5. Water Quality Impact

5.2 Relevant Legislations, Standards & Guidelines

Environmental Impact Assessment Ordinance (EIAO)

5.2.1 The Environmental Impact Assessment Ordinance (EIAO) provides a legislative framework to safeguard the environment by reducing and minimizing adverse environmental impacts from designated projects.

5.2.2 Annexes 6 and 14 of the EIAO – TM specify the general and project-specific criteria, and guidelines for water quality impact assessment.

Practice Note for Professional Persons

5.2.3 The Practice Note for Professional Persons (ProPECC Note PN1/94) on construction site drainage provides guidelines on good practice for dealing with discharges from construction sites. This note is applicable to this study for control of site runoff and wastewater generated during the construction phase of the Project.

5.2.4 The assessment will follow this practical note to recommend mitigation measures to minimize the potential water quality impact arising from construction activities.

Water Pollution Control Ordinance & Water Quality Objectives

5.2.5 The Water Pollution Control Ordinance (WPCO) (Cap. 358) enacted in 1980 is the principal legislation to safeguard the water quality in Hong Kong. Under the WPCO, the Hong Kong waters are divided into 10 Water Control Zones (WCZs). Water Quality Objectives (WQOs) are specified for each of the 10 WCZs, respectively. The WQOs set the limits for different water quality parameters for maintaining the integrity of aquatic system within each of the WCZs.

5.2.6 The effluent discharge from the Cheung Chau STW has the potential to impact the marine water quality conditions in the Southern WCZ and adjacent areas. The WQOs for the Southern WCZ are listed in Table 5.1.

Table 5.1 : Summary of Water Quality Objectives for Southern WCZ

Parameter	Water Quality Objective	Sub-Zone
Aesthetic Appearance	Ÿ There should be no objectionable odours or discolouration of the water. Ÿ Tarry residues, floating wood, articles made of glass, plastic, rubber or of any other substances should be absent. Ÿ Mineral oil should not be visible on the surface. Ÿ Surfactants should not give rise to a lasting foam. Ÿ There should be no recognisable sewage-derived debris. Ÿ 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. Ÿ The waters should not contain substances which settle to form objectionable deposits.	Whole zone
Bacteria	Ÿ Annual geometric mean level of Escherichia coli not to exceed 610 cfu/100mL for secondary contact recreation subzones and fish culture zones. Ÿ Geometric mean level of E.coli of all samples collected from March to October exclusive not to exceed 180 per 100mL for bathing beaches.	Ÿ Secondary Contact Recreation Subzones & Fish Culture Zones Ÿ Bathing Beach Subzones
Dissolved Oxygen (DO)	Ÿ Bottom dissolved oxygen not less than 2 mg/L for 90% of samples; Ÿ Depth-averaged dissolved Oxygen not less than 4 mg/L for 90% of samples.	Whole zone
pH	In the range 6.5 - 8.5, change due to waste discharge not to exceed 0.2.	Whole zone
Temperature	Change due to waste discharge not to exceed 2°C.	Whole zone
Salinity	Change due to waste discharge not to exceed 10% of natural ambient level.	Whole zone
Suspended solids (SS)	Waste discharge not to raise the natural ambient level by 30% nor cause the accumulation of suspended solids which may adversely affect aquatic communities.	Marine waters of the whole zone
Ammonia	Annual mean of un-ionized ammonia nitrogen not to exceed 0.021 mg/L, calculated as the arithmetic mean.	Whole zone
Nutrients	Annual mean depth-averaged total inorganic nitrogen (TIN) not to exceed 0.1mg/L.	Whole zone
Toxins	Not to be present at levels producing significant toxic effect.	Whole zone

Technical Memorandum on Effluents Discharge Standard

5.2.7 Effluent discharges are subject to control under the WPCO. The Technical Memorandum (TM) on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters, issued under Section 21 of the WPCO, sets limits for permissible effluent discharges based on the types of receiving waters (foul sewers, storm water drains, inland and coastal waters).

5.2.8 The effluent standards are intended to control the physical, chemical and microbial quality of effluent, and vary with the effluent flow rate. The TM effluent standards relevant to this study are summarized in Table 5.2.

Table 5.2 : Standards for Effluent Discharge into Marine Waters of the Southern Water Control Zone

Flow rate (m³/day) Determinant	≤10	>10 & ≤200	>200 & ≤400	>400 & ≤600	>600 & ≤800	>800 & ≤1000	>1000 & ≤1500	>1500 & ≤2000	>2000 & ≤3000	>3000 & ≤4000	>4000 & ≤5000	>5000 & ≤6000
pH (pH units)	6–10	6–10	6–10	6–10	6–10	6–10	6–10	6–10	6–10	6–10	6–10	6–10
Temperature (°C)	45	45	45	45	45	45	45	45	45	45	45	45
Colour (lovibond units) (25mm cell length)	1	1	1	1	1	1	1	1	1	1	1	1
Suspended solids	500	500	500	300	200	200	100	100	50	50	40	30
BOD	500	500	500	300	200	200	100	100	50	50	40	30
COD	1000	1000	1000	700	500	400	300	200	150	100	80	80
Oil & Grease	50	50	50	30	25	20	20	20	20	20	20	20
Iron	20	15	13	10	7	6	4	3	2	1.5	1.2	1
Boron	6	5	4	3.5	2.5	2	1.5	1	0.7	0.5	0.4	0.3
Barium	6	5	4	3.5	2.5	2	1.5	1	0.7	0.5	0.4	0.3
Mercury	0.1	0.1	0.1	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001
Cadmium	0.1	0.1	0.1	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001
Other toxic metals individually	2	1.5	1.2	0.8	0.6	0.5	0.32	0.24	0.16	0.12	0.1	0.1
Total toxic metals	4	3	2.4	1.6	1.2	1	0.64	0.48	0.32	0.24	0.2	0.14
Cyanide	1	0.5	0.5	0.5	0.4	0.3	0.2	0.15	0.1	0.08	0.06	0.04
Phenols	0.5	0.5	0.5	0.3	0.25	0.2	0.13	0.1	0.1	0.1	0.1	0.1
Sulphide	5	5	5	5	5	5	2.5	2.5	1.5	1	1	0.5
Total residual chlorine	1	1	1	1	1	1	1	1	1	1	1	1
Total nitrogen	100	100	80	80	80	80	50	50	50	50	50	50
Total phosphorus	10	10	8	8	8	8	5	5	5	5	5	5
Surfactants (total)	30	20	20	20	15	15	15	15	15	15	15	15
E.Coli (count/100 mL)	4000	4000	4000	4000	4000	4000	4000	4000	4000	4000	4000	4000

Notes:

(1) All units in mg/L unless otherwise stated; and

(2) All figures are upper limits unless otherwise indicated.

Assessment Criteria for Specific Sensitive Receivers

Water Supplies Department’s Water Quality Criteria

5.2.9 Water Supplies Department (WSD) has specified a set of water quality criteria for flushing seawater intakes. The WSD parameters relevant to this EIA are shown in Table 5.3 along with their target limits.

Table 5.3 : WSD Water Quality Standards for Flushing Water Intakes

Parameter (in mg/L unless otherwise stated)	WSD Target Limit
Colour (Hazen Unit)	< 20
Turbidity (NTU)	< 10
Threshold Odour Number (odour unit)	< 100
Ammonical Nitrogen	< 1
Suspended Solids (SS)	< 10
Dissolved Oxygen (DO)	> 2
Biochemical Oxygen Demand (BOD)	< 10
Synthetic Detergents	< 5
E.Coli (no. / 100 ml)	< 20,000

Assessment Criterion for Cooling Water Intake

5.2.10 The assessment criterion to address the impact on cooling water intakes is a limit of 40 mg/L for suspended solids (SS).

Assessment Criteria for Corals

5.2.11 The assessment criteria for corals are based on both the sedimentation rate and WQO for SS. A 3-dimensional hydrodynamic and water quality mathematical model was used in the EIA to predict the SS concentrations in receiving waters.

5.2.12 With reference to studies by (Pastorok & Bilyard, 1985) and (Hawker & Connel, 1992) on coral reef communities, the recommended sedimentation rate for providing sufficient protection and avoiding unacceptable impacts to corals is less than 0.1 kg/m²/day.

5.2.13 The WQO for SS specifies that human activities or waste discharges shall not raise the ambient SS level by 30% and shall not affect aquatic communities. The ambient SS concentrations at each of the identified WSRs were determined using the latest field data collected at the EPD’s marine water monitoring stations located near the WSRs. The level of increase in SS due to the Project was predicted by a mathematical modelling approach.

5.4 Study Area and Sensitive Receivers

5.4.1 The study area for this EIA study is shown in Figure 5.1 and Figure 5.2. It covers the Cheung Chau Island and its surrounding areas to have a sufficient aerial coverage of potentially affected marine waters. Identified water sensitive receivers (WSRs) in the study area that could be potentially affected by the Cheung Chau STW works include the following:

Ÿ bathing beaches including Cheung Chau Tung Wan, Kwun Yam Wan, Tai Kwai Wan, Tai Long Wan, and Yi Long Wan;

Ÿ secondary contact recreational zones;

Ÿ recreation and tourism related uses;

Ÿ coastal protection areas;

Ÿ natural coastal shore, water courses, and natural streams and rivers in and near Cheung Chau;

Ÿ Cheung Chau Typhoon Shelter;

Ÿ areas of ecological or conservation value in Cheung Chau;

Ÿ habitats of marine mammals (e.g. Finless Porpoise and Chinese White Dolphin);

Ÿ marine benthic communities;

Ÿ intertidal habitats;

Ÿ corals;

Ÿ fish spawning and nursery grounds and fish culture zones; and

Ÿ sea water intakes.

5.4.2 Potential adverse impacts on the above WSRs due to the proposed Cheung Chau Upgrading Project were evaluated in this EIA study.

5.5 Baseline Water Quality

5.5.1 To assess the existing water quality conditions in the study area, the most recently published monitoring data collected at the EPD marine water monitoring stations near Cheung Chau were compiled and reviewed.

5.5.2 The data collected at the EPD marine water quality monitoring station, SM12, and Cheung Chau Typhoon Shelter water monitoring station, ST1, in the Southern WCZ were used to represent the baseline water quality conditions of the study area. The station, SM12, is close to the project site and is considered to reflect the typical condition and trend of baseline water quality in Adamasta Channel, west of Cheung Chau Island. Data monitored at the Cheung Chau Typhoon Shelter reflect the local baseline water quality condition in the typhoon shelter area. The latest water quality data for the Tung Wan and Kwun Yam Wan beaches were also reviewed. The collected water quality data are presented in Table 5.7 to Table 5.10.

Table 5.7 : Water Quality Monitoring Results at SM12 from 2006 to 2010

Parameter	2006	2007	2008	2009	2010	Average
Temperature (°C)	24.2	24.1	23.0	24.3	23.7	23.9
Temperature (°C)	(18.0 - 28.1)	(19.4 - 27.9)	(13.7 - 28.9)	(16.4 - 28.9)	(16.7 - 28.6)
Salinity (ppt)	31.2	31.3	29.5	30.7	29.9	30.5
Salinity (ppt)	(24.8 - 33.1)	(27.1 - 33.6)	(14.9 - 32.9)	(25.6 - 33.3)	(23.1 - 33.4)
DO (mg/L) (Depth-average)	7.3	7.1	6.8	6.3	7.0	6.9
DO (mg/L) (Depth-average)	(6.1 - 9.2)	(5.2 - 9.7)	(4.5 - 9.8)	(5.3 - 8.0)	(5.4 - 8.6)
DO (mg/L) Bottom	7.3	6.9	6.6	6.2	6.5	6.7
DO (mg/L) Bottom	(5.8 - 9.6)	(5.6 - 8.8)	(3.0 - 9.7)	(5.0 - 8.0)	(4.7 - 7.9)
pH	8.1	8.2	8.1	8.1	8.0	8.1
pH	(7.9 - 8.3)	(7.5 - 8.9)	(7.7 - 8.3)	(7.9 - 8.5)	(7.7 - 8.3)
Secchi Disc Depth (m)	1.5	1.5	1.8	2.0	2.1	1.8
Secchi Disc Depth (m)	(0.4 - 2.1)	(1.0 - 2.2)	(1.0 - 3.2)	(1.0 - 3.3)	(1.1 - 3.5)
Turbidity (NTU)	15.7	12.9	13.5	9.7	5.0	11.4
Turbidity (NTU)	(4.9 - 27.7)	(4.2-20.5)	(8.8-21.9)	(1.7-23.5)	(1.8 - 9.5)
SS (mg/L)	7.8	7.7	11.6	13.1	6.5	9.3
SS (mg/L)	(2.3-14.9)	(2.1 - 18.7)	(2.8-21.0)	(2.3-36.0)	(1.5 - 14.2)
BOD₅ (mg/L)	0.8	1.4	0.7	1.0	1.1	1.0
BOD₅ (mg/L)	(0.2 - 2.2)	(0.1 - 3.4)	(0.2 - 1.8)	(0.3 - 2.2)	(0.4 - 2.2)
NH₄(mg/L)	0.06	0.09	0.07	0.049	0.070	0.068
NH₄(mg/L)	(0.02 - 0.14)	(0.03 - 0.16)	(0.02 - 0.21)	(0.020 - 0.120)	(0.030 - 0.163)
NH₃-N (mg/L)	0.004	0.007	0.004	0.003	0.003	0.004
NH₃-N (mg/L)	(0.001 - 0.009)	(0.001 - 0.028)	(<0.001 - 0.018)	(0.001 - 0.007)	(<0.001 - 0.009)
NO₂ (mg/L)	0.031	0.034	0.041	0.033	0.038	0.035
NO₂ (mg/L)	(0.002 - 0.086)	(0.009 - 0.081)	(0.006 - 0.160)	(0.013 - 0.120)	(0.006 - 0.088)
NO₃ (mg/L)	0.12	0.131	0.194	0.140	0.178	0.153
NO₃ (mg/L)	(<0.01 - 0.42)	(0.051 - 0.280)	(0.019 - 0.940)	(0.052 - 0.303)	(0.029 - 0.477)
TIN (mg/L)	0.22	0.25	0.31	0.22	0.29	0.26
TIN (mg/L)	(0.03 - 0.52)	(0.12 - 0.44)	(0.07 - 1.31)	(0.13 - 0.36)	(0.07 - 0.61)
TKN (mg/L)	0.23	0.30	0.27	0.21	0.22	0.25
TKN (mg/L)	(0.14 - 0.32)	(0.15 - 0.63)	(0.15 - 0.46)	(0.13 - 0.28)	(0.13 - 0.32)
TN (mg/L)	0.38	0.47	0.51	0.38	0.44	0.44
TN (mg/L)	(0.22 - 0.72)	(0.29 - 0.98)	(0.19 - 1.56)	(0.26 - 0.55)	(0.20 - 0.78)
PO₄-P mg/L)	0.01	0.014	0.016	0.011	0.013	0.013
PO₄-P mg/L)	(<0.01 - 0.03)	(0.004 - 0.027)	(0.004 - 0.028)	(<0.002 - 0.026)	(0.003 - 0.030)
TP (mg/L)	0.04	0.04	0.03	0.03	0.03	0.03
TP (mg/L)	(0.02 - 0.06)	(0.02 - 0.09)	(0.02 - 0.05)	(<0.02 - 0.05)	(<0.02 - 0.04)
Silica (mg/L)	0.86	0.9	1.3	0.81	0.90	0.95
Silica (mg/L)	(0.1 - 2.4)	(0.1 - 1.4)	(0.1 - 4.9)	(0.15 - 1.50)	(0.21 - 2.00)
Chlorophyll-a (µg/L)	7.3	11.8	3.5	5.3	7.9	7.2
Chlorophyll-a (µg/L)	(1.8 - 25.7)	(1.0 - 46.0)	(1.2 - 10.4)	(1.6 - 17.3)	(1.3 - 27.3)
E.Coli (count/100mL)	26	25	21	36	43	30
E.Coli (count/100mL)	(1 - 580)	(2 - 290)	(2 - 200)	(1 - 2600)	(1 - 840)
Fecal Coliforms (count/100mL)	63	54	43	79	93	66
Fecal Coliforms (count/100mL)	(1 - 1500)	(2 - 490)	(3 - 480)	(3 - 3500)	(2 - 2100)

Table 5.8 : Water Quality Monitoring Results at ST1 from 2006 to 2010

Parameter	2006	2007	2008	2009	2010	Average
Temperature (°C)	24.1	24.0	23.1	23.8	23.7	23.7
Temperature (°C)	(18.4 - 27.7)	(19.4 - 28.2)	(16.2 - 28.9)	(16.1 - 29.4)	(16.7 - 29.5)
Salinity (ppt)	31.8	31.6	29.4	30.7	29.7	30.6
Salinity (ppt)	(29.1 - 32.9)	(26.7 - 33.4)	(21.9 - 33.1)	(27.0 - 33.3)	(22.2 - 33.1)
DO (mg/L) (Depth-average)	6.7	6.4	6.8	6.2	6.9	6.6
DO (mg/L) (Depth-average)	(5.7 - 7.8)	(5.4 - 7.9)	(5.2 - 10.0)	(4.7 - 7.5)	(6.1 - 8.8)
DO (mg/L) Bottom	6.9	6.3	6.8	6.3	5.7	6.4
DO (mg/L) Bottom	(5.7 - 8.2)	(5.4 - 7.3)	(4.7 - 9.9)	(4.6 - 7.4)	(0.8 - 7.8)
pH	8.1	8.1	8.1	8.1	8.0	8.1
pH	(8.0 - 8.3)	(7.6 - 8.9)	(8.0 - 8.2)	(7.9 - 8.4)	(7.7 - 8.3)
Secchi Disc Depth (m)	1.6	1.8	2.2	2.0	2	1.9
Secchi Disc Depth (m)	(1.4 - 1.8)	(1.0 - 2.5)	(1.1 - 3.1)	(1.0 - 2.5)	(1.5 - 2.6)
Turbidity (NTU)	14.3	11.5	9.5	7.2	4.5	9.4
Turbidity (NTU)	(5.0-25.2)	(4.2 - 15.1)	(7.2 - 15.0)	(3.3 - 10.9)	(1.9 - 8.1)
SS (mg/L)	7.7	5.0	6.1	6.5	5.1
SS (mg/L)	(2.8-19.1)	(1.9 - 7.1)	(2.1 - 12.0)	(4.4 - 9.9)	(2.7 - 9.4)
BOD₅ (mg/L)	1.1	1.4	1.1	1.2	1.6	1.3
BOD₅ (mg/L)	(0.2 - 2.6)	(0.6 - 3.0)	(0.7 - 1.8)	(0.3 - 2.1)	(0.7 - 2.3)
NH₄(mg/L)	0.09	0.11	0.08	0.08	0.067	0.085
NH₄(mg/L)	(0.04 - 0.13)	(0.07 - 0.15)	(0.04 - 0.14)	(0.039 - 0.153)	(0.034 - 0.15)
NH₃-N (mg/L)	0.005	0.008	0.004	0.004	0.003	0.005
NH₃-N (mg/L)	(0.002 - 0.008)	(0.001 - 0.027)	(0.002 - 0.006)	(0.002 - 0.008)	(0.001 - 0.006)
NO₂ (mg/L)	0.025	0.022	0.024	0.017	0.034	0.024
NO₂ (mg/L)	(0.002 - 0.072)	(0.011 - 0.033)	(0.007 - 0.057)	(0.010 - 0.034)	(0.007 - 0.069)
NO₃ (mg/L)	0.10	0.116	0.181	0.124	0.159	0.136
NO₃ (mg/L)	(0.01 - 0.20)	(0.090 - 0.167)	(0.033 - 0.465)	(0.065 - 0.280)	(0.051 - 0.33)
TIN (mg/L)	0.22	0.25	0.29	0.22	0.26	0.25
TIN (mg/L)	(0.05 - 0.36)	(0.19 - 0.31)	(0.08 - 0.63)	(0.17 - 0.34)	(0.13 - 0.4)
TKN (mg/L)	0.25	0.31	0.29	0.23	0.24	0.26
TKN (mg/L)	(0.21 - 0.28)	(0.25 - 0.47)	(0.24 - 0.35)	(0.13 - 0.31)	(0.18 - 0.37)
TN (mg/L)	0.38	0.44	0.50	0.37	0.44	0.43
TN (mg/L)	(0.21 - 0.51)	(0.37 - 0.64)	(0.28 - 0.78)	(0.28 - 0.55)	(0.23 - 0.57)
PO₄-P (mg/L)	0.016	0.016	0.018	0.014	0.013	0.015
PO₄-P (mg/L)	(0.003 - 0.026)	(0.003 - 0.028)	(0.006 - 0.039)	(<0.002 - 0.022)	(0.003 - 0.026)
TP (mg/L)	0.04	0.04	0.04	0.03	0.03	0.04
TP (mg/L)	(0.03 - 0.04)	(0.04 - 0.05)	(0.02 - 0.05)	(0.02 - 0.04)	(0.02 - 0.04)
Silica (mg/L)	0.8	0.7	1.0	0.76	0.68	0.79
Silica (mg/L)	(0.1 - 1.3)	(0.3 - 1.0)	(0.1 - 3.1)	(0.09 - 1.37)	(0.09 - 1.47)
Chlorophyll-a (µg/L)	6.7	14.2	5.3	0.55	10	7.35
Chlorophyll-a (µg/L)	(1.5 - 22.7)	(2.1 - 52.7)	(1.4 - 9.8)	(1.9 - 12.5)	(0.9 - 27)
E.Coli (count/100mL)	45	110	79	80	63	75
E.Coli (count/100mL)	(2 - 200)	(14 - 240)	(19 - 270)	(32 - 290)	(15 - 150)
Fecal Coliforms (count/100mL)	150	390	260	210	220	246
Fecal Coliforms (count/100mL)	(5 - 1000)	(68 - 1000)	(77 - 1100)	(50 - 1000)	(32 - 530)

Table 5.9 : Beach Water Quality Monitoring Results at Cheung Chau Tung Wan from 2006 to 2010

Parameter	2006	2007	2008	2009	2010	Average
Temperature (°C)	25.8	26.0	25.7	26.4	25.9	26.0
Temperature (°C)	(16.9 - 30.9)	(17.6 - 29.8)	(17.0 - 29.6)	(19.5 - 31.0)	(18.0 - 31.0)
Salinity (ppt)	27.7	28.9	27.7	29.0	28.5	28.4
Salinity (ppt)	(18.9 - 32.3.)	(22.3 - 32.7)	(15.2 - 31.4)	(23.7 - 32.7)	(22.5 - 33.1)
DO (mg/L)	6.8	6.8	7.0	6.9	6.9	6.9
DO (mg/L)	(5.1 - 9.1)	(5.5 - 8.6)	(6.0 - 8.3)	(5.5 - 8.6)	(5.9 - 8.3)
pH	8.1	8.2	8.3	8.4	8.4	8.3
pH	(7.9 - 8.1)	(7.9 - 8.6)	(8.0 - 8.8)	(8.1 - 8.7)	(8.2 - 8.9)
Turbidity (NTU)	4.4	5.3	7.9	6.7	4.2	5.7
Turbidity (NTU)	(1.6 - 13.6)	(1.2 - 18.2)	(4.4 - 20.1)	(2.8 - 16.4)	(1.5 - 9.9)
E.Coli (count/100mL)	37	21	15	17	12	20

Table 5.10 : Beach Water Quality Monitoring Results at Cheung Chau Kwun Yam Wan from 2006 to 2010

Parameter	2006	2007	2008	2009	2010	Average
Temperature (°C)	25.7	25.9	25.6	26.4	25.7	25.9
Temperature (°C)	(16.6-31.3)	(17.9 - 29.8)	(17.0 - 29.3)	(19.5 - 30.4)	(18.0 - 31.0)
Salinity (ppt)	27.7	28.7	27.4	28.7	28.3	28.2
Salinity (ppt)	(19.6-32.3)	(21.7 - 32.7)	(15.0 - 31.4)	(23.4 - 32.6)	(21.7 - 33.1)
DO (mg/L)	6.9	6.9	7.0	6.8	6.8	6.9
DO (mg/L)	(5.6-9.6)	(4.7 - 8.9)	(6.0 - 8.5)	(5.6 - 9.7)	(5.6 - 8.0)
pH	8.1	8.2	8.3	8.4	8.4	8.3
pH	(7.9-8.4)	(7.7 - 8.7)	(7.9 - 8.8)	(8.1 - 8.7)	(8.1 - 8.9)
Turbidity (NTU)	4.3	5.1	8.3	7.2	3.8	5.7
Turbidity (NTU)	(1.6-11.5)	(1.2 - 16.2)	(4.1 - 32.0)	(3.1 - 19.5)	(1.3- 10.1)
E.Coli (count/100mL)	13	9	10	9	8	10

5.5.3 Water quality data collected at majority of the marine water monitoring stations show that total inorganic nitrogen (TIN) frequently exceeds the water quality objective (WQO) for TIN. The TIN level at SM12 has been consistently higher than the WQO limit since 2006. The TIN concentration at the Cheung Chau shelter (ST1) is similar to that at SM12, and is also in non-compliance with the WQO.

5.5.4 The pH level monitored from 2006 to 2010 was in full compliance with the WQO at the Adamasta Channel (SM12) and the Cheung Chau Typhoon Shelter (ST1). From 2006 to 2010, pH at Tung Wan ranged from 7.9 to 8.9 and varied from 7.7 to 8.9 at Kwun Yam Wan, which is close to the WQO range of 6.5-8.5.

5.5.5 Salinity ranged from 23.1 to 33.4 ppt for SM12 and 22.2 to 33.1 ppt for ST1 in 2010. Salinity conditions at these two locations were very close. The annual average salinity remains to be stable since 2006. Salinity at the Tung Wan and Kwun Yam Beaches was in a similar range from 22.5 to 33.1 and 21.7 to 33.1, respectively in 2010. The annual average salinity levels at the two gazetted beaches were very close.

5.5.6 Depth-averaged DO ranged from 6.1 to 8.8 mg/L and 5.4 to 8.6 mg/L at ST1 and SM12, respectively, in 2010. The DO levels have been within the WQO limits since 2006 without a significant annual variation. The bottom DO monitored in 2010 at ST1 showed a wider range of 0.8-7.8 mg/L than the range of 4.7-7.9 mg/L at SM12. The lower DO levels occurred at Cheung Chau Shelter (ST1) might be influenced by human activities. DO levels at Tung Wan and Kwun Yam Wan varied from 5.9-8.3 mg/L and 5.6-8.0 mg/L in 2010, respectively, and were in compliance with the WQOs for DO. No significant changes in the annual average DO since 2006 were detected at the two beaches.

5.5.7 SS concentration monitored in 2010 ranged from 2.7 to 9.4 mg/L at ST1 in the Cheung Chau Typhoon Shelter and 1.5 to 14.2 mg/L at SM12 in Adamasta Channel. The slightly smaller range of SS concentration in the typhoon shelter might be attributable to less tidal disturbance inside the typhoon shelter. A similar pattern of SS variation was also observed in other years since 2006 that ST1 has a smaller range of SS variation than SM12.

5.5.8 E.coli levels at SM12 were fairly low in 2006 to 2010, which were in good compliance with the WQO for E.coli. The E.coli levels at the typhoon shelter (ST1) were relatively higher than those at SM12, but still in compliance with the WQO. The relatively higher E.coli levels at ST1 might be attributable to sewage generation in the typhoon shelter area.

5.5.9 For the Tung Wan and Kwun Yam Wan beaches, E.coli levels monitored in 2006 to 2010 were fairly low as shown in Table 5.9 and Table 5.10, indicating a low risk to the public health.

5.6 Water Quality Impact Assessment

Construction Phase

Site Runoff

5.6.1 Site runoff from construction sites that are subject to excavation or earth works might lead to surface erosion and would carry high sediment concentrations. Without any control, sediments in runoff might be discharged to adjacent marine waters near the Cheung Chau STW, the Cheung Chau Typhoon Shelter, and the Tung Wan and Kwun Yam Wan areas through drainage channels. If this was the case, it had the potential to cause increased concentrations of suspended solid in local waters and reduce light penetration into the water, which might affect aquatic organisms.

5.6.2 However, it should be noted that all the construction works are land-based and no works will be carried out in the marine or intertidal environment. Effluents from these construction sites, including any site runoff into the drainage channels, are controlled to comply with the WPCO. The relevant water pollution control and mitigation measures as recommended in ProPECC Note (PN1/94) for Construction Site Drainage are required to be implemented, and any effluents have to meet the WPCO standards. With the implementation of good site practice and the precautionary measures, polluting effluents from the construction sites are avoided and adverse water quality impact on the marine environment is not anticipated.

General Construction Activities

5.6.3 Wastewaters generated from general construction activities may contain high SS concentrations and a certain amount of grease and oil. Nonetheless, potential impact due to such site wastewater discharges can be minimized if good construction and site management practices are implemented to ensure that litter, fuels, and solvents would not enter public drainage systems. With implementation of good management practices, no adverse impact is expected to occur to drainage systems and receiving waters.

5.6.4 The upgrading works of the Cheung Chau STW will not involve marine dredging activities and marine based construction works. Release of sediments and contaminants from the sea bed into the water column would not occur during the construction phase of the Project. The potential impact of the Cheung Chau STW Upgrading Project on sediment quality is not anticipated to be significant.

Domestic Sewage from Workforce

5.6.5 Domestic sewage generated from the workforce during the construction phase would be forbidden to be directly discharged into public drainage systems or adjacent waterbodies. Portable chemical toilets would be provided at the construction sites as necessary. Wastewater generated from site office would be discharged to public foul sewers or collected in temporary storage tanks. With a good control of domestic sewage, no adverse water quality impact from the workforce swage is anticipated to occur.

Accidental Spillage of Chemicals

5.6.6 Accidental spillage and illegal disposal of chemicals would cause soil contamination, which could potentially impose groundwater pollution. It could also impose pollution to nearby drainage channels or waterbodies through leaching to surface water. The Code of Practice on Packaging, Labelling and Storage of Chemical Wastes published under the Waste Disposal Ordinance would be used as a guideline for handling chemical wastes. The disposal of chemical wastes would be conducted following the rules stipulated in the Waste Disposal Ordinance.

5.6.7 With effective controls through good operation and management practices, no adverse impact is anticipated to occur to water quality due to the accidental spillage of chemicals from construction activities.

Operational Phase

Scenario 1 - Baseline Condition

5.6.8 Associated construction works is scheduled to commence in 2014 and to be completed in early 2019. Simulated marine water quality for 2019 with the existing Cheung Chau STW discharge was used as the baseline water quality condition for assessing the potential impact arising from the operation of the upgraded Cheung Chau STW.

5.6.9 The simulated baseline water quality results for 2019 at locations where the WSRs were identified are summarized in Table 5.11 and Table 5.12 for dry and wet seasons, respectively.

Table 5.11 : Baseline Water Quality (Scenario 1) at Identified WSRs in Dry Season

Location	DO (mg/L)	Bottom DO (mg/L)	TIN (mg/L)	UIA (mg/L)	SS (mg/L)	BOD₅ (mg/L)	E.Coli (count/100mL)
WQO	>4	>2	<0.1	<0.021	Change <30%	<10 (WQC)	< 610 cfu/100mL
C1	6.4	6.3	0.12	0.002	3.9	0.1	6
C2	6.4	6.3	0.12	0.002	3.9	0.1	6
C3	6.2	6.2	0.12	0.002	4.0	0.1	1
C4	6.2	6.2	0.12	0.002	4.0	0.1	2
C5	6.2	6.2	0.12	0.002	3.9	0.1	1
C6	6.3	6.3	0.12	0.002	4.0	0.1	1
C7	6.4	6.4	0.11	0.002	4.1	0.1	2
C8	6.3	6.2	0.12	0.002	4.0	0.1	1
C9	6.3	6.1	0.12	0.002	4.0	0.1	1
C10	6.4	6.3	0.11	0.002	4.1	0.1	5
C11	6.5	6.5	0.11	0.002	4.1	0.2	15
C12	6.3	6.2	0.12	0.002	4.1	0.1	1
C13	7.3	7.3	0.07	0.001	4.5	0.7	391
C14	6.7	6.7	0.10	0.002	4.2	0.3	7
C15	6.6	6.6	0.11	0.002	4.2	0.3	5
B1	6.5	6.4	0.11	0.002	3.9	0.1	10
B2	6.5	6.5	0.11	0.002	3.9	0.1	36
B3	6.3	6.2	0.11	0.002	3.9	0.1	1
B4	6.2	5.8	0.12	0.002	3.9	0.1	1
B5	7.4	7.4	0.07	0.001	4.5	0.8	265
B6	6.6	6.6	0.11	0.001	4.1	0.2	16
B7	6.7	6.6	0.10	0.001	4.2	0.2	9
B8	6.4	6.3	0.11	0.002	4.1	0.1	52
B9	6.3	6.1	0.11	0.002	4.1	0.2	3
ST1	6.5	6.3	0.13	0.003	4.8	0.6	80
ST2	6.3	6.2	0.11	0.002	4.1	0.1	1
FP1	6.3	6.2	0.11	0.002	4.0	0.1	47
S1	6.3	6.2	0.11	0.002	3.9	0.1	29
S2	6.2	6.2	0.11	0.002	4.0	0.1	1
S3	6.3	6.2	0.11	0.002	4.0	0.1	1
CW1	6.2	6.1	0.11	0.002	3.9	0.1	1
MH1	6.2	6.2	0.11	0.002	4.0	0.1	1
F1	6.7	6.7	0.10	0.002	4.2	0.3	7
M1	6.9	6.9	0.09	0.001	4.3	0.4	15
M2	6.7	6.7	0.10	0.001	4.2	0.2	37
U1	6.4	6.3	0.11	0.002	4.0	0.1	4
SM5	6.3	6.2	0.11	0.002	3.9	0.1	1
SM6	6.3	6.2	0.11	0.002	4.0	0.1	1
SM7	6.2	6.1	0.12	0.002	4.0	0.1	1
SM9	6.3	6.3	0.12	0.002	4.0	0.1	1
SM11	6.6	6.5	0.11	0.002	4.1	0.2	4
SM12	6.4	6.2	0.11	0.002	4.1	0.1	24
SM13	6.5	6.4	0.11	0.002	4.1	0.2	14
SM17	6.3	6.3	0.11	0.002	4.0	0.1	1
SM18	6.3	6.2	0.11	0.002	4.0	0.1	1
Average	6.4	6.4	0.11	0.002	4.1	0.2	25
Max	7.4	7.4	0.13	0.003	4.8	0.8	391
Min	6.2	5.8	0.07	0.001	3.9	0.1	1

Note:

(1) Numbers in shade exceed WQO standards.

Table 5.12 : Baseline Water Quality (Scenario 1) at Identified WSRs in Wet Season

Location	DO (mg/L)	Bottom DO (mg/L)	TIN (mg/L)	UIA (mg/L)	SS (mg/L)	BOD₅ (mg/L)	E.Coli (count/100mL)
WQO	>4	>2	<0.1	<0.021	Change <30%	<10 (WQC)	< 610 cfu/100mL
C1	5.7	5.5	0.21	0.005	5.0	0.7	43
C2	5.7	5.5	0.21	0.005	5.0	0.7	43
C3	5.6	5.3	0.21	0.005	5.0	0.6	0
C4	5.6	5.4	0.20	0.005	4.8	0.6	1
C5	5.4	5.3	0.21	0.005	4.8	0.6	1
C6	5.8	5.4	0.19	0.005	4.9	0.7	1
C7	5.8	5.5	0.19	0.005	5.0	0.8	5
C8	5.7	5.4	0.20	0.005	5.0	0.7	3
C9	5.7	5.4	0.20	0.005	5.0	0.7	8
C10	5.8	5.3	0.19	0.005	5.1	0.8	5
C11	5.8	5.5	0.20	0.005	5.1	0.8	7
C12	5.8	5.3	0.20	0.005	4.9	0.7	29
C13	5.2	5.0	0.20	0.006	5.1	1.0	182
C14	5.6	5.0	0.20	0.006	5.1	0.9	7
C15	5.7	5.2	0.20	0.005	5.1	0.8	13
B1	5.6	5.3	0.20	0.005	5.0	0.7	18
B2	5.6	5.3	0.20	0.005	5.0	0.7	38
B3	5.8	5.3	0.20	0.005	4.9	0.7	1
B4	5.8	5.4	0.20	0.005	4.8	0.7	0
B5	5.0	4.6	0.19	0.006	5.2	1.0	100
B6	5.9	5.3	0.21	0.005	5.2	0.7	10
B7	5.9	5.4	0.21	0.005	5.1	0.7	12
B8	5.7	5.4	0.22	0.005	5.1	0.7	3
B9	5.7	5.3	0.22	0.005	5.0	0.7	1
ST1	5.9	5.5	0.23	0.005	5.3	0.8	65
ST2	6.0	5.4	0.20	0.005	4.9	0.7	17
FP1	5.7	5.3	0.22	0.005	5.1	0.7	25
S1	5.6	5.2	0.21	0.005	5.0	0.7	10
S2	5.4	4.8	0.22	0.005	5.0	0.6	0
S3	5.6	5.3	0.21	0.005	4.9	0.6	0
CW1	5.7	5.2	0.21	0.005	4.9	0.6	0
MH1	5.5	5.0	0.21	0.005	5.0	0.6	0
F1	5.6	5.0	0.20	0.006	5.1	0.9	7
M1	5.5	4.9	0.20	0.006	5.1	0.9	8
M2	5.9	5.4	0.21	0.005	5.1	0.7	23
U1	5.9	5.6	0.21	0.005	5.0	0.7	63
SM5	5.8	5.3	0.20	0.005	4.9	0.7	1
SM6	5.5	5.1	0.21	0.005	5.0	0.6	0
SM7	5.6	5.3	0.20	0.005	4.9	0.6	0
SM9	5.8	5.5	0.19	0.005	4.9	0.7	1
SM11	5.8	5.3	0.20	0.005	5.0	0.8	8
SM12	5.8	5.4	0.22	0.005	5.1	0.7	96
SM13	5.8	5.3	0.22	0.005	5.2	0.7	0
SM17	5.5	4.9	0.23	0.004	5.1	0.5	0
SM18	5.3	4.8	0.22	0.005	5.0	0.5	0
Average	5.7	5.3	0.21	0.005	5.0	0.7	19
Max	6.0	5.6	0.23	0.006	5.3	1.0	182
Min	5.0	4.6	0.19	0.004	4.8	0.5	0

Note:

(1) Numbers in shade exceed WQO standards.

Scenario 2 – Normal Operation of Upgraded Cheung Chau STW

5.6.10 The simulated water quality concentrations at the identified WSRs under Scenario 2 of normal operation of the upgraded STW are summarized in Table 5.13 and Table 5.14 for dry and wet seasons, respectively.

5.6.11 Figures 5C-3 to 5C-16 in Appendix 5C illustrate the aerial distributions (contour plots) of selected water quality parameters of concern including the 90% depth-averaged DO, 90% bottom DO, TIN, SS, UIA, E.coli and BOD₅ to compare the difference between Scenario 1 (baseline) and Scenario 2 (normal operation of the upgraded STW). The time-series model results are also presented in Figures 5C-17 to 5C-106 in Appendix 5C for comparing water quality conditions between the baseline and normal operation scenarios.

5.6.12 Incremental changes in concentration of the water quality parameters between Scenario 2 and Scenario 1 are presented in Table 5.15 and Table 5.16 for the identified WSRs.

Table 5.13 : Water Quality under Normal Operation (Scenario 2) at Identified WSRs in Dry Season

Location	DO (mg/L)	Bottom DO (mg/L)	TIN (mg/L)	UIA (mg/L)	SS (mg/L)	BOD₅ (mg/L)	E.Coli (count/100mL)
WQO	>4	>2	<0.1	<0.021	Change <30%	<10 (WQC)	< 610 cfu/100mL
C1	6.4	6.3	0.12	0.002	3.9	0.1	1
C2	6.4	6.3	0.12	0.002	3.9	0.1	1
C3	6.2	6.2	0.12	0.002	4.0	0.1	1
C4	6.2	6.2	0.12	0.002	4.0	0.1	2
C5	6.2	6.2	0.12	0.002	3.9	0.1	1
C6	6.3	6.3	0.12	0.002	4.0	0.1	1
C7	6.4	6.4	0.11	0.002	4.1	0.1	2
C8	6.3	6.2	0.12	0.002	4.0	0.1	1
C9	6.3	6.1	0.12	0.002	4.0	0.1	1
C10	6.4	6.3	0.11	0.002	4.1	0.1	5
C11	6.5	6.5	0.11	0.002	4.1	0.2	15
C12	6.3	6.2	0.12	0.002	4.1	0.1	1
C13	7.3	7.3	0.07	0.001	4.5	0.7	391
C14	6.7	6.7	0.10	0.002	4.2	0.3	7
C15	6.6	6.6	0.11	0.002	4.2	0.3	5
B1	6.5	6.4	0.11	0.002	3.9	0.1	1
B2	6.5	6.5	0.11	0.002	3.9	0.1	1
B3	6.3	6.2	0.11	0.002	3.9	0.1	1
B4	6.2	5.8	0.12	0.002	3.9	0.1	1
B5	7.4	7.4	0.07	0.001	4.5	0.8	265
B6	6.6	6.6	0.11	0.001	4.1	0.2	15
B7	6.7	6.6	0.10	0.001	4.2	0.2	9
B8	6.4	6.3	0.11	0.002	4.1	0.1	1
B9	6.3	6.1	0.11	0.002	4.1	0.2	1
ST1	6.6	6.4	0.11	0.002	4.5	0.4	29
ST2	6.3	6.2	0.11	0.002	4.1	0.1	1
FP1	6.3	6.2	0.11	0.002	4.0	0.1	3
S1	6.3	6.2	0.11	0.002	3.9	0.1	1
S2	6.2	6.2	0.11	0.002	4.0	0.1	1
S3	6.3	6.2	0.11	0.002	4.0	0.1	1
CW1	6.2	6.1	0.11	0.002	3.9	0.1	1
MH1	6.2	6.2	0.11	0.002	4.0	0.1	1
F1	6.7	6.7	0.10	0.002	4.2	0.3	7
M1	6.9	6.9	0.09	0.001	4.3	0.4	15
M2	6.7	6.7	0.10	0.001	4.2	0.2	36
U1	6.4	6.3	0.11	0.002	4.0	0.1	1
SM5	6.3	6.2	0.11	0.002	3.9	0.1	1
SM6	6.3	6.2	0.11	0.002	4.0	0.1	1
SM7	6.2	6.1	0.12	0.002	4.0	0.1	1
SM9	6.3	6.3	0.12	0.002	4.0	0.1	1
SM11	6.6	6.5	0.11	0.002	4.1	0.2	4
SM12	6.4	6.2	0.11	0.002	4.1	0.1	1
SM13	6.5	6.4	0.11	0.002	4.0	0.1	1
SM17	6.3	6.3	0.11	0.002	4.0	0.1	1
SM18	6.3	6.2	0.11	0.002	4.0	0.1	1
Average	6.4	6.4	0.11	0.002	4.1	0.2	19
Max	7.4	7.4	0.12	0.002	4.5	0.8	391
Min	6.2	5.8	0.07	0.001	3.9	0.1	1

Note:

(1) Numbers in shade exceed WQO Standards.

Table 5.14 : Water Quality under Normal Operation (Scenario 2) at Identified WSRs in Wet Season

Location	DO (mg/L)	Bottom DO (mg/L)	TIN (mg/L)	UIA (mg/L)	SS (mg/L)	BOD₅ (mg/L)	E.Coli (count/100mL)
WQO	>4	>2	<0.1	<0.021	Change <30%	<10 (WQC)	< 610 cfu/100mL
C1	5.7	5.5	0.20	0.005	5.0	0.7	1
C2	5.7	5.5	0.20	0.005	5.0	0.7	1
C3	5.6	5.3	0.21	0.005	5.0	0.6	1
C4	5.6	5.4	0.21	0.005	4.8	0.6	1
C5	5.4	5.3	0.22	0.005	4.8	0.6	1
C6	5.8	5.4	0.20	0.005	4.9	0.7	1
C7	5.8	5.5	0.19	0.005	5.0	0.8	3
C8	5.7	5.4	0.20	0.005	5.0	0.7	1
C9	5.7	5.4	0.20	0.005	5.0	0.7	1
C10	5.8	5.3	0.19	0.005	5.1	0.8	4
C11	5.8	5.5	0.19	0.005	5.1	0.8	2
C12	5.8	5.3	0.20	0.005	4.9	0.7	1
C13	5.2	5.0	0.19	0.006	5.1	1.0	181
C14	5.5	5.0	0.19	0.006	5.1	0.9	4
C15	5.7	5.2	0.19	0.005	5.1	0.8	4
B1	5.6	5.3	0.20	0.005	5.0	0.7	1
B2	5.6	5.3	0.20	0.005	5.0	0.7	1
B3	5.8	5.3	0.21	0.005	4.9	0.7	1
B4	5.8	5.4	0.20	0.005	4.8	0.7	1
B5	5.0	4.6	0.19	0.006	5.2	1.0	100
B6	5.9	5.3	0.22	0.005	5.2	0.7	10
B7	5.9	5.4	0.21	0.005	5.1	0.7	12
B8	5.7	5.4	0.22	0.005	5.1	0.7	1
B9	5.7	5.3	0.22	0.005	5.0	0.7	1
ST1	6.0	5.6	0.21	0.005	5.2	0.8	8
ST2	6.0	5.4	0.19	0.005	4.9	0.7	1
FP1	5.6	5.3	0.22	0.005	5.1	0.7	1
S1	5.6	5.2	0.21	0.005	5.0	0.7	1
S2	5.4	4.8	0.22	0.005	5.0	0.6	1
S3	5.6	5.3	0.21	0.005	4.9	0.6	1
CW1	5.7	5.2	0.21	0.005	4.9	0.6	1
MH1	5.5	5.0	0.22	0.005	5.0	0.6	1
F1	5.5	5.0	0.19	0.006	5.1	0.9	4
M1	5.5	4.9	0.19	0.006	5.1	0.9	7
M2	5.9	5.4	0.21	0.005	5.1	0.7	23
U1	5.9	5.5	0.21	0.005	5.0	0.7	8
SM5	5.8	5.3	0.21	0.005	4.9	0.7	1
SM6	5.5	5.0	0.22	0.005	5.0	0.6	1
SM7	5.6	5.3	0.21	0.005	4.9	0.6	1
SM9	5.8	5.5	0.20	0.005	4.9	0.7	1
SM11	5.8	5.3	0.19	0.005	5.0	0.8	1
SM12	5.8	5.4	0.22	0.005	5.1	0.7	1
SM13	5.8	5.3	0.23	0.005	5.2	0.7	1
SM17	5.5	4.9	0.23	0.004	5.1	0.5	1
SM18	5.3	4.8	0.23	0.005	5.0	0.5	1
Average	5.7	5.3	0.21	0.005	5.0	0.7	9
Max	6.0	5.5	0.23	0.006	5.2	1.0	181
Min	5.0	4.6	0.19	0.004	4.8	0.5	1

Note:

(1) Numbers in shade exceed WQO standards.

Table 5.15 : Incremental Changes in Water Quality from Scenario 1 (Baseline) to Scenario 2 (Normal Operation) at Identified WSRs in Dry Season

Location	DO (mg/L)	Bottom DO (mg/L)	TIN (mg/L)	UIA (mg/L)	SS (Percentage)	BOD₅ (mg/L)	E.Coli (count/100mL)
C1	0.0	0.0	0.00	0.000	0%	0.0	-5
C2	0.0	0.0	0.00	0.000	0%	0.0	-5
C3	0.0	0.0	0.00	0.000	0%	0.0	0
C4	0.0	0.0	0.00	0.000	0%	0.0	0
C5	0.0	0.0	0.00	0.000	0%	0.0	0
C6	0.0	0.0	0.00	0.000	0%	0.0	0
C7	0.0	0.0	0.00	0.000	0%	0.0	0
C8	0.0	0.0	0.00	0.000	0%	0.0	0
C9	0.0	0.0	0.00	0.000	0%	0.0	0
C10	0.0	0.0	0.00	0.000	0%	0.0	0
C11	0.0	0.0	0.00	0.000	0%	0.0	0
C12	0.0	0.0	0.00	0.000	0%	0.0	0
C13	0.0	0.0	0.00	0.000	0%	0.0	0
C14	0.0	0.0	0.00	0.000	0%	0.0	0
C15	0.0	0.0	0.00	0.000	0%	0.0	0
B1	0.0	0.0	0.00	0.000	0%	0.0	-9
B2	0.0	0.0	0.00	0.000	0%	0.0	-35
B3	0.0	0.0	0.00	0.000	0%	0.0	0
B4	0.0	0.0	0.00	0.000	0%	0.0	0
B5	0.0	0.0	0.00	0.000	0%	0.0	0
B6	0.0	0.0	0.00	0.000	0%	0.0	-2
B7	0.0	0.0	0.00	0.000	0%	0.0	-1
B8	0.0	0.0	0.00	0.000	0%	0.0	-51
B9	0.0	0.0	0.00	0.000	0%	0.0	-2
ST1	0.1	0.1	-0.03	-0.001	-5%	-0.2	-51
ST2	0.0	0.0	0.00	0.000	0%	0.0	0
FP1	0.0	0.0	0.00	0.000	0%	0.0	-44
S1	0.0	0.0	0.00	0.000	0%	0.0	-28
S2	0.0	0.0	0.00	0.000	0%	0.0	0
S3	0.0	0.0	0.00	0.000	0%	0.0	0
CW1	0.0	0.0	0.00	0.000	0%	0.0	0
MH1	0.0	0.0	0.00	0.000	0%	0.0	0
F1	0.0	0.0	0.00	0.000	0%	0.0	0
M1	0.0	0.0	0.00	0.000	0%	0.0	0
M2	0.0	0.0	0.00	0.000	0%	0.0	0
U1	0.0	0.0	0.00	0.000	0%	0.0	-3
SM5	0.0	0.0	0.00	0.000	0%	0.0	0
SM6	0.0	0.0	0.00	0.000	0%	0.0	0
SM7	0.0	0.0	0.00	0.000	0%	0.0	0
SM9	0.0	0.0	0.00	0.000	0%	0.0	0
SM11	0.0	0.0	0.00	0.000	0%	0.0	0
SM12	0.0	0.0	0.00	0.000	0%	0.0	-23
SM13	0.0	0.0	0.00	0.000	0%	0.0	-13
SM17	0.0	0.0	0.00	0.000	0%	0.0	0
SM18	0.0	0.0	0.00	0.000	0%	0.0	0
Average	0.0	0.0	0.00	0.000	0%	0.0	-6
Max	0.1	0.1	0.00	0.000	0%	0.0	0
Min	0.0	0.0	-0.03	-0.001	-5%	-0.2	-51

Notes:

(1) All absolute differences were calculated as Scenario 2 minus Scenario 1.

(2) SS was evaluated by percentage difference: (Scenario 2-Scenario1) / Scenario 1 × 100%.

Table 5.16: Incremental Changes in Water Quality from Scenario 1 (Baseline) to Scenario 2 (Normal Operation) at Identified WSRs in Wet Season

Location	DO (mg/L)	Bottom DO (mg/L)	TIN (mg/L)	UIA (mg/L)	SS (Percentage)	BOD₅ (mg/L)	E.Coli (count/100mL)
C1	0.0	0.0	0.00	0.000	0%	0.0	-42
C2	0.0	0.0	0.00	0.000	0%	0.0	-42
C3	0.0	0.0	0.00	0.000	0%	0.0	-1
C4	0.0	0.0	0.00	0.000	0%	0.0	0
C5	0.0	0.0	0.00	0.000	0%	0.0	0
C6	0.0	0.0	0.00	0.000	0%	0.0	0
C7	0.0	0.0	0.00	0.000	0%	0.0	-2
C8	0.0	0.0	0.00	0.000	0%	0.0	-2
C9	0.0	0.0	0.00	0.000	0%	0.0	-7
C10	0.0	0.0	0.00	0.000	0%	0.0	-1
C11	0.0	0.0	0.00	0.000	0%	0.0	-5
C12	0.0	0.0	0.00	0.000	0%	0.0	-28
C13	0.0	0.0	0.00	0.000	0%	0.0	0
C14	0.0	0.0	0.00	0.000	0%	0.0	-3
C15	0.0	0.0	0.00	0.000	0%	0.0	-9
B1	0.0	0.0	0.00	0.000	0%	0.0	-17
B2	0.0	0.0	0.00	0.000	0%	0.0	-37
B3	0.0	0.0	0.00	0.000	0%	0.0	0
B4	0.0	0.0	0.00	0.000	0%	0.0	-1
B5	0.0	0.0	0.00	0.000	0%	0.0	0
B6	0.0	0.0	0.00	0.000	0%	0.0	0
B7	0.0	0.0	0.00	0.000	0%	0.0	0
B8	0.0	0.0	0.00	0.000	0%	0.0	-2
B9	0.0	0.0	0.00	0.000	0%	0.0	0
ST1	0.1	0.1	-0.02	0.000	-2%	-0.1	-57
ST2	0.0	0.0	0.00	0.000	0%	0.0	-16
FP1	0.0	0.0	0.00	0.000	0%	0.0	-24
S1	0.0	0.0	0.00	0.000	0%	0.0	-9
S2	0.0	0.0	0.00	0.000	0%	0.0	-1
S3	0.0	0.0	0.00	0.000	0%	0.0	-1
CW1	0.0	0.0	0.00	0.000	0%	0.0	-1
MH1	0.0	0.0	0.00	0.000	0%	0.0	-1
F1	0.0	0.0	0.00	0.000	0%	0.0	-3
M1	0.0	0.0	0.00	0.000	0%	0.0	-1
M2	0.0	0.0	0.01	0.000	0%	0.0	0
U1	0.0	0.0	0.00	0.000	0%	0.0	-55
SM5	0.0	0.0	0.00	0.000	0%	0.0	0
SM6	0.0	0.0	0.00	0.000	0%	0.0	-1
SM7	0.0	0.0	0.00	0.000	0%	0.0	-1
SM9	0.0	0.0	0.00	0.000	0%	0.0	0
SM11	0.0	0.0	0.00	0.000	0%	0.0	-7
SM12	0.0	0.0	0.00	0.000	0%	0.0	-95
SM13	0.0	0.0	0.00	0.000	0%	0.0	-1
SM17	0.0	0.0	0.00	0.000	0%	0.0	-1
SM18	0.0	0.0	0.00	0.000	0%	0.0	-1
Average	0.0	0.0	0.00	0.000	0%	0.0	-10
Max	0.1	0.1	0.01	0.000	0%	0.0	-1
Min	0.0	0.0	-0.02	0.000	-2%	-0.1	-95

Notes:

(1) All absolute differences were calculated as Scenario 2 minus Scenario 1.

(2) SS was evaluated by percentage difference: (Scenario 2-Scenario1) / Scenario 1 × 100%.

5.6.13 Under the normal operation condition, predicted concentrations of depth-averaged DO (90%) range from 6.2 to 7.4 mg/L in dry season and from 5.0 to 6.0 mg/L in wet season, which are both in compliance to the WQO for DO. The bottom DO concentration (90%) ranges from 5.8 to 7.4 mg/L in dry season and from 4.6 to 5.5 mg/L in wet season, respectively, which are also both in compliance to the WQO. At the Cheung Chau Typhoon Shelter (ST1), the DO concentration was simulated to be slightly higher than the baseline condition for both dry and wet seasons (higher by 0.1mg/L for both the depth-averaged and the bottom DO concentrations). No significant changes in DO concentration at the other WSRs were predicted as indicated in Table 5.15 and Table 5.16. Overall, the upgraded Cheung Chau STW was predicted to result in a certain level of improvement in DO concentration near the Project area. No adverse impact is expected to occur with respect to DO concentration.

5.6.14 The baseline TIN ranges from 0.07 to 0.12 mg/L in dry season and from 0.19 to 0.23 mg/L in wet season at the identified WSRs as shown in Table 5.13 and Table 5.14, which both exceed the WQO standard of 0.1 mg/L. This indicates that the background TIN concentration in the southern WCZ has already been experiencing non-compliance to the TIN WQO.

5.6.15 With the commissioning of the upgraded Cheung Chau STW, TIN concentration in Cheung Chau Wan at ST1 was predicted to be lower by 0.03 mg/L in dry season and by 0.02 mg/L in wet season (indicating an improvement in water quality). As indicated in Table 5.15 and Table 5.16, an overall reduction in TIN concentration near the Project area was predicted, and no significant water quality changes in terms of TIN were simulated at the other WSR locations.

5.6.16 The simulated UIA concentration ranges from 0.001 to 0.002 mg/L in dry season and from 0.004 to 0.006 mg/L in wet season, which are both in compliance to the WQO. Compared to the baseline condition, the upgraded Cheung Chau STW would result in a decrease in UIA concentration by 0.001 mg/L in dry season near Cheung Chau Wan (ST1). As indicated in Table 5.15 and Table 5.16, no significant changes in UIA concentration were predicted at the other WSRs. The results indicate that no adverse impacts are likely to occur to the UIA concentration of receiving waters.

5.6.17 SS concentration varies from 3.9 to 4.5 mg/L in dry season and from 4.8 to 5.2 mg/L in wet season, respectively. A decrease by 5% in SS level was predicted for dry season and by 2% for wet season near Cheung Chau Wan at ST1. The predicted SS level is in compliance with the WQO. No significant changes in SS level were predicted at the other identified WSRs. No adverse water quality impacts were predicted to occur to the SS level after commissioning of the upgraded Cheung Chau STW.

5.6.18 BOD₅ was simulated to range from 0.1 to 0.8 mg/L in dry season and from 0.5 to 1.0 mg/L in wet season, respectively, which are below the WQO of 10 mg BOD₅/L for water intakes. BOD₅ was predicted to decrease near Cheung Chau Wan at ST1 by 0.2 mg/L in dry season and by 0.1mg/L in wet season, respectively after commissioning of the upgraded Cheung Chau STW. No significant changes in BOD₅ concentration were simulated at the other WSR locations. No adverse water quality impact was predicted to occur to the BOD₅ concentration in the marine water after commissioning of the upgraded Cheung Chau STW.

5.6.19 Predicted E.coli levels range from 1 to 391 cfu/100mL in dry season and from 1 to 181 cfu/100mL in wet season, which are compliant to the WQO of 610 cfu/100mL for E.coli. Compared to the baseline condition, the upgraded Cheung Chau STW would result in a decrease in the E.coli level at a number of locations including SM12 (Water Monitoring Station), ST1 (Cheung Chau Wan), FP1 (south to Cheung Chau), S1 (Spawning Nursery Grounds south of Cheung Chau), B1 (Tung Wan) and B2 (Kwun Yam Wan). Reduced E.coli levels were also predicted at other locations including Yi Long Wan, Hei Ling Chau (C12).

5.6.20 Generally, an improvement in water quality in terms of E.coli was predicted in the vicinity of Cheung Chau Island under the both dry and wet weather conditions. The improvement is mainly attributable to the enhanced treatment efficiency of the upgraded STW, which would significantly reduce the E.coli loading to the marine water. No adverse water quality impacts are anticipated to occur as far as E.coli is concerned under the normal operation of the upgraded Cheung Chau STW.

5.6.21 Overall, upon commissioning of the upgraded Cheung Chau STW, a relative improvement in water quality is expected to occur to the marine water near the Project area. The proposed Upgrading Project would result in a reduction in concentrations of a number of water quality parameters including TIN, UIA, SS, BOD₅ and E.coli, and would result in a certain level of improvement in DO concentration (by approximately 0.1 mg/L) near the Project area. Predicted differences (benefit) with the upgrading of the Cheung Chau STW are as follows for different water quality parameters: TIN concentration was predicted to be lower by 0.03 mg/L in dry season and by 0.02 mg/L in wet season; UIA concentration lower by 0.001 mg/L; SS level lower by 5% in dry season and by 2% in wet season; BOD5 lower by 0.2 mg/L in dry season and by 0.1 mg/L in wet season; and E. coli level lower at a number of locations including SM12 (Water Monitoring Station), ST1 (Cheung Chau Wan), FP1 (south to Cheung Chau), S1 (Spawning Nursery Grounds south of Cheung Chau), B1 (Tung Wan) and B2 (Kwun Yam Wan). Reduced E. coli levels were also predicted at other locations including Yi Long Wan, Hei Ling Chau (C12). No significant changes to water quality were predicted at the other WSR locations. No adverse water quality impacts are therefore anticipated to result from the normal operation of the upgraded Cheung Chau STW.

Scenario 3 - Emergency Discharge of Untreated Sewage from STW

5.6.22 Emergency discharge is a provisional situation for operation of the STW regardless of this upgrading works. The past operational record by DSD shows it normally takes about 6 hours to resume to the normal operation of a STW in case of an operational failure. A 6-hour period of emergency discharge was therefore simulated to assess the impact of emergency discharge of untreated sewage from the Cheung Chau STW. In general, a neap tide is expected to result in higer concentrations in the discharge outfall area, whereas a spring tide would result in a larger extent of the impact area due to higher tidal current speeds during the spring tide. Sensitive receivers at South Lantau, e.g. Chi Ma Wan, Cheung Sha Fish Cultural Zone, are one of the issues to look into for the emergency discharge assessment.

5.6.23 To cover different conditions of tidal movement, the potential impact of emergency discharge was assessed for the following six scenarios:

Ÿ Scenario 3a – emergency discharge occurs at beginning of a flood tide during the neap tide cycle in dry season (for consideration of the maximum concentration near the discharge outfall in dry season);

Ÿ Scenario 3b – emergency discharge occurs at beginning of a flood tide during the neap tide cycle in wet season (for consideration of the maximum concentration near the discharge outfall in wet season);

Ÿ Scenario 3c – emergency discharge occurs at beginning of a flood tide during the spring tide cycle in dry season (for consideration of the maximum aerial extent of impact in dry season);

Ÿ Scenario 3d – emergency discharge occurs at beginning of a flood tide during the spring tide cycle in wet season (for consideration of the maximum aerial extent of impact in wet season);

Ÿ Scenario 3e – emergency discharge occurs at beginning of an ebb tide during the spring tide cycle in dry season (for consideration of the maximum aerial extent of impact in dry season);

Ÿ Scenario 3f – emergency discharge occurs at beginning of an ebb tide during the spring tide cycle in wet season (for consideration of the maximum aerial extent of impact in wet season).

5.6.24 The purpose of Scenarios 3a and 3b is to evaluate the maximum emergency impact to local areas near the discharge outfall, e.g Cheung Chau Wan. The ebb tide flows away from Cheung Chau Island to the south-west direction and would result in a smaller impact than the flood tide to near field areas, such as Cheung Chau Wan.

5.6.25 The purpose of Scenarios 3c, 3d, 3e and 3f is to evaluate the maximum aerial impact to some sensitive receivers at South Lantau, e.g. Chi Ma Wan, Cheung Sha Fish Cultural Zone under both flood and ebb tide conditions.

Scenario - 3a, 3b

5.6.26 Figures 5C-107 to 5C-206 in Appendix 5C show time-series plots for depth-averaged 90% DO, bottom 90% DO, SS, UIA, E.Coli and BOD₅ concentrations at the identified WSRs for emergency discharge (Scenario 3) from the upgraded Cheung Chau STW in comparison to the baseline condition (Scenario 1) under the both dry and wet seasons.

5.6.27 The locations of ST9 and ST10 are close to the Cheung Chau STW discharge outfall. As shown in Figures 5C-156 and 5C-166 of Appendix 5C, the UIA concentration at ST10 increases slightly right after the emergency discharge, and the peak UIA level is up to 0.0018 mg/L (in comparison to the baseline level of 0.002 mg/L) in dry season, and 0.0058 mg/L (baseline is 0.0059 mg/L) in wet season. Nonetheless, the predicted UIA concentrations during the emergency discharge are still below the baseline levels during both dry and wet seasons. The impact of the Cheung Chau STW emergency discharge is short in duration and not significant with respect to UIA.

5.6.28 At more distant locations of B8 (Yi Long Wan), B9 (Tai Long Wan), S1 (Cheung Chau Spawning Nursery Grounds), FP1 (South of Cheung Chau) and SM12 (Adamasta Channel), the magnitude of impact on UIA from the Cheung Chau STW emergency discharge is significantly smaller than that at ST9 and ST10. The simulated UIA concentrations in these locations are at the same level as the baseline condition for both dry and wet seasons and the impact of emergency discharge on UIA is insignificant at these locations.

5.6.29 Similar to UIA, the potential impact of the Cheung Chau STW emergency discharge on E.coli is limited to local areas (such as ST9, ST10 and U1) as shown in Figures 5C-136 and 5C-142 in Appendix 5C. The E.Coli level would increase sharply at ST9 to a peak up to 10,000 cfu/100ml (baseline peak is about 15,000 cfu/100ml) in dry season, and to a peak up to 5,000 cfu/100ml in wet season (baseline peak for the wet season is in the range of 6,000 cfu/100ml as well). On the other hand, the E.Coli concentration would decrease sharply from its peak and return to its normal background level in less than 12 hours after the emergency discharge is ceased.

5.6.30 Apart from the local area (such as at ST9, ST10 and U1), no obvious changes to the E.Coli level were predicted to occur at other more distant locations such as at ST1 (Cheung Chau typhoon shelter), C1 (Tung Wan), C2 (Kwun Yam Wan), C12 (South of Hei Ling Chau), B8 (Yi Long Wan), B9 (Tai Long Wan), S1 (Cheung Chau Spawning Nursery Grounds), and F1 (Cheung Sha Wan Fish Culture Zone). Figures 5C-133 and 5C-143 in Appendix 5C show that the potential impact of the STW emergency discharge on Cheung Sha Wan Fish Culture Zone (F1) is insignificant. As shown in Table 5.4 and Table 5.6, the emergency discharge loading from the upgraded Cheung Chau STW (Scenario 3) is higher than, but still in the same order of magnitude as the baseline loading (Scenario 1). Given that the upgraded STW is expected to result in an improvement of overall water quality in local areas near the Project site compared to the existing baseline condition, and given that the duration of emergency discharge last only about 6 hours, it is anticipated that the potential impact due to the Cheung Chau STW emergency discharge is temporary and short lived.

Scenario - 3c, 3d, 3e, 3f

5.6.31 Four typical scenarios 3c, 3d, 3e, 3f were simulated to assess the maximum extent of impact during the spring tide cycle for both the flood and ebb tide conditions.

5.6.32 Figures 5C-208 and 5C-214 show that the emergency discharge would result in a sharp increase in the E.Coli level in the local area (Observation point U1) during a flood tide. The peak value at U1 is up to 2,000 cfu/100mL in dry season and 7,000 cfu/100mL in wet season. On the other hand, the E.Coli level would decrease quickly from its peak and return to its normal background level in about 10 hours after the emergency discharge is ceased. At more distant locations such as SM12 (EPD routine monitoring station), B1 (Tung Wan), B8 (Yi Long Wan), B9 (Tai Long Wan), C15 (Chi Ma Wan Peninsula) and F1 (Cheung Sha Wan Fish Culture Zone), the emergency discharge was predicted to have minimal influence in both dry and wet seasons as shown in Figures 5C-207 to 5C-218.

5.6.33 Similar to the flood tide condition, a sharp increase in the E.Coli level is expected in the local area near the emergency discharge outfall during an ebb tide as shown in Figures 5C-220 and 5C-226 (Observation points ST1 and U1). The peak value at U1 was simulated to reach up to 900 cfu/100mL in dry season and 5,000 cfu/100mL in wet season. The peak value at ST1 is up to 12,000 cfu/100mL in dry season and 5,000 cfu/100mL in wet season. Nonetheless, the E.Coli level would decrease quickly from its peak and return to its normal background level in about 12 hours after the emergency discharge is ceased. Figures 5C-219 to 5C-230 show that the emergency discharge would impose a minimal influence at more distant locations such as SM12 (EPD routine monitoring station), B1 (Tung Wan), B8 (Yi Long Wan), B9 (Tai Long Wan), C15 (Chi Ma Wan Peninsula) and F1 (Cheung Sha Wan Fish Culture Zone) under both the dry and wet season conditions.

5.6.34 The above four scenarios 3c, 3d, 3e, 3f show that the impact of emergency discharge would be limited only to local areas. The potential impact would disappear rapidly and sharp concentration of water quality variables such as E.Coli would return to their normal levels in less than 12 hours after the emergency discharge is ceased. Impact to more distant WSRs would be extremely small and insignificant.

Water Quality Impact on Cooling Water Intake at Lamma Island

5.6.35 As shown in Table 5.13 and Table 5.14, the commissioning of the upgraded Cheung Chau STW would not result in a significant change to water quality conditions in vicinity of the sea water intake at Lamma Island at CW1, SM5, and SM6 and SM7. The SS level at the intake location was simulated to be in the range of 3 to 5 mg/L, which is in compliance with the criteria of 40 mg SS/L for cooling water intakes. It is not anticipated that adverse water quality impact would occur at the sea water intake at Lamma Island.

Impact to Corals

5.6.36 As indicated in Table 5.15 and Table 5.16, changes in sediment concentration at the proximity of the identified coral sites are negligible near Stations of C1 to C15. No adverse impact on corals is predicted to result from the upgrading of the Cheung Chau STW.

Cumulative Impacts

5.6.37 Concurrent projects that are expected to be constructed or operated in the same period of time include:

Ÿ Improvement of fresh water supply to Cheung Chau;

Ÿ Replacement and Rehabilitation of Water Mains Stage 3, Mains On Hong Kong and Islands – Investigation, Design and Construction;

Ÿ Replacement and Rehabilitation of Water Mains Stage 4, Mains On Hong Kong and Islands – Investigation, Design and Construction;

Ÿ Improvement to Existing Roads and Drains in Cheung Chau Old Town, Remaining Engineering Works, Stage 3, Cheung Chau;

Ÿ Development of a 100MW Offshore Wind Farm in Hong Kong;

Ÿ Harbour Area Treatment Scheme (HATS) Stage 2A;

Ÿ Hong Kong - Zhuhai - Macao Bridge Hong Kong Link Road Tuen Mun Chek Lap Kok Link;

Ÿ New Contaminated Mud Marine Disposal Facility at Airport East / East Sha Chau Area Mud pits at North Brothers;

Ÿ Hong Kong - Zhuhai - Macao Bridge Hong Kong Boundary Crossing Facilities;

Ÿ Integrated Waste Management Facility at Shek Kwu Chau; and

Ÿ Upgrading of Tai O Sewage Collection, Treatment and Disposal Facilities (354DS – Outlying and Sewerage Stage 2).

5.6.38 Based on the results of relevant EIA studies, cumulative impacts by the concurrent projects listed above were concluded to be insignificant. Water quality impacts from the listed projects are limited to respective local areas near each project site. Given that the Cheung Chau STW Upgrading Project is sufficiently distant away from the listed project locations, it is anticipated that the potential cumulative water quality impacts due to the additive impacts from these projects and the Cheung Chau STW Upgrading Project would be insignificant.

5.6.39 The concurrent projects include the proposed works “Upgrading of Tai O Sewage Collection, Treatment and Disposal Facilities (354DS – Outlying and Sewerage Stage 2)”. As the sewerage works in Tai O is expected to have insignificant impact to marine water quality at Cheung Chau, the cumulative water quality impact from the Tai O STW works and the Cheung Chau STW works is anticipated to be negligible, provided that the recommended mitigation measures are properly implemented during construction.

5.7 Water Pollution Mitigation and Management

Construction Phase

5.7.1 The practices outlined in ProPECC PN 1/94 Construction Site Drainage should be adopted to minimize the potential water quality impacts from site runoff and other construction activities. The mitigation measures should cover, but not be limited to, the following practices:

Ÿ Perimeter channels are to be installed in works areas to intercept runoff at the site boundary prior to the commencement of any earthworks. Surface runoff should be discharged into storm drains via sand/ silt removal facilities with an adequate capacity;

Ÿ Works programme should be designed to minimize works areas to reduce soil exposure and site runoff;

Ÿ Silt removal facilities, channels and manholes should be maintained and cleaned regularly to ensure their proper functions;

Ÿ Works programme should be carefully planned to minimize the scale of soil excavation during the rainy season;

Ÿ Earthworks surfaces should be well compacted and subsequent permanent works or surface protection measures should be carried out immediately;

Ÿ All vehicles should be washed before they leave the construction site to avoid earth, mud, and debris being carried off from the site. Wash-water should be treated to remove sand and silt at least on a weekly basis to ensure the continued efficiency of the washing facility;

Ÿ Open stockpiles of construction materials on site should be covered with tarpaulin or similar fabric materials during storms;

Ÿ For sections of sewer mains that need to be laid underneath water courses with the open cut method, site works should be carried out during the dry season with a temporary drainage diversion; and

Ÿ Any construction works along Hak Pai Road immediately by the Kwun Yam beach and Cheung Chau Tung Wan beach should be avoided during the swimming season.

5.7.2 With implementation of the above measures, construction site runoff is not anticipated.

General Construction Activities

5.7.3 Good site practices should be adopted to regularly clean the construction sites to avoid rubbish, debris and litter from entering to nearby water bodies.

5.7.4 Wastewaters generated from construction activities may contain high SS concentrations and possibly a certain amount of grease and oil. Good construction and site management practices should be implemented to ensure that litter, fuels, and solvents would not enter the public drainage systems.

Sewage Arising from Workforces

5.7.5 Domestic sewage generated by the workforce on construction sites should be collected and disposed of to the STW for proper treatment. Portable toilets should be provided by the Contractor, where necessary, to handle sewage from the workforce. The Contractor should also be responsible for waste disposal.

Spillage of Chemicals

5.7.6 Illegal disposal of chemicals should be strictly prohibited. Registration to EPD as a Chemical Waste Producer is required if chemical wastes are generated and need to be disposed of. Disposal of chemical wastes should be carried out in compliance with the Waste Disposal Ordinance (WDO). The Code of Practice on Packaging, Labelling and Storage of Chemical Wastes published under the WDO should be used as a guideline for handling chemical wastes.

5.7.7 Oils and fuels should only be used and stored in designated areas that have pollution prevention facilities.

Operational Phase

Cheung Chau STW

5.7.8 Emergency discharge of raw sewage from the STW may be needed in case of failure of electrical power supply or treatment units. Under the circumstances of a possible emergency discharge, the STW operator should immediately notify the relevant governmental departments, such as EPD, LCSD and DSD. The STW operator should maintain a good line of communication with various parties involved in an emergency discharge event.

5.7.9 Standby facilities for the main treatment units and standby pumps, accessories/ equipment parts should be installed to avoid the occurrence of an emergency discharge. Storm Tanks would also be incorporated to provide temporary storage of flow under extremely high flow conditions and hence reduce the chance of emergency bypass. Dual power supply or standby power sources should also be implemented to minimize the possibility of power failure. It is expected that with the above mitigation measures, the chance of emergency discharge of untreated effluent is small.

5.7.10 A contingency plan should be developed to deal with the occurrence of an emergency discharge during the operation of the STW.

Sewage Overflow from the SPS

5.7.11 Although the emergency overflow of raw sewage from the Pak She SPS would be diverted into the Cheung Chau STW for treatment prior to discharge into the marine water, mitigation measures would still be recommended to reduce the possibility of an emergency bypass of sewage from the Pak She SPS:

Ÿ A standby pump is already provided and will continue to be provided after the upgrading to cater for breakdown and maintenance of duty pumps to reduce the chance of occurrence of sewage bypass;

Ÿ A backup power supply would be provided to secure electrical power supply; and

Ÿ Regular maintenance and checking of equipment should be carried out to prevent equipment failure.

Contingency Plan

5.7.12 A contingency plan each for the Cheung Chau STW and the Pak She SPS respectively, should be developed to deal with emergency discharge. The contingency plan should include the following:

Ÿ Locations of the sensitive receivers in vicinity of the emergency discharge;

Ÿ A list of relevant governmental bodies to inform of and to ask for assistance in the event of an emergency discharge, including key contact persons and telephone numbers;

Ÿ Reporting procedures required in the event of an emergency discharge;

Ÿ Responsibility and procedure for clean-up of the affected water body/sensitive receivers after the emergency discharge; and

Ÿ Procedures listing the most effective means in rectifying the breakdown of the pumping station to minimize the discharge duration.

5.8 Residual Impacts

5.8.1 With effective controls through good operation and management practices, no adverse water quality impact is anticipated to occur during the construction phase of both the upgrading works for the Cheung Chau STW and Pak She SPS and the sewers works under this Project.

5.8.2 For the operational phase, water quality in the study area is expected to be in compliance with the WQOs under the normal operation of the upgraded Cheung Chau STW (Scenario 2) with the exception for TIN, based on the modelling and analysis results. Given that the background TIN level in the Southern WCZ is already higher the WQO for TIN, the upgraded Cheung Chau STW was not predicted to impose additional adverse impacts to water quality conditions of the receiving marine environment.

5.8.3 In general, the results of model simulation indicate that the upgraded Cheung Chau STW would result in an improvement of water quality conditions in vicinity of the Project area. Lower concentrations of the key water quality parameters including UIA, TIN, E.Coli, SS and BOD₅ in areas near the Project site are expected. This is mainly attributable to the improved treatment efficiency of upgraded facility and better quality of the upgraded Cheung Chau STW effluent, resulting in a reduction in pollutant loads into the marine water. The predicted pollutants level during normal operation of the upgraded Cheung Chau STW generally in compliance with WQO for the Southern Water Control Zones except the TIN content, where the background TIN concentration has already exceeded the standard as specified in the WQO.

5.8.4 The simulation results show that the impact of emergency discharge of Cheung Chau STW would be limited only to the local areas. The potential impact would disappear rapidly and sharp concentration of water quality variables such as E.Coli would return to their normal levels in less than 12 hours after the emergency discharge is ceased. Impact to more distant WSRs would be extremely small and insignificant.

5.8.5 Upgrading of the Pak She SPS will reduce the chance of sewage overflow from the SPS and eliminate any direct emergency discharge to marine waters. Adverse water quality impacts due to this upgrading works is not anticipated.

5.9 Environmental Monitoring and Audit Requirements

5.9.1 The need for EM&A activities during the construction and operation phases of the Project was identified in this EIA study. Details of the EM&A requirements are provided in a separate EM&A manual.

5.10 Conclusion

5.10.1 Overall, no significantly adverse impacts from the effluent discharge of the upgraded Cheung Chau STW are anticipated at the identified water sensitive receivers in the study area. An overall improvement in water quality is expected for marine waters near the Project site, as a result of the enhanced coverage of sewer systems to collect raw sewage on Cheung Chau and the improved treatment efficiency of the upgraded Cheung Chau STW.

Flow (m³/d)	BOD (kg/d)	TSS (kg/d)	E. coil (CFU/d)	NH3-N (kg/d)	TKN (kg/d)	Org-N (kg/d)
8,817	797	571	2.56x10¹⁵	97	194	97

Flow (m³/d)	BOD (kg/d)	TSS (kg/d)	E. coil (CFU/d)	NH₃-N (kg/d)	TKN (kg/d)	NO₃ (kg/d)
9,800	196	294	9.80×10¹⁰	49	73.5	24.5

Flow (m³/d)	BOD (kg/d)	TSS (kg/d)	E. coil (CFU/d)	NH₃-N (kg/d)	TKN (kg/d)	Org-N (kg/d)
9,800	1881.6	1636.6	5.68×10¹⁵	186.2	323.4	137.2

5.1.1 This section presents the results of water quality impact assessment associated with the construction and operation phases of both the Cheung Chau STW and Pak She SPS upgrading works and the sewers works of the Project.

5.2.1 The Environmental Impact Assessment Ordinance (EIAO) provides a legislative framework to safeguard the environment by reducing and minimizing adverse environmental impacts from designated projects.

5.2.2 Annexes 6 and 14 of the EIAO – TM specify the general and project-specific criteria, and guidelines for water quality impact assessment.

5.2.4 The assessment will follow this practical note to recommend mitigation measures to minimize the potential water quality impact arising from construction activities.

5.2.6 The effluent discharge from the Cheung Chau STW has the potential to impact the marine water quality conditions in the Southern WCZ and adjacent areas. The WQOs for the Southern WCZ are listed in Table 5.1.

5.2.8 The effluent standards are intended to control the physical, chemical and microbial quality of effluent, and vary with the effluent flow rate. The TM effluent standards relevant to this study are summarized in Table 5.2.

5.2.9 Water Supplies Department (WSD) has specified a set of water quality criteria for flushing seawater intakes. The WSD parameters relevant to this EIA are shown in Table 5.3 along with their target limits.

5.2.10 The assessment criterion to address the impact on cooling water intakes is a limit of 40 mg/L for suspended solids (SS).

5.2.11 The assessment criteria for corals are based on both the sedimentation rate and WQO for SS. A 3-dimensional hydrodynamic and water quality mathematical model was used in the EIA to predict the SS concentrations in receiving waters.

5.2.12 With reference to studies by (Pastorok & Bilyard, 1985) and (Hawker & Connel, 1992) on coral reef communities, the recommended sedimentation rate for providing sufficient protection and avoiding unacceptable impacts to corals is less than 0.1 kg/m2/day.

5.3.1 Main water quality issues related to the construction and operational phases of the Project include:

5.3.2 During the construction phase, runoff and potential wastewater releases from construction sites are the major issues related to water quality impact.

5.3.3 During the operational phase, water quality impact is mainly related to the effluent discharge from the upgraded Cheung Chau STW. A key concern is that the effluent discharge may affect the quality of waterbodies at the nearby water quality sensitive receivers.

5.3.6 The study area for water quality impact assessment mainly includes the Southern WCZ as required in the EIA Study Brief. Adjacent areas in other WCZs were evaluated where appropriate.

5.3.7 Potential sources of water quality impact under the construction phase include:

5.3.11 The potential impact of emergency discharge was assessed using the 3-dimensional water quality model, Delft3D. The existing submarine outfall and emergency discharge of Cheung Chau STW is shown in Figure 5.1a.

5.3.15 Appropriate mitigation measures were identified where necessary to mitigate the potential water quality impact to an acceptable level for both the construction and operational phases of the Project.

5.3.16 With the implementation of recommended mitigation measures, the residual water quality impact was assessed to examine the potential impact to water sensitive receivers in the study area.

5.3.21 Table 5.4 shows the pollutant loads into the marine water under the baseline condition without upgrading of the Cheung Chau STW. Details on the pollutant load calculation are illustrated in Appendix 5B.

5.3.23 The pollutant loads from the upgraded Cheung Chau STW effluent into the marine water under the normal operational condition are summarized in Table 5.5.

5.3.24 In case of operational failure of the Cheung Chau STW, raw sewage would bypass the wastewater treatment unit and be directly discharged into the marine water as an emergency discharge. The STW emergency discharge loads are shown in Table 5.6.

5.3.25 To assess the existing water quality conditions in the study area, the most recently published monitoring data collected at the EPD marine water monitoring stations near the proposed Project site were compiled and reviewed.

5.4 Study Area and Sensitive Receivers

5.4.2 Potential adverse impacts on the above WSRs due to the proposed Cheung Chau Upgrading Project were evaluated in this EIA study.

5.5.1 To assess the existing water quality conditions in the study area, the most recently published monitoring data collected at the EPD marine water monitoring stations near Cheung Chau were compiled and reviewed.

5.5.9 For the Tung Wan and Kwun Yam Wan beaches, E.coli levels monitored in 2006 to 2010 were fairly low as shown in Table 5.9 and Table 5.10, indicating a low risk to the public health.

5.6.7 With effective controls through good operation and management practices, no adverse impact is anticipated to occur to water quality due to the accidental spillage of chemicals from construction activities.

5.6.9 The simulated baseline water quality results for 2019 at locations where the WSRs were identified are summarized in Table 5.11 and Table 5.12 for dry and wet seasons, respectively.

5.6.10 The simulated water quality concentrations at the identified WSRs under Scenario 2 of normal operation of the upgraded STW are summarized in Table 5.13 and Table 5.14 for dry and wet seasons, respectively.

5.6.12 Incremental changes in concentration of the water quality parameters between Scenario 2 and Scenario 1 are presented in Table 5.15 and Table 5.16 for the identified WSRs.

5.6.23 To cover different conditions of tidal movement, the potential impact of emergency discharge was assessed for the following six scenarios:

5.6.25 The purpose of Scenarios 3c, 3d, 3e and 3f is to evaluate the maximum aerial impact to some sensitive receivers at South Lantau, e.g. Chi Ma Wan, Cheung Sha Fish Cultural Zone under both flood and ebb tide conditions.

5.6.31 Four typical scenarios 3c, 3d, 3e, 3f were simulated to assess the maximum extent of impact during the spring tide cycle for both the flood and ebb tide conditions.

5.6.36 As indicated in Table 5.15 and Table 5.16, changes in sediment concentration at the proximity of the identified coral sites are negligible near Stations of C1 to C15. No adverse impact on corals is predicted to result from the upgrading of the Cheung Chau STW.

5.6.37 Concurrent projects that are expected to be constructed or operated in the same period of time include:

5.7.1 The practices outlined in ProPECC PN 1/94 Construction Site Drainage should be adopted to minimize the potential water quality impacts from site runoff and other construction activities. The mitigation measures should cover, but not be limited to, the following practices:

5.7.2 With implementation of the above measures, construction site runoff is not anticipated.

5.7.3 Good site practices should be adopted to regularly clean the construction sites to avoid rubbish, debris and litter from entering to nearby water bodies.

5.7.4 Wastewaters generated from construction activities may contain high SS concentrations and possibly a certain amount of grease and oil. Good construction and site management practices should be implemented to ensure that litter, fuels, and solvents would not enter the public drainage systems.

5.7.7 Oils and fuels should only be used and stored in designated areas that have pollution prevention facilities.

5.7.10 A contingency plan should be developed to deal with the occurrence of an emergency discharge during the operation of the STW.

5.7.12 A contingency plan each for the Cheung Chau STW and the Pak She SPS respectively, should be developed to deal with emergency discharge. The contingency plan should include the following:

5.8.1 With effective controls through good operation and management practices, no adverse water quality impact is anticipated to occur during the construction phase of both the upgrading works for the Cheung Chau STW and Pak She SPS and the sewers works under this Project.

5.8.5 Upgrading of the Pak She SPS will reduce the chance of sewage overflow from the SPS and eliminate any direct emergency discharge to marine waters. Adverse water quality impacts due to this upgrading works is not anticipated.

5.9 Environmental Monitoring and Audit Requirements

5.9.1 The need for EM&A activities during the construction and operation phases of the Project was identified in this EIA study. Details of the EM&A requirements are provided in a separate EM&A manual.

5.10 Conclusion

5.2.12 With reference to studies by (Pastorok & Bilyard, 1985) and (Hawker & Connel, 1992) on coral reef communities, the recommended sedimentation rate for providing sufficient protection and avoiding unacceptable impacts to corals is less than 0.1 kg/m²/day.