be initiated at the earliest possible time. As the plant would be manned during operation, it is believed that any necessary response actions in emergency situations would be promptly initiated and completed within 6-hour of emergency storage ⁽¹⁹⁾.

Pane 5.4 Hierarchy of Design Measures to Prevent Emergency Discharge for the TSTP

5.8.33 As required under the Appendix D of the Study Brief, this EIA should include the water quality impacts of temporary, accidental and emergency discharges at the TSTP and the expanded STKSTW. Water quality modelling exercise has been conducted for selected scenario as specified under Annex 5I. Detailed finding of the emergency discharges at the TSTP is provided below, together with the modelling assessment of the emergency discharge scenario from the expanded STKSTW.

Sewage Effluents from Construction Workforce

5.8.34 If needed, appropriate numbers of portable toilets shall be provided by a licensed contractor to serve the construction workers over the construction site to prevent direct disposal of sewage into the water environment. The Contractor shall also be responsible for waste disposal and maintenance practices.

Spillage of Chemicals

5.8.35 Site drainage should be well maintained and good construction practices should be observed to ensure that drilling fluid, oil, fuels, solvents and other chemicals are managed, stored and handled properly and do not enter the nearby marine waters or streams. No adverse water quality impacts are expected with proper implementation of the recommended mitigation measures.

Operation Phase

5.8.36 The potential impacts to water quality arising from the operation of the expanded STKSTW would be the effluent discharge from the proposed submarine outfall. When compared with the baseline scenario (no STKSTW expansion), effluent flow discharged from the expanded STKSTW would be much higher (from 1,660 m³/day ADWF for baseline to 10,000 m³/day ADWF for project operation). The effluent water quality would be improved for certain water quality parameters due to the adoption of more advanced treatment technology. Also the effluent would be discharged at the proposed outfall location near the opening of Starling Inlet via a diffuser for project operation. The potential change in water quality would be assessed by comparing the difference between the baseline scenario and the project scenario.

5.8.37 As discussed in the previous sections, between the commencement of construction works at the existing STKSTW and the commencement of the expanded STKSTW, a TSTP would be set up to provide continuous sewage treatment within the sewage catchment. Moving Bed Biofilm Reactors (MBBR) would be adopted for TSTP and primary treatment with chemical dosing would be provided such that TSTP can comply with the existing discharge license of STKSTW.

Selection of Assessment Years and Modelling Scenarios

5.8.38 Based on the tentative timeline for the expanded STKSTW and TSTP under this Project is summarized in Table 5.18 below.

Table 5.18 Tentative Timeline for the Expanded STKSTW and TSTP

Year / Month	TSTP	STKSTW
2017	Construction of TSTP	-
2018	Commissioning of TSTP (ADWF: 2,500 m3/day)	-
2019	TSTP Operation	Construction of Expanded STKSTW
2020	TSTP Operation	Construction of Expanded STKSTW
2021	TSTP Operation	Commissioning of Expanded STKSTW Phase 1 (ADWF: 5,000 m^3/day)
2022	TSTP Operation	Operation of the Expanded STKSTW Phase 1 operation
2023-2029	-	Expanded STKSTW Phase 1 operation
2030	-	Commissioning of Expanded STKSTW Phase 2 (ADWF: 10,000 m^3/day)

5.8.39 As shown in the tentative timeline shown above, the construction of the TSTP (ADWF: 2,500 m³/day) and the expanded STKSTW would be commenced by 2017. The TSTP would be completed and commissioned by end 2018. The expanded STKSTW would be commissioned by phases, with the phase 1 expansion (ADWF: 5,000 m³/day) commission in 2021 and the phase 2 expansion (ADWF: 10,000 m³/day) commission in 2030. The operation of the TSTP is expected to discontinue in 2022 when the phase 1 expansion of the STKSTW is in its full operation.

5.8.40 Water quality modelling assessment for the operation of the phase 2 expansion of the STKSTW would be conducted for 2030, assuming maximum design effluent pollutant concentration at design ADWF. In view of the much lower ADWF for the phase 1 expansion of the STKSTW, it is considered not necessary to model the operation of phase 1 expansion of the STKSTW.

5.8.41 It should be highlighted that this Project aims to provide sufficient and appropriate sewage treatment capacity to cope with the growth of community within the sewage catchment. Population will grow regardless of the presence of Project. Without the sewage treatment capacity provided by the expansion of the STKSTW, increased sewage from population growth cannot be appropriately treated. This additional loading will potentially be discharged back into Starling Inlet indirectly via (1) inappropriate discharge of sewage to storm drain and (2) overflow of untreated sewage from the existing STKSTW when the maximum treatment capacity is reached. The above effect has not been taken into account in the baseline scenarios (no expansion of STKFCZ) and the potential benefits in water quality from the Project would therefore not be observable in the modelling assessment.

5.8.42 An additional water quality model has been conducted for year 2011. The results of this model are used for water quality model verification and are presented in Annex 5C.

Sewage Loading from STKSTW

5.8.43 Sewage effluent concentration and designed ADWF of the existing and expanded STKSTW are shown in Table 5.12 above. The modelled effluent loadings are shown in Table 5.19 below. The annual average of actual loading discharge from STKSTW in 2011 is also provided in Table 5.19.

Table 5.19 Pollution Load of Effluent Discharged from the STKSTW in Modelled Scenarios

Parameters	Unit	Baseline	Operation (% change against baseline)	Year 2011 Actual Loading (Annual Average) ^(1)
BOD5	g/day	66400	400000 (502.4%)	2463
SS	g/day	99600	600000 (502.4%)	2342
TN	g/day	142760	240000 (68.1%)	8690
TIN	g/day	104691	160000 (52.8%)	5699
NH3-N	g/day	38069	40000 (5.1%)	2194
Organic Nitrogen	g/day	38069	80000 (110.1%)	2992 ^(2)
NO3 and NO2	g/day	66621	120000 (80.1%)	3504
E. coli	count/day	2.49×10^7	1.50×10^8 (502.4%)	8.69×10^5

Note: Loading for all parameters are calculated based on the upper limit level stipulated in Table 5.12 and the corresponding ADWF.

⁽¹⁾ Actual loading compiled based on measurements taking at STKSTW in 2011. Annual average of actual loading from STKSTW in 2011 is presented here. The actual loading from STKSTW (daily-varying) is used for water quality model verification. See Annex 5C for details.

⁽²⁾ Actual measurements data not available. Reference has been made to the approved EIA of Tai Po Sewage Treatment Works Stage 5. As given in Table A4.7 of Appendix 4.1 of the approved EIA, the daily loading of organic nitrogen was assumed to be 16,083 g/day, based on daily flow rate of 4,785 m³/day. This means the assumed concentration of organic nitrogen from STKSTW is about 3.354 mg/L.

5.8.44 As shown in Table 5.19 above, the increase in loading for operation scenario is not necessarily proportional to the increase in effluent flow. It is because the effluent quality could be improved by the adoption of new treatment technology in the expanded STKSTW. Improvements on effluent concentration for certain parameters are expected for the operation of the expanded STKSTW (when compared with the existing STKSTW). Improvements on effluent concentration are expected for Org-N, NH₃-N, NO₃, NO₂, OP and TP and their corresponding increase in loading is much lower than the increase in effluent flow rate (1,660 m³/day to 10,000 m³/day, 502.4% increase). The levels of 5-day biochemical oxygen demand (BOD₅), suspended solids (SS) and E.coli in effluent from the expanded STKSTW remain the same as the existing STKSTW and the corresponding loadings increase proportionally.

5.8.45 It should also be highlighted that the expanded STKSTW will adopt membrane bio-reactor (MBR) technology which does not require disinfection for achieving the proposed E. coli effluent quality standard. As such, there will not be any discharge of biocide, residual chlorine and chlorination by-products from the operation of the expanded STKSTW.

5.8.46 It should be highlighted that the assumed loadings from STKSTW for the model scenarios under baseline and operation are very conservative. As shown in Table 5.19, the modelled loadings from STKSTW are at least a few times greater than the actual loading in 2011. It is because the pollution loadings from STKSTW are calculated from the maximum effluent concentration and the maximum design ADWF. In reality, the existing plant and expanded plant would operate with effluent discharge below the proposed ADWF. The effluent quality would also be generally below the mean level stated in Table 5.19 above. As such, the modelled scenarios represent the reasonably worst case scenarios for the operation of the existing STKSTW and expanded STKSTW for the purpose of this EIA.

Background Pollution Loading

5.8.47 Other than pollution loading from the STKSTW, other water pollution sources are taken into account in the water quality modelling exercise. Background pollution loadings from runoff, rainfall related loading and other point source discharge within the model domain were compiled for each of the assessment years according to the method adopted in Update on Cumulative Water Quality and Hydrological Effect of Coastal Developments and Upgrading of Assessment Tool by EPD in 1997 (referred as the Update Study hereafter). In view of the proximity to the Mainland, the pollution loadings of Mainland discharges were from the approved EIA of the Tai Po Sewage Treatment Works Stage 5 Expansion. Details on the compilation of background pollution loading are stipulated in the water quality modelling methodology statement in Annex 5A and would not be provided here.

Verification of Water Quality Model

5.8.48 In addition to the water quality modelling scenario in 2030 for the assessment of potential change in water quality from the operation of the expanded STKSTW, an additional water quality model has been conducted for 2011. Actual flow rate and effluent quality from the existing STKSTW would be adopted in this model (in contrast to the use of design ADWF and maximum effluent pollutant concentration for modelling scenarios in 2030). Also, background pollution loadings were also compiled based on the corresponding population data from the 2011-based Territorial Population & Employment Data Matrices (TPEDM). The modelling predictions were compared against the marine water quality monitoring results of 2011 to ensure the pollution loadings and water quality modelling parameters are appropriately set to provide a reasonably conservative estimation of the actual situation. The year 2011 was chosen for verification of the water quality model performance in view of the following: (1) the availability of 2011-based TPEDM (from Population Census 2011), which means interpolation to other year is not required; (2) while population prediction of year 2016 is also available in the 2011-based TPEDM, the EPD water quality monitoring data of 2016 was not available when the verification was commenced. The approach and results of the water quality model verification exercise were provided in detail in Annex 5C and would not be further detailed here. The predicted water quality of modelling verification shown in Annex 5C are in generally slightly worse than the corresponding field data of the same period. This means same modelling settings and approach for compiling the pollution loading would result in conservative predictions and are therefore acceptable for the purpose of this assessment.

Near Field Dispersion Modelling

5.8.49 To determine the near field behavior of the effluent plume (which cannot be resolved by far field modelling tools like Delft3D) from the location of the existing and the proposed new outfall, near field dispersion modelling has been conducted using CORMIX suite of model. CORMIX is a USEPA-supported mixing zone model and decision support system for environmental impact assessment of regulatory mixing zones resulting from continuous point source discharges. The CORMIX suite of model is widely used in various kinds of near field simulation for river and marine discharge world-wide. The details of near field dispersion modelling are provided in Annex 5B and would not be provided here. The predictions of near field dispersion modelling of effluent discharge at the proposed submarine outfall are summarized below in Table 5.20.

Table 5.20 Summary of Near Field Dispersion Modelling Results

Scenario	Ambient Flow Velocity	Probability	Horizontal Distance from the Outfall to the Edge of Near Field Region (m)	Plume Top Level (m from seabed level)	Plume Bottom Level (m from seabed level)	Plume Thickness (m)
For Existing STKSTW
Dry-10^th	10%ile of ambient	0.2	841.38	2.00	1.01	0.99
Dry-50^th	50%ile of ambient	0.6	84.49	2.00	1.95	0.05
Dry-90^th	90%ile of ambient	0.2	14.61	2.00	1.94	0.06
Wet-10^th	10%ile of ambient	0.2	401.55	2.00	1.60	0.40
Wet -50^th	50%ile of ambient	0.6	36.27	2.00	1.96	0.04
Wet -90^th	90%ile of ambient	0.2	5.02	2.00	1.90	0.10
Weighted Average (Dry Season)				2.00	1.760	0.240
Weighted Average (Wet Season)				2.00	1.876	0.124
For Proposed Expanded STKSTW
Dry-10^th	10%ile of ambient	0.2	1011.08	7.15	6.52	0.63
Dry-50^th	50%ile of ambient	0.6	2.57	7.15	3.58	3.57
Dry-90^th	90%ile of ambient	0.2	15.37	7.15	1.14	6.01
Wet-10^th	10%ile of ambient	0.2	168.31	6.31	5.68	0.62
Wet -50^th	50%ile of ambient	0.6	103.94	4.84	3.56	1.28
Wet -90^th	90%ile of ambient	0.2	65.00	4.29	1.72	2.57
Weighted Average (Dry Season)				7.15	3.680	3.470
Weighted Average (Wet Season)				5.02	3.616	1.408

5.8.50 According to the near field dispersion modelling prediction, the effluent plume tends to be buoyant and float to the surface layer of the water column in dry season. It is because the effluent from STKSTW is generally of lower salinity than that of seawater, and therefore has slightly lower density than the environment when first emerge from the diffuser. In contrast, the effluent plume is much less buoyant in wet season, as a result of lower salinity of ambient seawater. None of the effluent plume modelled reaches the water surface in wet season. Also, it is observed that the plume height of the effluent plume decreases as ambient current velocity increases. The reason is that entrainment, which dilute the effluent plumes and reduces its buoyance, increases according to current velocity. As shown in Table 5.20 above, the bottom level of the effluent plume drops from 5.68 m above seabed level for 10^th-percentile ambient current velocity to 3.56 m for median ambient current velocity, and then to 1.72 m for 90^th-percentile ambient current velocity in dry season. The top level of the effluent plume remains at the water surface at all modelled dry season scenario due to relatively stronger buoyance. For wet season, reduction of plume height is also predicted to increase, with observed reduction of both plume top and bottom level as ambient flow velocity increases.

5.8.51 The near field dispersion model prediction would be taken into account in the far field Delft3D model according the requirement stipulated in the EIA Study Brief Appendix D-1 under this Project. As shown in Table 5.20 above, the weighted average plume top and bottom level at the proposed submarine outfall in dry season would be 7.15 m and 3.68 m above seabed level respectively. Since the average water depth at the outfall is 7.15 m in dry season, this means the effluent would be located in the top 50% of the water column at the near field region. Similarly, the average top and bottom level of the effluent plume at the proposed submarine in wet season would be 5.02 m and 3.62 m respectively, which correspond to about 50% to 70% of the water depth above seabed level. The predicted vertical effluent profile would be taken into account in the water quality modelling scenarios in the corresponding seasons respectively.

5.8.52 Near field dispersion modelling has also been conducted for discharge from the existing STKSTW. Results (provided in Table 5.20) for modelling indicate the effluent plume would stay at the surface of the water column in both seasons. The predicted vertical effluent profile would also be taken into account in the Delft3D WAQ modelling. Considering the near field plume behaviour, no change in hydrology and flow regime is expected from the proposed outfall location. Please refer to Annex 5B for the detailed results of the near field dispersion modelling.

Outfall Tracer Modelling

5.8.53 Simple modelling has been conducted using Delft3D WAQ conservative tracers. This exercise aims to (1) visualize the dispersion of pollutants for discharge from different outfall locations, (2) identify major zones of impact from discharge at different sources, (3) identify effect of different hydrodynamic on pollutant dispersion and (4) identify seasonal difference in dispersion pattern. This modelling exercise provides some insights on dispersion pattern that eventually help the interpretation of water quality modelling results. Three types of conservative tracers (Ctr1, Ctr2 and Ctr3) have been used to simulate the dispersion of pollutants discharge from the STKSTW (existing/expanded), the STKFCZ and other sources of pollution (dry weather load plus rainfall load) respectively. Brief approach on this exercise is provided below and contour plots showing the results are provided in Annex 5G.

Approach

5.8.54 For each type of pollution sources, a fixed discharge rate of 1000 g/s of conservative tracer is assumed in the model. For tracer discharge from existing/expanded STKSTW, tracer is assumed to discharge at the outfall location. For tracer discharge from the STKFCZ, it is assumed the tracer would be evenly distributed across the whole gazetted area of the STKFCZ (which covers a total 32 horizontal grid cells in the STK Model), which is the same as the arrangement for actual water quality modelling exercise. Similarly for other sources of pollution (dry weather load plus rainfall load), the tracer loading is assumed to be evenly distributed to 6 major discharge locations within Starling Inlet. For easy comparison, all tracers are assumed to be discharged at the surface layer of the water column.

5.8.55 A total of 2 tracer modelling scenarios have been conducted based on the corresponding hydrodynamic. They include:

· 2030 water quality baseline scenarios with 1,660 m³/day ADWF from existing STKSTW

· 2030 operation scenario with 10,000 m³/day ADWF from expanded STKSTW (at new submarine outfall)

5.8.56 The mean surface tracer concentration within the Study area is presented in Annex 5G. Annex 5G-1 to 5G-3 shows the tracer dispersion for discharge from STKSTW (existing/expanded), the STKFCZ and other sources of pollution (dry weather load plus rainfall load) respectively in dry season. The corresponding plots for wet season are provided in Annex 5G-7 to 5G-9. Comparison for tracer dispersion pattern is also provided for baseline vs. operation of the expanded STKSTW for two kinds of tracers representing different sources. Comparison plots for dry season are provided in Annex 5G-4 to 5G-6 and that for wet season are provided in Annex 5G-10 to 5G-12. The value presented in the comparison plots are the fraction of predicted level of tracer under the two stipulated scenarios. For instance, plots showing “Baseline/Operation” show the fraction of predicted tracer levels, with the tracer level from the baseline as numerator and that from the operation scenario as denominator. Values below 1 (one) indicate decrease in tracer level (i.e. improvement) for changing from baseline scenario to operation scenario, and vice versa.

Change in Pollutant Dispersion Pattern from Different Discharge Locations

5.8.57 Contour plots showing the distribution of conservative tracer for discharge from STKSTW (existing/expanded) are provided in Annex 5G-1 and 5G-7 in dry and wet seasons respectively. The discharge from the existing STKSTW and the expanded STKSTW are shown in the top and bottom plots respectively. As shown, the dispersion at the proposed submarine outfall site is significantly better than the existing outfall location in both seasons. Much lower level of tracer is predicted within Starling Inlet. Also, the tracer plume is also predicted to expand outward slightly. Both of these indicate higher rate of material exchange with the northern Mirs Bay for discharge at the proposed submarine outfall.

Major Zone of Impact from Different Sources

5.8.58 As shown in Annex 5G-1 and 5G-7, the tracer level is relatively high at the northwestern side of Starling Inlet for discharge from the existing in both seasons, with a smaller extent in wet season. For discharge from the proposed submarine outfall for the expanded STKSTW, the tracer plume is quite localized around the outfall, with slight elongation in the northeast to southwest direction (which is the direction of tidal current).

5.8.59 For the discharge from the STKFCZ, the tracer plume is quite localized around the boundary of the FCZ. As shown in Annex 5G-2, the tracer plume extends to the west significantly in dry season. Since dispersion of tracer is more favorable on the east side, the tracer plume does not extend to the east because of faster dilution. As shown in Annex 5G-8, the westward extension of the plume becomes smaller, but extensions to the north and the east emerge in wet season. It is considered a result of increase in storm discharge “pushing” the tracer plume outward (i.e. eastward) of Starling Inlet.

5.8.60 As shown in Annex 5G-3 and 5G-9, the tracer plumes from other sources in the STK catchment occupy similar location in both seasons. The size of tracer plume is slightly bigger in wet season. It should also be highlighted that tracer plumes from these other sources overlap significantly with the tracer plumes from the existing / TSTP outfall. This means the WSRs at these locations is significantly affected by the pollution loading from both sources.

Seasonal Difference

5.8.61 Differences in size of tracer plumes from STKSTW outfall are notable in dry (Annex 5G-1) and wet season (Annex 5G-8). The dispersion at existing outfall is significantly smaller in wet season. It is expected the increase in runoff in wet season contributes to this by increased dilution of tracer. Also, increased runoff also “pushes” tracer out of Starling Inlet, thus enhance material exchange.

Results of Modelling Prediction

5.8.62 Annual results of water quality modelling for relevant water quality parameters are summarized in Table 5.21 below. Corresponding contour plots and predictions by seasons are provided in Annex 5H.

Table 5.21 Predicted Water Quality at WSRs - 2030 Baseline and 2030 Operation

WSR Name (WSR ID)	Scenario	Mean DO (mg/L)	10^th-Percentile DO (mg/L)	Mean TIN (mg/L)	Mean UIA (mg/L)	Mean SS (mg/L)	Geometric Mean E.coli (cfu./100 mL)
Fish Culture Zones	WQO (If applicable)	N/A	5	0.3	0.021	Increase < 30%	610
Sha Tau Kok (FCZ1)	2030 Baseline	6.38	5.03	0.22	0.007	14.1	24
	2030 Operation	6.36	5.01	0.21	0.006	15.6	23
Temporary Relocation Zone of Fish Raft for the Sha Tau Kok Fish Culture Zone 1 (FCZ7)	2030 Baseline	6.82	5.82	0.15	0.004	11.9	7
	2030 Operation	6.67	5.72	0.20	0.005	13.4	7
Temporary Relocation Zone of Fish Raft for the Sha Tau Kok Fish Culture Zone 2 (FCZ8)	2030 Baseline	6.78	5.77	0.16	0.004	13.7	3
	2030 Operation	6.80	5.74	0.18	0.004	16.1	3
Ap Chau (FCZ2)	2030 Baseline	6.62	5.89	0.10	0.002	6.3	1
	2030 Operation	6.59	5.94	0.11	0.003	6.1	1
Kat O (FCZ3)	2030 Baseline	6.17	5.50	0.11	0.003	4.9	2
	2030 Operation	6.19	5.60	0.11	0.003	4.9	2
O Pui Tong (FCZ4)	2030 Baseline	6.13	5.31	0.14	0.003	3.0	1
	2030 Operation	6.10	5.33	0.15	0.003	2.9	1
Sai Lau Kong (FCZ5)	2030 Baseline	7.20	6.21	0.05	0.002	7.3	1
	2030 Operation	7.23	6.22	0.05	0.002	7.4	1
Wong Wan (FCZ6)	2030 Baseline	6.61	5.59	0.10	0.002	4.0	9
	2030 Operation	6.59	5.60	0.09	0.002	4.0	9
Spawning and Nursery Grounds of Commercial Fisheries Resources	2030 Baseline	6.62	5.89	0.10	0.002	6.3	1
	2030 Operation	6.59	5.94	0.11	0.003	6.1	1
Seagrass	WQO (If applicable)	N/A	4	0.3	0.021	Increase < 30%	N/A
Seagrass (SG)	2030 Baseline	6.79	5.62	0.27	0.007	16.2	125
	2030 Operation	6.89	5.64	0.19	0.005	16.9	124
Mangrove stand	WQO (If applicable)	N/A	4	0.3	0.021	N/A	N/A
Off STKSTW (M1)	2030 Baseline	7.07	6.20	0.40	0.010	14.9	2474
	2030 Operation	7.26	6.30	0.16	0.005	15.6	2513
Off Wu Shek Kok (M2)	2030 Baseline	7.00	6.47	0.17	0.005	17.5	455
	2030 Operation	7.15	6.57	0.13	0.004	20.3	460
Off Tai Wan (M3)	2030 Baseline	6.45	5.54	0.22	0.010	25.2	149
	2030 Operation	6.51	5.53	0.21	0.010	27.6	153
Off Luk Keng (M4)	2030 Baseline	6.70	5.99	0.12	0.004	26.4	1
	2030 Operation	6.84	6.06	0.11	0.004	30.7	1
Off Kuk Po (M5)	2030 Baseline	7.33	6.86	0.11	0.003	13.3	370
	2030 Operation	7.53	6.89	0.12	0.003	16.0	400
Kei Shan Tsui (M6)	2030 Baseline	7.67	7.05	0.07	0.002	12.3	1
	2030 Operation	7.76	7.39	0.10	0.002	13.2	2
Tai Sham Chung (M7)	2030 Baseline	7.75	7.06	0.09	0.002	9.9	1
	2030 Operation	7.83	7.27	0.13	0.002	10.5	1
So Lo Pun (M8)	2030 Baseline	7.34	6.35	0.05	0.001	5.7	6
	2030 Operation	7.47	6.39	0.05	0.001	6.6	6
Pak Kok Wan (M9)	2030 Baseline	7.40	6.58	0.05	0.001	5.2	1
	2030 Operation	7.50	6.60	0.05	0.001	5.7	1
Yan Chau Tong Marine Park (M10)	2030 Baseline	7.02	6.22	0.04	0.002	5.9	456
	2030 Operation	7.07	6.26	0.03	0.002	6.1	458
Yan Chau Tong Marine Park (M11)	2030 Baseline	7.06	6.05	0.04	0.002	8.7	56
	2030 Operation	7.13	6.10	0.04	0.002	8.9	56
Ngau Shi Wu Wan (M12)	2030 Baseline	7.28	6.33	0.04	0.001	5.8	1
	2030 Operation	7.31	6.41	0.03	0.001	5.9	1
Yan Chau Tong Marine Park (M13)	2030 Baseline	6.67	6.10	0.03	0.001	1.4	1
	2030 Operation	6.66	6.09	0.03	0.001	1.5	1
Yan Chau Tong Marine Park (M14)	2030 Baseline	6.71	6.07	0.03	0.001	1.8	1
	2030 Operation	6.69	6.07	0.03	0.001	1.9	1
Coral sites identified under this EIA	WQO (If applicable)	N/A	4	0.3	0.021	Increase < 30%	N/A
Off Ah Kung Au (T1)	2030 Baseline	6.52	5.77	0.11	0.004	14.3	3
	2030 Operation	6.50	5.75	0.10	0.004	14.7	3
Coral sites identified under this EIA (T2)	2030 Baseline	6.53	5.78	0.12	0.004	14.5	3
	2030 Operation	6.51	5.74	0.10	0.004	14.9	3
Coral sites identified under this EIA (T3)	2030 Baseline	6.53	5.76	0.13	0.004	14.7	3
	2030 Operation	6.52	5.72	0.11	0.003	15.0	3
Horseshoe crab	WQO (If applicable)	N/A	4	0.3	0.021	N/A	N/A
Off Muk Min Tau (H1)	2030 Baseline	6.97	6.12	0.28	0.007	13.8	265
	2030 Operation	7.12	6.16	0.15	0.004	15.9	277
Off Pak Hok Lam (H2)	2030 Baseline	6.93	6.19	0.24	0.006	10.6	224
	2030 Operation	7.07	6.21	0.15	0.004	12.6	203
Off Nga Yiu Tau (H3)	2030 Baseline	7.00	6.47	0.17	0.005	17.5	455
	2030 Operation	7.15	6.57	0.13	0.004	20.3	460
A Chau (H4)	2030 Baseline	5.86	4.24	0.26	0.013	38.2	902
	2030 Operation	5.92	4.22	0.25	0.012	41.3	916
Off Luk Keng (H5)	2030 Baseline	6.70	5.99	0.12	0.004	26.4	1
	2030 Operation	6.84	6.06	0.11	0.004	30.7	1
Marine Park	WQO (If applicable)	N/A	4	0.3	0.021	Increase < 30%	N/A
Yan Chau Tong (MP1)	2030 Baseline	6.45	5.39	0.08	0.003	6.4	1
	2030 Operation	6.49	5.43	0.08	0.003	6.5	1
Yan Chau Tong (MP2)	2030 Baseline	6.59	5.72	0.07	0.002	5.2	1
	2030 Operation	6.61	5.75	0.07	0.002	5.3	1

Values not fulfilling the relevant WQO criteria are underlined and bolded.

5.8.63 As shown in Table 5.21 above, the operation of the expanded STKSTW would not result in significant changes in water quality in the Study Area. The predicted water quality under the 2030 operation scenario is expected to fully comply with the relevant WQO criteria.

5.8.64 The predicted maximum decrease in mean and 10^th-percentile DO are 0.15 and 0.10 mg/L, and are predicted at FCZ7, which is the potential relocation site for fish rafts of the STKFCZ when the potential concurrent dredging project stated in EIA project profile (PP-350/2008) proceeds. Further details on the considerations of this concurrent project are provided under Sections 5.11.3 to 5.11.5. Other than FCZ7, the decrease in mean and 10^th-percentile DO at all other WSRs are equal to or below 0.03 and 0.04 mg/L respectively. It should also be highlighted that while the predicted DO level decreases slightly for some of the WSRs close to the new outfall under the operation of the expanded STKSTW, significant improvement in DO levels are predicted at WSRs near the existing outfall. For instance, the increase in mean DO level at mangrove stand M1 near STKSTW is 0.19 mg/L. Overall, the predicted water quality complies with the corresponding DO criteria of WQO at all WSRs.

5.8.65 Similar prediction is observed for TIN and UIA. Maximum increase in TIN of 0.05 mg/L is predicted at FCZ7, and the predicted increases at all other WSRs near the new outfall are generally lower. Also, reduction of TIN level is predicted at some WSRs near the existing outfall. Notably, the level of TIN at M1 is expected to decrease from 0.40 mg/L of the baseline level to 0.16 mg/L during the operation of the expanded STKSTW, owing to the relocation of the STKSTW outfall to a better flushed location. Reduction in TIN and UIA are also predicted at seagrass location (SG) and all horseshoe crab habitats (H1 to H5). For existing FCZ1 to FCZ6, the predicted change in TIN and UIA are within ±0.01 and ±0.001 mg/L respectively. Compliance of TIN and UIA criteria of the WQO is predicted at all WSRs under the 2030 operation scenario of the expanded STKSTW.

5.8.66 The predicted changes of SS at WSRs are limited as well. Among WSRs which are considered sensitive to SS, the maximum percentage increase of average SS level is predicted at FCZ8 (17.5% increase), which is also the potential relocation site for fish rafts of STKFCZ, maintaining compliance with the corresponding WQO criteria for SS. The percentage increases of SS level at other WSRs are similar or lower and compliance to the corresponding WQO criteria is predicted as well.

5.8.67 The predicted change on level of E. coli is also limited. Among all designated FCZs, potential relocation sites and spawning and nursery grounds of commercial fisheries resources, no notable increase in E. coli level is expected. The predicted E. coli levels in both 2030 baseline and operation scenarios are below the corresponding WQO criteria of 610 cfu/100ml. Full compliance on the E. coli criteria of WQO is expected at these fisheries-related WSRs under the 2030 operation scenario. There is no WQO criterion for E. coli other WSRs.

5.8.68 The predicted water quality at WSRs for future baseline and expanded STKSTW operation scenarios are presented in Table 5.21 above. Contour plots are also provided in Annex 5H to show general water quality of baseline and with project scenarios in 2030. The potential change in water quality due to the operation of the expanded STKSTW as well as the corresponding evaluation against the WQO criteria is discussed in Sections 5.8.63 to 5.8.67 above. General improvement over the inner Starling Inlet (particularly at the existing outfall) is predicted and discussed. Also, full WQO compliance is expected from the operation phase of the STKSTW.

Potential Discharge of Untreated Sewage from TSTP / Expanded STKSTW in case of Overflow, Power Failure and Plant Failure

5.8.69 An emergency discharge from the TSTP and the expanded STKSTW could result in elevation in pollution level and turbidity as well as reduction in dissolved oxygen or other secondary effects on water quality. An avoidance approach should be taken to minimize the risk of emergency discharge from the expanded STKSTW as far as practicable through design measures. A number of precautionary measures have been taken into account in the design of the expanded STKSTW to minimize the risk of discharge of untreated or incompletely-treated sewage into the marine water of Starling Inlet under emergency situation. These measures include:

· Routine/ regular checking to the equipment

· Provision of dual power supply and backup generator to eliminate the risk of power failure;

· Provision of standby equipment (online and on-shelf) for all treatment units;

· Operation of STKSTW is under 24-hour monitoring by Shift Team of Sha Tau Kok (for new STKSTW) and/or Shek Wu Hui STW in order to allow inspection and any necessary repair works by DSD at the earliest possible time;

· A remote control and monitoring system (SCADA) will also be installed to allow off-site DSD staff (Shift Team) to monitor the operation of STKSTW; and

· Provision of on-site storage of raw sewage up to 6 hours for the TSTP and STKSTW ^([20]).

5.8.70 The hierarchy of the above design measures for preventing emergency discharge from the expanded STKSTW is illustrated in Pane 5.5 below. As shown, there is a series of design measures to avoid any emergency discharge from the expanded STKSTW. Continuous failure of these design measures is highly unlikely. A remote control and monitoring system (SCADA) for sewage treatment facilities in expanded STKSTW would be provided, allowing inspection and any necessary repair works by DSD be initiated at the earliest possible time. As the plant would be manned during operation, it is believed that any necessary response actions in emergency situations would be promptly initiated and completed within 6-hour of emergency storage ⁽²⁰⁾. The potential risk of emergency discharge from the expanded STKSTW is considered significantly reduced with the implementation of such measures.

Pane 5.5 Hierarchy of Design Measures to Prevent Emergency Discharge for the Expanded STKSTW

5.8.71 In general, the initial commissioning of a new sewage treatment plant is considered more prone to operation error and malfunction. As shown in the tentatively programme shown in Pane 5.1 above, a gradual phase-in of new plant and gradual phase-out of old plant is allowed for both the initial commissioning of the TSTP and the expanded STKSTW. By allowing a short period of overlapped operation, the new plant is allowed to run in the initial period at low influent flow. The old plant also acts as a backup when needed. This minimizes the risk associated with the initial commissioning of the TSTP and the expanded STKSTW.

Comparison with the Most Recent Emergency Discharge Event at the Pillar Point Sewage Treatment Works

5.8.72 An emergency discharge event was recorded on 25 and 26 August 2014 at the Pillar Point Sewage Treatment Works (PPSTW). As a most recent example of emergency discharge event from a sewage treatment works, the event was studied by the EPD ⁽[21]⁾. A comparison is drawn from the PPSTW emergency discharge event and the design measures of the TSTP and the expanded STKSTW:

5.8.73 The emergency discharge of PPSTW case is the maintenance problem of the fine screen. The chains of the fine screens were broken due to lack of maintenance. As a lesson learnt, in order to minimize the risk at STKSTW, the following measures, in addition to measures specified Section 5.8.69, would be provided:

· 2 duties + 1 standby fine screens would be provided;

· Uninstalled spare parts would be provided;

· Monitoring equipment of fine screens would be installed;

· Routine inspection and scheduled maintenance works would be strengthened and carried out regularly; and

· Equipment and necessary measures such as lifting opening would be provided to shorten the time required for replacement of screen.

5.8.74 In addition, the following measures/ evidence show that STKSTW will have very rare chance to have emergency discharge due to similar failure:

· Operation of STKSTW will be under 24-hour monitoring by Shift Team of STKSTW or Shek Wu Hui Sewage Treatment Works (SWHSTW). A remote control and monitoring system (SCADA) will also be installed to allow off-site DSD staff (Shift Team) to on-line monitor the operation of STKSTW and take swift action if needed.

· Proper and regular maintenance will be conducted, and additional spare parts for replacement of the chain for the fine screen will be provided.

· 6 hours of emergency storage ⁽[22]⁾ will be provided which is sufficient for replacing the chains provided that the spare parts are already available on site.

· The travelling time from SWHSTW to STKSTW is within 30 minutes. Any inspection and necessary supporting works by DSD in-house staff could be able to reach STKSTW very soon.

· From the commissioning day of STKSTW, there is no overflow at STKSTW so far.

5.8.75 As required under Appendix D of the Study Brief, this EIA should include an assessment on the potential impacts of temporary, accidental and emergency discharges at the TSTP and the expanded STKSTW on water quality. Water quality modelling exercise has been conducted for selected scenario as specified under Annex 5I. The following sections present the predicted maximum extent of potential water quality impact as well as estimated time required for the marine water quality near the Project to return to its baseline level for the selected emergency discharge scenarios for the TSTP and the expanded STKSTW. Contour plots showing the 1^st, 2^nd, 4^th, 8^th and 16^th hour after the start of the emergency discharge from the TSTP and the expanded STKSTW for DO, UIA, SS and E. coli are provided in Annex 5J. Time-series plots showing selected water quality parameters (TSTP for DO, UIA, SS and E. coli) at major nearby WSRs (including the FCZ1, FCZ2, FCZ7 and FCZ8) from 0^th to 72^th hours after the start of the emergency discharge from the TSTP and the expanded STKSTW are provided in Annex 5K to provide an indication on predicted changes in water quality following such discharge.

Emergency Discharge from the TSTP

5.8.76 The 2-hour emergency discharge from the safety outlet of the TSTP (which is the same existing outfall) is predicted to result in a notable localized change in water quality (Annex 5J). A localized drop of DO level is predicted around the safety outlet, but the extent is limited and would not result in exceedance of WQO criteria at the nearby WSRs. Similar localized plume of elevated UIA level is also predicted. Significant elevation of SS level is not predicted as a result of relatively high background level. Elevation of E. coli level is also limited as well. As shown in Annex 5J, the mixing zones of the untreated effluent (if any) are not predicted to encroach into the nearby WSRs, namely H1, M1 and SG.

5.8.77 The above observations can also be identified in the time-series plots shown in Annex 5K. The two lines, each representing the baseline and emergency discharge water quality, are generally close together. As expected, the predicted water quality is slightly worse for the emergency discharge water quality, but the difference is very limited at the FCZs and would not result in WQO exceedance at the FCZs. In view of the relatively small change in water quality, the water quality at the FCZs generally is predicted to recover within 3 days after the emergency discharge (Annex 5K). The marine water quality at these WSRs remains similar to the corresponding level during normal operation of the TSTP. This is likely to be due to the fact that these WSRs are either far away from the safety outlet (i.e. FCZ2, FCZ7 and FCZ8) and / or located at direction perpendicular to the direction of the tidal current (NE-SW for the Starling Inlet) (FCZ1, FCZ7 and FCZ8), which means longer path and more diffusion during transportation of pollutants from the safety outlet to these WSRs. In view of above, it is considered the potential change in water quality at these WSRs would be transient and reversible.

5.8.78 While the tidal conditions for emergency discharge is expected to be of some importance to determine its influence on water quality as demonstrated in the tracer dispersion analysis in Annex 5I, there is no significant difference between the discharge in spring tide and neap tide in both seasons. It is because the effect of tidal conditions is significantly masked by the contribution by other sources. Also, the pollutants from the emergency discharge would be gradually assimilated in the water body (e.g. sedimentation, biological uptake of nutrients, removal of nitrogen through biochemical activities, NH₃ à NO₃^- à N₂), which is not considered in the tracer dispersion model using conservative tracer.

Emergency Discharge from the Expanded STKSTW

5.8.79 Similar to the case of the TSTP, the 2-hour emergency discharge from the safety outlet of the expanded STKSTW is predicted to result in a notable localized change in water quality. The extent of effluent plume is larger than that of the TSTP. A small area of localized drop in DO level is predicted near the safety outlet, and at the boat shelter nearby. Similar elevation is predicted for UIA, SS and E. coli levels, which are all quite localized, yet larger than that of the TSTP. Unlike the emergency discharge from the TSTP, the predicted change in water quality due to emergency discharge from the expanded STKSTW is much more significant because of the relatively large flow of the expanded STKSTW (which is 4 times of that of the TSTP). More importantly, the untreated sewage is discharged back in the Starling Inlet (v.s. treated effluent is discharged at the new outfall outside the Starling Inlet during normal operation of the expanded STKSTW). The safety outlet for the expanded STKSTW is also close to the shoreline and the untreated effluent may be retained in the nearby boat shelter, resulting in slower dispersion. As shown in Annex 5J, the effluent plume from the expanded STKSTW generally encroaches into the nearby boat shelter, and remains there for more than 8 hours after the start of the emergency discharge.

5.8.80 As shown in Annex 5K, the water quality impact of the emergency discharge from the expanded STKSTW is more significant than that of the TSTP. Yet the predicted impact on the nearby FCZs is still quite limited due to the factors stipulated in Section 5.8.77 and no WQO exceedance would be resulted. The predicted change in DO, UIA, SS and E.coli levels is much more observable for both spring tide and neap tide conditions in both seasons when compared with that of the TSTP. The elevation of pollutant as well as decrease in oxygen level also persists for a longer period of time. Modelling results indicated that the level of UIA at FCZ1 would return to its baseline level by the 9^th and 10^th day after the emergency discharge in the worst case (neap tide) scenario in wet and dry season respectively. In view of the above, it is considered the potential change in water quality at these WSRs would be transient and reversible.

Chemical Spillage

5.8.81 Similar to the existing STKSTW, all chemicals required for sewage treatment would be stored indoor. All handling of chemicals would be conducted indoor as well. This means any spillage of chemical would only occur indoor and would not get into the nearby marine waters. Any spillage of chemical would be clean up with appropriate tool(s) stipulated in the corresponding material safety data sheet and the cleanup materials and wastes would be properly disposed to the Chemical Waste Treatment Facilities. No unacceptable water quality impact would be expected.

Effluent Reuse

5.8.82 Based on the latest design information, about 50 m³ of treated effluent (i.e. 0.5% of the ultimate design capacity of the expanded STKSTW) would be reused for non-potable purposes within the expanded STKSTW (which is not accessible to the general public), including cleansing, toilet flushing and landscape irrigation. It should be noted that the effluent quality from the proposed MBR treatment for the expanded STKSTW is generally good enough for direct reuse after additional chlorination (to fulfil the TRC requirement specified in Table 5.10). Part of the treated effluent would be diverted for chlorination using liquid bleach (aqueous sodium hypochlorite solution) to at least 1 mg/L level. Afterwards, the chlorinated effluent (reclaimed effluent) would be stored in a storage tank and be ready for non-portable reuse purposes within the expanded STKSTW. It should be noted that the proposed treatment level specified in Table 5.10 are extracted from Ngong Ping STW (for toilet flushing and controlled irrigation), North District (for toilet flushing, unrestricted irrigation & water features), Lo Wu Correction Institution (for toilet flushing) and WSD’s water quality objectives for toilet flushing.It is considered the proposed standard is sufficiently stringent for the protection of users of reclaimed effluent for the proposed purposes.

5.8.83 A number of design measures have been included to avoid cross-connection of the reclaimed water supply to the potable water supply. These measures are detailed in Section 5.9 below.

5.8.84 It should be noted that the proposed reuse would not result in any undesirable change in water quality impact. The diversion of treated effluent from MBR would be activated on when needed and the storage capacity of the storage tank for reclaimed effluent would be larger than the maximum daily usage of 50 m³. Furthermore, the proposed non-potable reuse on cleansing, toilet flushing and landscape irrigation would not result in direct discharge of the reclaimed effluent into the marine environment or nearby water bodies. Therefore, potential change in water quality due to the proposed reuse of effluent is not expected.

5.9 Mitigation Measures

Construction Phases

5.9.1 Construction phase water quality modelling results indicated that the potential water quality impact from the proposed sheetpile installation and removal would be very minimal. No exceedance in SS, DO, TIN, UIA, arsenic, PCBs and PAHs is predicted for marine construction in both seasons. Therefore, no mitigation measures would be required for the sheetpiling installation and removal works.

5.9.2 As discussed in section 5.8.26, the trenchless HDD construction of outfall pipeline would proceed from the landside. Also, the construction of diffuser would be conducted after the dry excavation of marine sediment in the cofferdam. No release of suspended solids into the water column from the excavation of sediment would be expected. It is expected that the construction of diffuser would be conducted similar to other land-based construction works. Appropriate site practices and mitigation measures for land-based construction works are provided below in Section 5.9.4.

5.9.3 Furthermore, a number of standard measures and good site practices should be implemented to avoid / minimize the potential impacts from marine construction. These measures include:

· All vessels should be well maintained and inspected before use to limit any potential discharges to the marine environment;

· All vessels must have a clean ballast system;

· No discharge of sewage/grey wastewater should be allowed. Wastewater from potentially contaminated area on working vessels should be minimized and collected. These kinds of wastewater should be brought back to port and discharged at appropriate collection and treatment system; and

· No soil waste is allowed to be disposed overboard.

General Construction Activities

5.9.4 Standard site practices outlined in ProPECC PN 1/94 “Construction Site Drainage” will be followed as far as practicable in order to reduce surface runoff, minimize erosion, and also to retain and reduce any SS prior to discharge. These practices include the following:

· Silt removal facilities such as silt traps or sedimentation facilities will be provided to remove silt particles from runoff to meet the requirements of the TM standard under the WPCO. The design of silt removal facilities will be based on the guidelines provided in ProPECC PN 1/94. All drainage facilities and erosion and sediment control structures will be inspected on a regular basis and maintained to confirm proper and efficient operation at all times and particularly during rainstorms. Deposited silt and grit will be removed regularly.

· Earthworks to form the final surfaces will be followed up with surface protection and drainage works to prevent erosion caused by rainstorms.

· Appropriate surface drainage will be designed and provided where necessary.

· The precautions to be taken at any time of year when rainstorms are likely together with the actions to be taken when a rainstorm is imminent or forecasted and actions to be taken during or after rainstorms are summarised in Appendix A2 of ProPECC PN 1/94.

· Oil interceptors will be provided in the drainage system where necessary and regularly emptied to prevent the release of oil and grease into the storm water drainage system after accidental spillages.

· Temporary and permanent drainage pipes and culverts provided to facilitate runoff discharge, if any, will be adequately designed for the controlled release of storm flows.

· The temporary diverted drainage, if any, will be reinstated to the original condition when the construction work has finished or when the temporary diversion is no longer required.

5.9.5 As the Project site is next to the shoreline, infiltration of seawater during excavation is anticipated. Appropriate infiltration control, such as cofferdam wall, should be adopted to limit groundwater inflow to the excavation works areas in the Project site. Groundwater pumped out from excavation area should be discharged into the storm system via silt removal facilities.

5.9.6 If needed, appropriate numbers of portable toilets shall be provided by a licensed contractor to serve the construction workers over the construction site to prevent direct disposal of sewage into the water environment.

Spillage of Chemicals

5.9.7 Site drainage should be well maintained and good construction practices should be observed to ensure that oil, fuels, solvents and other chemicals are managed, stored and handled properly and do not enter the nearby streams or marine water.

Operation Phase

5.9.8 It is considered that the total pollution loading from TSTP is the same as the baseline scenario, hence no unacceptable change in water quality would be expected from the operation of TSTP. For the operation of the expanded STKSTW, water quality modelling exercise indicated that no unacceptable adverse water quality would be expected. No additional water quality mitigation measures would be required. The assumed design on flow rate, effluent quality, outfall location and diffuser / outfall shall be taken into account into the final design to ensure water quality performance on the expanded STKSTW operation.

5.9.9 The following design measures are also provided in the TSTP and the expanded STKSTW to avoid the risk of emergency discharge:

· Routine/ regular checking to the equipment

· Provision of dual power supply and backup generator to eliminate the risk of power failure;

· Provision of standby equipment (online and on-shelf) for all treatment units;

· Operation of STKSTW is under 24-hour monitoring by Shift Team of Sha Tau Kok (for new STKSTW) and/or Shek Wu Hui STW in order to allow inspection and any necessary repair works by DSD at the earliest possible time;

· A remote control and monitoring system (SCADA) will also be installed to allow off-site DSD staff (Shift Team) to monitor the operation of STKSTW; and

· Provision of on-site storage of raw sewage up to 6 hours for the TSTP and STKSTW ^([23]).

5.9.10 Additional measures provided to avoid plant failure associated fine screen include:

· 2 duties + 1 standby fine screens would be provided;

· Uninstalled spare parts would be provided;

· Monitoring equipment of fine screens would be installed;

· Routine inspection and scheduled maintenance works would be strengthened and carried out regularly; and

· Equipment and necessary measures such as lifting opening would be provided to shorten the time required for replacement of screen.

5.9.11 The provided onsite storage ⁽²³⁾ should also be sufficient for DSD crew to handle the issue with the plant and resume back to normal operation. Emergency discharge modelling exercise has been conducted for the emergency discharge scenario of the TSTP and the expanded STKSTW to advise on the extent of impact, time required for nearby waters to return to its baseline level. An Emergency Response Plan shall be prepared and implemented in the event of emergency. Relevant government departments shall be informed as soon as possible of any emergency conditions. Appropriate actions, including necessary follow-up monitoring, shall be outlined in the Emergency Response Plan. A follow-up water quality monitoring exercise shall also be conducted after every emergency discharge event to monitor the recovery of water quality.

5.9.12 The reclaimed effluent would be treated to the quality standard stipulated in Table 5.10. The reclaimed water pipeline will be a separate system and will not be connected with the potable water pipeline system. To avoid cross-connection of the reclaimed water supply to the potable water supply, the pipes for the reclaimed water will be specially arranged to differentiate them from that of the potable water pipe, e.g. clearly labelled with warning signs and notices, colour-coded, and/or using different pipe size, so that physical connection of the reclaimed water pipes with the potable water fittings would not be possible. The reuse of reclaimed effluent would be limited to non-potable purposes within the expanded STKSTW and would not be accessible to the general public., caution would also be taken to avoid the use of high pressure jet in cleansing and landscape irrigation to minimize aerosol formation from the reclaimed effluent.

5.9.13 No secondary impact is expected from the mitigation measures for both construction and operation phase of the Project proposed in the EIA report.

5.10 Residual Impacts

Construction Phase

5.10.1 Construction phase water quality modelling results indicated that the potential water quality impact from the proposed sheetpile installation and removal would be very minimal. No exceedance in SS, DO, TIN, UIA, arsenic, PCBs and PAHs is predicted for marine construction in both seasons. No adverse water quality impact from sheetpile installation and removal is expected.

5.10.2 With the implementation of mitigation measures recommended in Sections 5.9.4 to 5.9.7, no adverse water quality impact would be expected from other land-based construction works under this Study.

Operation Phase

5.10.3 Water quality modelling exercise indicated that no unacceptable water quality would be expected from the operation of the expanded STKSTW. Also, there would not be any exceedance of WQO criteria under the operation of the expanded STKSTW.

5.10.4 A number of design measures have been taken into account to minimize the risk of emergency discharge from the TSTP and the expanded STKSTW. In case of emergency discharge from the TSTP or the expanded STKSTW, there would be a short term elevation of pollutant level near the corresponding safety outlets. Based on the modelling prediction, the water quality at the nearby FCZs is expected to recover by the 10^th day after the emergency discharge event, and such emergency discharge event would unlikely result in exceedance of WQO, which considers the long term (annual mean or 10^th-percentile level) water quality of the water body.

5.10.5 Other WSRs which are even closer to the safety outlet of the expanded STKSTW would be affected more significantly upon an emergency discharge from the expanded STKSTW and elevated level of UIA may exceed the corresponding WQO level for a brief period of time after the emergency discharge. The residual impact on these WSRs are considered acceptable in view of the followings with respect to Section 4.4.3 of the EIAO TM:

· Emergency discharge events are considered accident, which should rarely occurs. The incidence should be even lower with the built-in design preventive measures. Therefore, the frequency of such even is extremely low.

· The provision of temporary storage ⁽[24]⁾ should effectively eliminate any need of emergency discharge. The assumed 2-hour discharge scenario is already a conservative assumption. Therefore, the duration of such even is short.

· In case such event happens, the discharge of untreated sewage would be ended after the maintenance works. Based on the modelling prediction, the water quality would recover in short period of time (<10 days).

· The mixing of UIA from such event is quite localized and covers only a stretch of shoreline along the STKSTW. The mixing zone would gradually shrink after the end of emergency discharge.

· The WSRs are not considered particularly vulnerable to a short term elevation of UIA.

· The impacted area / WSRs are not considered to be of international and regional importance.

· The temporary elevation of UIA would not result in exceedance of WQO UIA criteria, which considers annual average of the parameter.

5.10.6 Since no residual impact is identified in the EIA report, no additional study is required.

5.11 Cumulative Impacts

5.11.1 Potential cumulative water quality impacts from concurrent projects within the Study area as well as the area covered by the STK Model have been taken into account under this assessment. A list of identified project at the vicinity (i.e. geodesic distance < 7 km) of Starling Inlet is summarized below in Table 5.22 based publicly available sources.

Table 5.22 Nearby Projects Identified

Project	Duration	Location	Major Marine Activity / Sewage Discharge
North District sewerage, stage 2 part 2A - Pak Hok Lam trunk sewer and Sha Tau Kok village sewerage	2012 - 2017	Pak Hok Lam and Sha Tau Kok Village	Increase sewage flow to STKSTW
Sediment Removal at Sha Tau Kok Fish Culture Zone, Boat Shelter and Approach Channel	2017-2018 (Tentative)	Sha Tau Kok Fish Culture Zone, Boat Shelter and Approach Channel	(1) Sediment dredging (2) Temporary relocation of fish rafts
Drainage Improvement Works at North District, including various drainage improvement measures in Sha Tau Kok.	2016 – 2020	Sha Tau Kok Town area	Drainage improvement works in Sha Tau Kok Town area

North District sewerage, stage 2 part 2A - Pak Hok Lam trunk sewer and Sha Tau Kok village sewerage

5.11.2 The scope of the North District Sewerage Stage 2 Part 2A project comprises the construction of about 2 km of trunk sewers along Sha Tau Kok Road; about 7.5 km of sewers for the nine unsewered areas; and one sewage pumping station near Wu Shek Kok with associated twin rising mains. This project would significantly increase the amount of sewage collected in the STKSTW catchment and is the main drive for the expansion of the STKSTW. The increased sewage flow to the STKSTW from 1,660 m³/day ADWF to 10,000 m³/day ADWF would be taken into account in this Study and be modelled as the project scenario.

Sediment Removal at Sha Tau Kok Fish Culture Zone, Boat Shelter and Approach Channel

5.11.3 An EIA project profile (PP-350/2008) of the captioned project is submitted to EPD on 2008 and an EIA study brief (ESB-186/2008) was issued. No approved EIA is available at the time of preparation of this EIA. Based on the latest information provided by CEDD, the construction works are scheduled to commence in the 1st half of 2017 for completion in the 1st half of 2018 tentatively, which would potentially be concurrent with the marine construction period under this Project. In view of this, the sediment removal project has been assessed explicitly as a concurrent project under Section 5.8.

5.11.4 Under the sediment removal project, dredging operation would be conducted at the STKFCZ, Sha Tau Kok boat shelter, approach channel and dredging area between the shore and the island. Tentative dredging rate would be 2600 (for STKFCZ), 800 (for boat shelter and approach channel) and 300 (for dredging area between the shore and the island) m³/day respectively. During the dredging operation, the fish rafts of the STKFCZ would be relocated to two proposed temporary relocation zones. One of the relocation zones is about 800 m east to the existing STKFCZ (FCZ7 shown in Figure 5.1) and the other would be about 250 m south to the existing STKFCZ (FCZ8 shown in Figure 5.1). In view of this, the sediment contribution from the sediment removal works has already been considered in the construction phase sediment dispersion modelling under Section 5.8. The temporary relocation sites for fish rafts of the STKFCZ are also considered as WSR (FCZ7 and FCZ8) in the construction phase sediment dispersion modelling as well as operation phase water quality modelling. The dredging areas and relocation zone under the sediment removal project in relation with the marine dredging area under this Project are shown in Figure 5.3.

5.11.5 Since the marine construction and plant operation under this Project would be carried out after the after the sediment removal project, the seabed level at the dredging areas covered under sediment removal project is assumed to be the same as the proposed dredging level under that project for construction phase and operation phase scenarios, with the exception of operation phase scenario for model verification (year 2011).

Drainage Improvement Works at North District, including various drainage improvement measures in Sha Tau Kok

5.11.6 This drainage improvement project is tentatively scheduled from 2016 to 2020. Under this drainage improvement project, a 600 mm diameter covered U-channel would be constructed along the access road to STKSTW and a pair of 1350 mm diameter drainage pipes are proposed to be constructed along Sha Tau Kok Road – Shek Chung Au. There will be minor interface between the drainage pipes by Drainage Projects Division and the proposed gravity sewer. No marine construction is required and no additional pollution loading from this Project is expected. Therefore no cumulative water quality impact from this drainage improvement project is expected.

5.12 Environmental Monitoring and Audit

Construction Phase

5.12.1 Marine water quality monitoring at selected WSRs is recommended for installation, maintenance and removal of sheetpile ^([25]) and sediment removal works under this Project. Site audit would also be conducted throughout the marine and land-based construction under this Project. Details environmental monitoring procedures and audit requirements are provided in the standalone EM&A manual.

Operation Phase

5.12.2 Marine water quality monitoring at selected WSRs is recommended for the first year of (1) interim operation of the TSTP, (2) operation of phase 1 expansion, (3) operation of phase 2 expansion of the STKSTW. Monitoring of effluent quality would also be required for the first year of operation for the TSTP as well as Stage 1 and Stage 2 expansion of the STKSTW. Also, follow-up water quality monitoring exercise shall be conducted after each emergency discharge event to monitor the recovery of water quality in the vicinity. Detailed environmental monitoring procedures are provided in the standalone EM&A manual.

5.13 Conclusion

5.13.1 Water quality assessment has been conducted for construction phase activities under this Project. Modelling assessment results indicated that the construction phase water quality impact from SS elevation, DO depletion as well as nutrient and contaminant release from marine construction works for cofferdam at submarine outfall would be minimal. Potential water quality impacts from land-based construction activities have also been assessed and found to be minimal. Appropriate site measures and standard practices are recommended. Water quality monitoring and audit exercise for sheetpiling activities is also recommended to ensure protection to nearby WSRs.

5.13.2 It is considered that the total pollution loading of TSTP is the same as the baseline scenario, hence no unacceptable water quality impact would be expected from the operation of the TSTP. Modelling assessment has been conducted to predict the potential change in water quality impact from the expanded STKSTW. The worst case discharge condition (maximum average dry weather flow with maximum effluent concentration) has been modelled, taking into account the pollution loading from background sources including the FCZs, dry weather load from drainage system and rainfall-related loading. Modelling results indicate compliance of DO, TIN, UIA and E.coli level in both all WSRs under the operation of the expanded STKSTW. Increase in SS level in predicted in some WSRs under the operation of the expanded STKSTW, yet the change in SS levels at all WSR are predicted to be below the corresponding WQO criteria. No unacceptable adverse water quality impact would be expected from the operation of the expanded STKSTW.

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([1]) CEDD (2008). Project Profile for Sediment Removal at Sha Tau Kok Fish Culture Zone, Boat Shelter and Approach Channel. Submitted under EIAO with Application No. ESB-186/2008.

([2]) ERM – Hong Kong, Ltd (2007) EIA for Development of a Bathing Beach at Lung Mei, Tai Po. For Civil Engineering Department, Hong Kong SAR Government.

([3]) ERM - HK Ltd (2010). Development of an Offshore Wind Farm in Hong Kong. Final Environmental Impact Assessment. For the Hong Kong Electric Company

([4]) Black & Veatch Hong Kong Ltd - Desalination Plant at Tseung Kwan O – Feasibility Study, For Water Supplies Department, Hong Kong SAR Government.

([5]) ERM – Hong Kong, Ltd (2002) EIA for the Proposed Submarine Gas Pipeline from Cheng Tou Jiao Liquefied Natural Gas Receiving Terminal, Shenzhen to Tai Po Gas Production Plank, Hong Kong. Final EIA Report. For the Hong Kong and China Gas Co., Ltd.

([6]) Maunsell Consultant Asia Ltd, (2007). Wan Chai Development Phase II and Central-Wan Chai Bypass. EIA-141/2007.

([7]) Maunsell (2002). EIA for Decommissioning of Cheoy Lee Shipyard at Penny's Bay. For Civil Engineering Department, Hong Kong SAR Government.

([8]) ERM – Hong Kong (1997). EIA for Disposal of Contaminated Mud in the East Sha Chau Marine Borrow Pit. For Civil Engineering Department, Hong Kong SAR Government.

([9]) Maunsell (2001). EIA for Wanchai Development Phase II - Comprehensive Feasibility Study. For Territory Development Department, Hong Kong SAR Government.

([10]) ERM - Hong Kong, Ltd (2006) Op Cit

([11]) BMT Asia Pacific Ltd (2009). EIA for Hong Kong Offshore Wind Farm in Southeastern Waters. For HK Offshore Wind Limited

([12]) ERM - HK Ltd (2010). Development of an Offshore Wind Farm in Hong Kong. Final Environmental Impact Assessment. For the Hong Kong Electric Company

^([13]) In 2008, Hong Kong and Shenzhen initiated the first review study of the “Mirs Bay Water Quality Regional Control Strategy”. The study was completed on schedule in 2011. The study findings indicated that, with progressive extension and improvement of sewerage infrastructure, the overall pollution load to Mirs Bay would hold steady in the next five to ten years and that the water quality of the bay could remain good. Both sides agreed to continue to implement the jointly formulated water quality control strategy for protecting the Mirs Bay water environment and meeting the sustainable development objectives. With the continuous efforts by both sides of the government to implement pollution control measures to reduce wastewater discharge, it is expected that the pollution situations in the future would not be worse than the 2004 conditions as assumed in the approved EIA for Tai Po STW Stage V. Source: EPD (http://wqrc.epd.gov.hk/en/regional-collaboration/deep-bay.aspx)

^([14]) AECOM (2011) Development of the Integrated Waste Management Facilities Phase 1, for EPD. Register No.: AEIAR-163/2012, http://www.epd.gov.hk/eia/register/report/eiareport/eia_2012011/index.htm

^([15]) Mott MacDonald (2010) South Island Line (East) Environmental Impact Assessment, for MTR. Register No.: AEIAR-155/2010 http://www.epd.gov.hk/eia/register/report/eiareport/eia_1852010/Index.html

^([16]) AECOM (2009) Tuen Mun – Chek Lap Kok Link, for HyD. Register No.: AEIAR-146/2009 http://www.epd.gov.hk/eia/register/report/eiareport/eia_1742009/index.html

([17]) The dimension provided are rounded-up for conservative calculation. Based on the latest available information, the actual footprint is 54 m × 22 m, which is slightly smaller.

^([18])Juliane Finn and Rosie Berger, 2007. Measurement of Dissolved Oxygen in Lakes and Ponds during Light and Dark Conditions. http://depts.alverno.edu/nsmt/archive/bergerfinn.htm

([19]) The storage volume for the TSTP is 625 m³.

([20]) The storage volume for the TSTP and the expanded STKSTW are 625 m³ and 2,500 m³ respectively.

([21]) EPD’s submission to The Legislative Council - Panel on Environmental. Affairs, September 2014. Available at: http://www.legco.gov.hk/yr13-14/english/panels/ea/papers/eacb1-1989-1-e.pdf

([22]) The storage volume for the TSTP and the expanded STKSTW are 625 m³ and 2,500 m³ respectively.

([23]) The storage volume for the TSTP and the expanded STKSTW are 625 m³ and 2,500 m³ respectively.

([24]) The storage volume for the TSTP and the expanded STKSTW are 625 m³ and 2,500 m³ respectively.

([25]) This means the construction phase water quality monitoring would cover the whole period of marine works associated with the proposed outfall.