6          WATER QUALITY IMPACT ASSESSMENT

 

6.1       Introduction

 

This Section describes the impacts on water quality associated with the construction and the operation of the Proposed Beach Development.  The sediment transport and sedimentation resulting from dredging and sandfilling for the beach have been simulated through computer modelling.  The purpose of the assessment is to evaluate the acceptability of predicted impacts to water quality from the construction and operation of the Proposed Beach Development.  Impacts have been assessed with reference to the relevant environmental legislation and standards.

 

6.2       Relevant Legislation and Guidelines

The following relevant pieces of legislation and associated guidance are applicable to the evaluation of water quality impacts associated with the Proposed Beach Development.

 

·                 Water Pollution Control Ordinance (WPCO);

 

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

 

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

 

6.2.1    Water Pollution Control Ordinance (WPCO)

 

Under the WPCO, Hong Kong waters are divided into 10 Water Control Zones (WCZs) each of which has a designated set of statutory Water Quality Objectives (WQOs) designed to protect the marine environment and its users.  The Proposed Beach Development is located within the Buffer Subzone of the Tolo Harbour and Channel WCZ (which is divided into three sub-zones, Harbour, Buffer and Channel).  The applicable WQOs associated with the Proposed Beach Development are presented in Table 6.1.   

Apart from the parameters shown in Table 6.1, WQOs are also provided for temperature, pH and salinity.  However, construction of the beach area, ie dredging and backfilling, will only result in elevation of suspended solids (SS) concentration but will not change temperature, pH and salinity.  In addition, there will not be any sewage discharges from the Proposed Beach Development to the marine water in the WCZ during its construction and operation, the criteria for these parameters are therefore not considered applicable and are not discussed further.

 

Table 6.1: Relevant Water Quality Objectives for Tolo Harbour and Channel WCZ (Buffer Subzone)

Parameter		Tolo Harbour and Channel WCZ
		Buffer Subzone
Suspended Solids (SS)		No criteria established
Dissolved Oxygen (DO)	2 m above the seabed	Not less than 4 mg L-1
	within 2 m above the seabed	Not less than 3 mg L-1
	Fish Culture Zone (a)	At Fish Culture Zones the DO levels should not be less than 5 mg L-1
Nutrients (measured as inorganic nitrogen)		No criteria established
Unionised Ammonia (UIA)		No criteria established
Chlorophyll-a		Not exceed 10 mg L-1 (mg m-3) (calculated as a running arithmetic mean of 5 daily measurements for any location and depth) (b)
Toxicants		Toxicants are not to be present at levels producing significant toxic effect
E. coli		Not exceed 610 cfu per 100 mL, calculated as annual geometric mean

Notes:

(a)     DO level of 5 mg L^-1 is WQO parameter generally applied for FCZ at other WCZs but there is no specific DO level established for FCZ for Tolo Harbour and Channel WCZ.

(b)     Chlorophyll-a WQOs at Tolo Habour and Tolo Channel Subzones are 20 µg L^-1 and 6 µg L^-1 respectively. 

 

Besides the WQOs stipulated for the WCZ, a WQO for bathing beaches has been set under the WPCO.  The WQO states that the level of E. coli should not exceed 180 cfu per 100 mL, calculated as the geometric mean for all samples collected from March to October inclusive.  Samples have to be taken at least 3 times a month at intervals of between 3 and 14 days.  This WQO applies to all bathing beaches in Hong Kong waters.

 

6.2.2        Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM)

 

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

 

The EIAO-TM recognises that, in the application of the above water quality criteria, it may not be possible to achieve the WQOs at the source as there are areas which are subjected to greater impacts (which are termed by EPD as the mixing zones) where the initial dilution of a pollution input takes place.  The definition of this area is determined on a case-by-case basis.  In general, the criteria for acceptance of the initial dilution area is that it must not impair the integrity of the water body as a whole and must not damage the ecosystem or impact marine sensitive receivers (including migratory pathways of important species, beaches, breeding grounds or other beneficial uses).

 

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

 

Apart from the above statutory requirements, ProPECC PN 1/94 was issued by ProPECC in 1994 and it provides useful guidance on the management of construction site drainage and the prevention of water pollution associated with construction activities.

 

6.3              Assessment Methodology

 

The construction method and sequence as described in Section 3 were reviewed to assess the remoteness of the construction works of the Proposed Beach Development to existing and committed Water Sensitive Receivers (WSRs).  The WSRs were identified according to guidance provided in the EIAO-TM.

 

The design of the beach, construction sequence, duration and activities, and the operation activities were reviewed to identify activities likely to impact upon identified WSRs and other water courses during construction and operation phases. Following the identification of WSRs and potential water quality impacts, the scale, extent and severity of potential net (ie unmitigated) construction, operation impacts were evaluated against the assessment criteria defined for this Proposed Beach Development, as discussed in the following section.  The evaluation was also taken into account all potential cumulative effects including those of adjacent projects, with reference to the WPCO criteria.

 

Where net water quality impacts exceed the appropriate WPCO criteria, practical water pollution control measures/mitigation proposals were identified to ensure compliance with reference to the WPCO criteria.  Water quality monitoring and audit requirements were developed, if necessary, to ensure the effectiveness of the water pollution control and mitigation measures.

 

6.3.1        Defined Assessment Criteria

 

The WQO for the Tolo Harbour and Channel WCZ has been discussed in Section 6.2.1.  The WQO has stipulated criteria for pH, temperature, salinity, DO, chlorophyll-a and toxicants.  As discussed before, no sewage will be discharged into the marine water within the WCZ and hence it is not necessary to define assessment criteria for pH, temperature and salinity.  There is, however, no WQO for SS, Total Inorganic Nitrogen (TIN) and unionised ammonia (UIA) in the WCZ.

 

Assessment Criteria for Suspended Solids and Dissolved Oxygen

 

In view of the low background concentrations in the north eastern waters of Hong Kong (see discussion in Section 6.4), an alternate assessment criterion of allowable increases in SS concentrations of 10 mg L^-1 has been adopted in previous projects.  These projects included the EIAs for sand dredging at the proposed Eastern Waters Marine Borrow Area ^([1]), for uncontaminated mud disposal at the exhausted East Tung Lung Chau MBA ^([2]), for jetting and dredging carried out in Tolo Harbour and Channel and Mirs Bay ^([3]).  In addition, a 10 mg L^-1 criterion was used as the limit level in the EM&A programme for SS impacts to coral areas during sand dredging at the West Po Toi MBA ^([4])  and the dredging for a gas pipeline in Tolo Harbour and Channel ^([5]).

 

There are several fish culture zones (FCZ) located within the Study area and they are considered to be sensitive receivers.  The FCZs within the area of expected influence of the Proposed Beach Development include Yim Tin Tsai West and Yim Tin Tsai East.  The only WQO that is specific to FCZs is for dissolved oxygen.  However, there is no specific DO level established for FCZs for Tolo Harbour and Channel WCZ.  In addition to dissolved oxygen there is a general water quality protection guideline for SS, which has been proposed by AFCD.  The guideline requires that SS levels remain below 50 mg L^-1.  With regard to the water quality modelling, the FCZs were included as discrete points for evaluation in the assessment against the above criteria and guideline.  This criterion has been adopted in the EIA for dredging for Towngas gas pipeline ^([6]).

 

It is particularly relevant to make use of the abovementioned assessment criteria for this Study because these previous projects were primarily concerned with the potential for impacts to ecologically sensitive areas (ie areas of high coral coverage, fish spawning areas, fish culture zones), which are the main concern with regard to SS impacts during the construction phase.

 

Assessment Criteria for Sediment Deposition

 

A coral site has been identified at Pak Sha Tau (Figure 6.1) and it was evaluated as of medium ecological value in the approved EIA for Towngas gas pipeline ^([7]).  Coral communities have been identified in the vicinity of the Project Site during the dive surveys conducted for this Proposed Beach Development.  Sediment deposition is the key parameter to evaluate the impacts to these corals.

 

Hard or hermatypic corals are susceptible to increased rates of deposition, with the species sensitivities to sedimentation being determined largely by the particle-trapping properties of the colony and ability of individual polyps to reject settled materials.  Horizontal platelike colonies and massive growth forms present large stable surfaces for the interception and retention of settling solids while vertical plates and upright branching forms are less likely to retain sediments.  Tall polyps and convex colonies are also less susceptible to sediment accumulation than other growth forms.  It is also acknowledged that sensitivities to sediment loads can also vary markedly between species within the same genus ^([8]). 

 

Information presented by Pastorok and Bilyard (1985) ^([9]) has been regarded as the aforementioned when discussing the effects of sedimentation on corals.  Pastorok and Bilyard have suggested the following criteria:

 

* 10 - 100 g m^-2 day^-1              slight to moderate impacts

* 100 - 500 g m^-2 day^-1            moderate to severe impacts

* > 500 g m^-2 day^-1                  severe to catastrophic impacts

 

Fringing and inshore reefal environments, however, are known to experience sedimentation events in exceedance of the 500 g m^-2 day^-1 criterion and support flourishing coral communities ^([10]).  It is clear from the above that the adoption of strict criteria for impact assessment based on Pastorok & Bilyard's system of assessment for open water communities may well be overly protective in an environment such as Hong Kong.  However, using a precautionary approach, it is proposed to adopt a value of 100 g m^-2 day^-1 as the assessment criterion for deposition, which is at the lower end of the range for moderate to severe impacts specified above, for the purposes of this Study.   This criterion has been utilised in Hong Kong (Eastern Waters, West Po Toi) before and deemed to be sufficiently protective during EM&A ^{([11]) ([12]) ([13])}.  It should be noted that exceedance of this value should trigger further assessment and should not be deemed to imply that damage would necessarily occur.  The results from EM&A programmes in Hong Kong that have adopted 100 g m^-2 day^-1 have indicated that no adverse impacts to corals have occurred.

 

Assessment Criteria for E. coli

 

In accordance with Study Brief Conditions 3.4.3.5 (xv), it is necessary to assess the suitability of Lung Mei Beach to be operated as a gazetted bathing beach.  In this regard, the WQO for bathing beach, ie E. coli levels less than 180 cfu per 100 mL (calculated as the geometric mean for all samples collected from March to October inclusive), is used for this purpose.   

 

Assessment Criteria for Seawater Intakes

 

Seawater intakes at WSD Tai Po Industrial Estate and at the Marine Science Laboratory (MSL) of Chinese University have their specific standards and these are presented in Table 6.2.  With respect to SS, 5-day Biological Oxygen Demand (BOD₅) and E. coli, target quality standards for MSL seawater intake are more stringent than those of WSD due to the reported purpose of the MSL intake which supplies the MSL fish and invertebrate stock holding, culture and experimental facilities.  The SS concentration for WSD and MSL intakes should not be more than 10 mg L^-1 (with an upper tolerance level of 20 mg L^-1) and 5 mg L^-1 respectively.

 

Table 6.2:    Water Quality Criteria for Seawater to be used by MSL and WSD (for Flushing Water) Intakes

Parameter	MSL Target Limit	WSD Flushing Target Limit
Colour (HU)	-	<20
Secchi Disc Depth (m)	>2	-
Salinity (ppt)	>25	-
pH	>7.5	-
Turbidity (NTU)	-	<10
Threshold Odour Number	-	<100
Ammoniacal Nitrogen (mg L-1)	-	<1
Total Nitrogen (mg L-1)	<1	-
Total Phosphate (mg L-1)	<0.1	-
Suspended Solids (mg L-1)	<5	<10 (20 – upper threshold)
Dissolved Oxygen (mg L-1)	-	>2
Dissolved Oxygen (% saturation)	>10-30	-
Biochemical Oxygen Demand (mg L-1)	<5	<10
Synthetic Detergents (mg L-1)	-	<5
Chlorophyll-a (mg L-1)	<10-50	-
E. coli (cfu 100 mL-1)	<100-1,000	<20,000

 

Assessment Criteria for Dissolved Metals and Organic Compounds

 

There are no existing legislative standards or guidelines for dissolved metals and organic compounds in the marine waters of Hong Kong.  It is thus proposed to make reference to the relevant water quality standards in the UK, Australia and USEPA.  The proposed assessment criteria are summarised in Table 6.3 and these values are total concentrations of the pollutants which takes the ambient concentrations into account. 

 

In general, the proposed criterion is taken as the most stringent value among the nation’s standards.  When compared with the Australian Assessment Criterion, it is noted that such criteria are broken down into four categories, according to the level of protection desired (% of species).  It is unclear, however, which of the above Australian criterion would be appropriate for application to Hong Kong.  With exception of Polychlorinated Aromatic Hydrocarbons (PAHs), for which only Australian criterion is available, the UK and US standards are more appropriate to be adopted as the assessment criterion. 

 

Criterion for Tributyltin (TBT) which was suggested in an international literature (see Table 6.3) is proposed to be used in this Study.  This value has been adopted in the previous approved EIAs such as EIA for Decommissioning of Cheoy Lee Shipyard at Penny’s Bay^([14]), EIA for Disposal of Contaminated Mud in the East Sha Chau Marine Borrow Pit ^([15]), EIA for Wanchai Development Phase II ^([16]) and EIA for Emissions Control Project at Castle Peak Power Station "B" Units ^([17]).

 

It is also considered important to note that the assessment criteria presented in Table 6.3 are based on long term exposure.  Works associated with the construction of the Proposed Beach Development are, however, relatively short term and in localised areas.  Hence USEPA’s Criteria Maximum Concentration (CMC) criteria are considered to be more suitable than Criteria Continuous Concentration (CCC) since CCC is designed for the long term exposure.

 

    Table 6.3:    Proposed Assessment Criteria for Dissolved Metals and Micro-Pollutants with Reference to Standards Adopted by Other Countries

 

6.4              Baseline Conditions and Water Sensitive Receivers

 

6.4.1        Water Sensitive Receivers

 

The Project Site is located in the Tolo Harbour, near Ting Kok Sites of Special Scientific Interest (SSSI) & Coastal Protection Areas, Yim Tin Tsai East & West Fish Culture Zones, Sha Lan Non-gazetted Beach, Lung Mei & Yim Tin Tsai Mangroves, Pak Sha Tau coral, WSD Seawater Intakes for Tai Po Industrial Estate and Chinese University.  Table 6.4 shows the identified water sensitive receivers (WSRs) (including ecological sensitive receivers) in the vicinity of the Project Site and gives the WSRs’ shortest distance to the Project Site.  The water sensitive receivers have been identified in reference with the Tai Po Outline Zoning Plan (No. S/TP/19 – Tai Po), gazetted on 18^th November 2005, Outline Development and Layout Plan and relevant published landuse plans.  Figure 6.1 illustrates the surrounding environment for the WSRs.  These WSRs are regarded as the output points in the water quality modelling in order to quantify the impacts to these WSRs.

 

As aforementioned, Pak Sha Tau coral was classified as medium ecological values in the approved EIA ^([18]).  Marine ecological dive surveys for this Proposed Beach Development were undertaken in October 2006 and the results are presented in Section 8 – Ecological Impact Assessment.  Very low number of coral colonies was found during the marine ecological study area and all of the identified coral species (totally 3 species) are either common or abundant in Hong Kong waters and they are considered to be low ecological value.   Hence no specific output points are designated for these coral communities in the water quality model.  However these coral communities will also be assessed by examining the contour plots of the water quality modelling results.

 

The WSRs, SR 8 to SR12, are marked in Figure 6.1.  They represent the corners of the Proposed Land Requirement Boundary (SR8, SR9, SR11 and SR12) and the middle of the Project Site (SR10).  The Project Site itself is not a sensitive area and hence is not defined as the WSR for construction phase impact assessment.  However, the WSRs are used to predict the water quality during the operation phase of the Proposed Beach Development and to determine the suitability of being a gazetted bathing beach.

 

Table 6.4: Water Sensitive Receivers (WSRs) in the Vicinity of the Beach

Sensitive Receiver	Name	ID	Shortest Distance to the Project Site (km)
Fisheries Resources
Fish Culture Zone	Yim Tin Tsai West Fish Culture Zone	SR1	2.8
	Yim Tin Tsai East Fish Culture Zone	SR2	1.5
Marine Ecological Resources
SSSI/Coastal Protection Area	Ting Kok SSSI, near Ting Kok	SR3	0.5
	Ting Kok SSSI, near Shuen Wan	SR4	1.6
Mangrove	Ting Kok	SR5 (a)	0.5
	Yim Tin Tsai, next to Yim Tin Tsai West Fish Culture Zone	SR6	1.9
Coral	Pak Sha Tau	SR7	3
Water Resources
Non-gazetted Beaches	Sha Lan	SR13	2.1
Seawater Intakes	MSL of Chinese University	SR14	4.4
	WSD at Tai Po Industrial Estate	SR15	4.3
Other Recreational Areas	Tai Mei Tuk Water Sports Centre	SR16	immediate vicinity
EPD Monitoring Stations	Tolo Harbour & Channel WCZ	TM3, TM5, TM6	3.5, 1.4, 3.1
Notes: (a) The modelling station shown in Figure 6.1 for SR5 is taken the same as SR3 which is approximately 100m offshore.

 

6.4.2        Hydrodynamics

 

Tolo Harbour is a long narrow tidal inlet, which expands to form a large bay at the landward end, into which a number of rivers and streams discharge.  Shan Liu River and Lo Tsz River are adjacent to the Project Site.  Tidal currents are generally low, 0.1 to 0.2 ms^-1, within Tolo Harbour, particularly towards the landward end.  Along the outer, channel section, of the harbour the tidal currents are generally aligned parallel with the channel, while in the bay at the landward end large circulation patterns form.  As a result of the low current speeds the flushing rate of the Tolo Harbour is low and residence times for the waters are long.  Wave activity within Tolo Harbour is not significant as the waters are sheltered by the surrounding hills and because waves from the open sea are not able to propagate up the Tolo Harbour, due to the narrow seaward entrance.

 

6.4.3        Water Quality

 

EPD monitoring results (1998-2005) indicate that there is a gradient of improving water quality from the inner Tolo Harbour Subzone through to the outer Tolo Channel Subzone.  Three monitoring stations are located in the vicinity of the Project Site, namely TM3, TM5 and TM6 (Figure 6.1).  Among these three stations, TM5 is the closest to the Project Site and hence has similar characteristics to the baseline conditions in the Study Area.  The water body in the vicinity of TM5 is reported as having water quality in-between TM3 and TM6 (Table 6.5).

 

At TM5, DO level at the bottom layer (2m above the seabed) has exceeded the WQO once in 1999 whereas the annual mean value was above the WQO.   EPD data (1998-2005) also indicates that chlorophyll-a levels occasionally exceeded the WQO but no non-compliances were recorded during 2005.  As mentioned above, there is no WQO for SS for the Tolo Harbour and Channel WCZ.  The EPD data indicate that mean SS concentrations ranged between ~1 and 8 mg L^-1.  Full compliance with the WQO was achieved for E. coli and relatively low E. coli levels (maximum 56 cfu 100mL^-1) were recorded.

 

Table 6.5: EPD Routine Water Quality Monitoring Data (1998-2005) in the Vicinity of the Project Area

 

WQ Parameter	Harbour Subzone	Buffer Subzone	Buffer Subzone
	TM3	TM5	TM6
Temperature (oC)	24.1	24.4	23.5
	(14.2-32.0)	(14.2-31.7)	(14.0-30.9)
Salinity (ppt)	31.2	31.1	31.8
	(25.7-34.6)	(25.2-34.3)	(25.0-35.6)
Dissolved Oxygen (Surface to 2m above Bottom) (mg L-1)	7.3	6.5	6.7
	(3.7-12.3)	(4.0-9.9)	(2.3-11.4)
Dissolved Oxygen (Bottom) (mg L-1)	6.3	6.7	5.3
	(0.7-11.7)	(2.9-13.3)	(0.6-11.9)
5-Day Biochemical Oxygen Demand (mg L-1)	2.1	1.9	1.5
	(0.2-5.6)	(0.6-5.6)	(0.1-5.5)
Suspended Solids (mg L-1)	2.5	2.8	2.1
	(0.5-8.0)	(0.6-8.0)	(0.5-8.2)
Total Inorganic Nitrogen (mg L-1)	0.10	0.07	0.08
	(0.01-0.34)	(0.01-0.37)	(0.01-0.34)
Unionised Ammonia (mg L-1)	0.005	0.004	0.003
	(0.000-0.017)	(0.000-0.017)	(0.000-0.016)
Chlorophyll a (µg L-1)	8.3	6.3	5.5
	(0.2-27.0)	(0.6-25.0)	(0.3-23.0)
E. coli (cfu 100mL-1)	3	3	1
	(1-58)	(1-56)	(1-53)
Notes: 1. Data presented are depth averaged, except as specified. 2. Data presented are annual arithmetic mean except for E. coli, which are geometric means. 3. Data enclosed in brackets indicate the ranges. 4. Bolded numbers indicate non-compliance with the WQOs. 5. Outliers have been removed.

6.4.4        Beach Water Quality

 

Although Lung Mei Beach is a non-gazetted beach, EPD routinely monitors the beach water quality at least twice per month during bathing seasons (March to October) and once per month during non-bathing season.  During each beach monitoring visit, beach water samples are collected at three locations (see Figure 6.2) where the water depth is between thigh to waist depth, ie about 0.6 to 1 metre depth, for analysis of E. coli. 

 

The monitoring data obtained between 2000 and 2006 are summarised in Table 6.6.  The data showed that there is an overall increasing trend in E. coli concentrations throughout 2000-2005.  In 2005, the level reached its highest in record but remained below the beach WQO, ie 180 cfu per 100mL.  A slight decrease in E. coli concentrations has been recorded in 2006.  According to EPD’s annual ranking system, the rank of Lung Mei throughout 2000 – 2006 was classified as ‘Fair’, which refers to the geometric mean of E. coli between 25 and 180 cfu per 100mL.  

 

Table 6.6: EPD Routine Beach Quality Monitoring Data (2000-2006) at Lung Mei Beach

Year	Geometric Mean of E. coli (counts/100mL) during March to October
2000	26.5
2001	64.1
2002	47.4
2003	91.4
2004	80.1
2005	164.6
2006	147.9

Further routine monitoring data for 2007 (up to September) was obtained from EPD.  Since it does not cover the whole bathing season at the time of completion of this EIA Report, the annual geometric mean is not able to be derived and hence is not presented in Table 6.6.  The geometric mean during March 2007 to September 2007 was calculated as 345 cfu per 100mL. 

 

6.4.5        Existing Watercourses and Drainage System

 

Within the Study Area, there are several stormwater drainage outfalls along the coast of Lung Mei.  The locations of these drains are shown in Figure 6.3.  

 

There are two box culverts just beside the Project Site.  One of them is a single cell box culvert, which is located at the western side of the Project Site and aligned underneath Ting Kok Road, carries surface runoff from the existing natural stream, Lo Tze River (W1-W3) and eventually discharges into the sea.  Another single cell box culvert (W4), which is situated at the eastern side of the Project Site, collects surface runoff from Ting Kok Road and the natural hillside at Lung Mei. 

 

In order to characterise the water quality of the drains and watercourses, the water from the stormwater drains (W4-W6), Lo Tsz River (W3) and upstream/midstream (W1 and W2) of Lo Tsz River has been sampled and tested.  Water sampling surveys were conducted on 28 December 2006, and 4, 11, 16, 23, 31 January 2007 to measure the flow rate, temperature, pH and E. coli of the effluents.   During these site surveys, the water in W4-W6 were found dirty and having some sewage odour.  The water sampling results are summarised in Table 6.7.  In the case that the flow could not be measured (it was too little and slow), the flow rate was recorded as less than 0.01 m s^-1.

 

Table 6.7: Water Sampling Results for the Existing Drains and Watercourses measured between December 2006 and January 2007

ID	Location	Purity	Odour	Averaged Flow Velocity (a) (m s-1)	Temperature (°C)	pH	Geometric Mean of E. coli (counts per 100mL)
W1	Upstream of Lo Tsz River	Clear	No	<0.01	22.0	6.8	6.9E+02
W2	Midstream of Lo Tsz River	Mostly clear but found dirty during one survey	Mostly without odour but with sewage smell during one survey	0.49	22.1	7.3	7.2E+03
W3	Downstream of Lo Tsz River	Clear	No	0.03	21.9	6.9	9.1E+02
W4	Box culvert at the immediate east of the Project Site	Dirty	With sewage smell	0.03	21.5	7.1	2.3E+04
W5	Box culvert from Lung Mei village	Dirty	With sewage smell	0.01	22.0	8.1	5.9E+03
W6	Box culvert from Tai Mei Tuk	Dirty	With sewage smell	<0.01	22.1	7.7	6.0E+01
Notes: (a) The averaged flow velocity is taken as the mean of measured data during all surveys.

 

Based on the water quality sampling results, there is a potential that the identified watercourses may have contributed to E. coli concentrations in the existing Lung Mei Beach.  As such, the beach water quality monitoring data obtained by EPD during the same period, ie from December 2006 to January 2007, have been reviewed.  The EPD monitoring results are taken as geometric mean of E. coli data at three sampling stations within Lung Mei Beach and the E. coli concentrations are 3.4E+02 and 1.9E+02 counts per 100 mL in December 2006 and January 2007 respectively.  These concentrations are found less than the geometric mean of E. coli at the two nearby drains, ie W3 and W4.  This indicates that the discharges from the drains may be diluted by the seawater before reaching Lung Mei Beach.

 

6.4.6        Sediment Quality                                                                                                                                                

 

EPD Monitoring

 

EPD collects sediment quality data as part of the marine water quality monitoring programme.  There are two monitoring stations in the vicinity of Lung Mei Beach, namely TS3 and TS4 (see Figure 6.1), whereas TS4 is the closest to the Project Site.  Monitoring data obtained during 1998 to 2005 for these stations have been published and are summarised in Table 6.8. 

 

The routine EPD sediment quality monitoring data do not include values for TBT.  The values for metals, PAHs and Total Polychlorinated Biphenyls (PCBs) can be compared to the relevant sediment quality criteria specified in Environment Transport & Works Bureau Technical Circular No 34/2002 Management of Dredged/Excavated Sediment (ETWBTC 34/2002), in order to determine whether the concentrations of these parameters is a cause for concern. 

 

The EPD monitoring results indicate that few parameters have exceeded the Lower Chemical Exceedance Level (LCEL) in the reporting period (see Table 6.8).   They include arsenic at TS3, chromium at TS4, copper at TS3, nickel at TS4 and lead, zinc and total PCBs at both stations.  Among these, the maximum values of lead and zinc at TS3 and nickel at TS4 have exceeded Upper Chemical Exceedance Level (UCEL).

 

Table 6.8: EPD Routine Sediment Quality Monitoring Data (1998-2005) in the Vicinity of the Project Area

Parameters	Lower Chemical Exceedance Level (LCEL)	Upper Chemical Exceedance Level (UCEL)	Harbour Subzone TS3	Buffer Subzone TS4
Chemical Oxygen Demand	-	-	22,118	21,029
(mg/kg)			(15,000-28,000)	(15,000-26,000)
Ammoniacal Nitrogen	-	-	5.8	10.7
(mg kg-1)			(0.0-23.0)	(0.0-24.0)
Total Kjeldahl Nitrogen	-	-	527	643
(mg kg-1)			(330-640)	(370-850)
Total Phosphorus	-	-	162	185
(mg kg-1)			(140-200)	(140-230)
Total Sulphide	-	-	122	130
(mg kg-1)			(1-320)	(4-330)
Arsenic	12	42	10.3	9.2
(mg kg-1)			(5.9-15.0)	(6.6-12.0)
Cadmium	1.5	4	0.5	0.4
(mg kg-1)			(0.2-0.7)	(0.1-0.7)
Chromium	80	160	26	32
(mg kg-1)			(14-36)	(19-81)
Copper	65	110	47	32
(mg kg-1)			(22-97)	(15-42)
Lead	75	110	105	77
(mg kg-1)			(75-130)	(55-93)
Mercury	0.5	1	0.07	0.07
(mg kg-1)			(0.03-0.17)	(0.03-0.15)
Nickel	40	40	16	21
(mg kg-1)			(8-28)	(13-46)
Silver	1	2	0.6	0.3
(mg kg-1)			(0.0-1.0)	(0.0-0.5)
Zinc	200	270	233	168
(mg kg-1)			(170-380)	(75-240)
Total PCBs	23	180	10	8
(µg kg-1) (e)			(3-52)	(3-36)
Low Molecular Wt PAHs	550	3,160	86	85
(µg kg-1) (f)			(60-185)	(60-191)
High Molecular Wt PAHs	1,700	9,600	47	56
(µg kg-1) (g)			(21-100)	(20-137)
Notes: (a)     Data presented are arithmetic mean. (b)     Data enclosed in brackets indicate the ranges. (c)      The shaded cell indicates exceedance of LCEL. (d)     The bolded text indicates exceedance of UCEL (e)     The Total PCBs results only cover 2002-2005 since the ETWB (W) No. 34/2002 was issued in 2002.  If the value is below the reporting limit (RL), it will be taken as 0.5 x RL in the calculation. (f)       Low Molecular Wt PAHs include acenaphthene, acenaphthylene, anthracene, fluorine and phenanthrene.  If the value is below the reporting limit (RL), it will be taken as 0.5 x RL in the calculation. (g)     High Molecular Wt PAHs include benzo[a]anthracene, benzo[a]pyrene, chrysene, dibenzo[a,h]anthracene, fluoranthene, pyrene, benzo[b]fluoranthene, benzo[k]fluoranthene, indeno[1,2,3-c,d]pyrene  and benzo[g,h,I]perylene.  If the value is below the reporting limit (RL), it will be taken as 0.5 x RL in the calculation.

 

Marine Sediment Sampling

 

Sediment sampling has been undertaken to examine the quality of the sediment within the proposed dredging area.  Section 7 details the sampling programme and the sediment testing results. 

 

Vibrocores were taken from nine sampling locations (SS1 – SS9) for chemical analysis.  The locations of the sampling locations are illustrated in Figure 7.2.  Chemical testing results indicate that all the contaminants contained in the sediment were found below the LCEL, with exception of arsenic.  Arsenic concentrations in the sediment samples taken from locations SS1, SS2, SS4, SS7 and SS8 exceeded the LCEL but remained below the UCEL.  These samples were classified as Category M and required biological screening.  The biological tests were performed and all the sediment samples passed the biological test, with exception of SS1.   

 

6.5              Evaluation of Impacts

6.5.1        Construction Phase

 

Construction methods and sequence of the Proposed Beach Development has been reviewed.   During the construction phase, the following water impacts will potentially arise:

 

·       Increase in SS, sediment deposition, and release of heavy metals and toxic chemicals from the sediment may result from dredging at the proposed beach area, two groynes and proposed new eastern box culvert;

 

·       Increase in SS and sediment deposition may be resulted due to sandfilling at the proposed beach area;

 

·       Change in chlorophyll-a levels due to dredging operations;

 

·       Construction site runoff, if uncontrolled, may enter nearby watercourses; and

 

·       Sewage generated from the workforce, if uncontrolled, may affect the quality of the surrounding water.

 

Impacts from the dispersion of fine sediment in suspension from the dredging and sandfilling have been simulated using Delft 3D Model.  The construction sequence and construction methods were reviewed to work out the modelling assumptions. Working Paper 2.0 – Methodology for Water Quality Modelling, as enclosed in Appendix E, details modelling methodology and key model assumptions.  The results are presented and discussed in the following sections.  

 

Note that, “pre-development”, thereinafter, refers to the existing baseline conditions, ie there is no proposed Lung Mei beach development.  “Operation phase”, thereinafter, refers to the situation that the proposed Lung Mei beach development and the bathing beach that will be operated by Leisure and Cultural Services Department (LCSD).

 

SS Elevations due to Dredging and Sandfilling Operations

 

Minimization of dredging quantities has been considered in the preliminary design of the proposed Bathing Beach Development.  The rationale of the dredging requirements is elaborated in Section 2.5.2.  The proposed dredging depth at the groynes is around 0.5 m to 1 m, which is for the levelling of the groyne foundation and ensuring groyne stability.  It is assumed that one dredger will be used at a time during the dredging work.

 

Dredging for the beach, two groynes and the eastern box culvert will be conducted by a closed grab dredger.  Half of the beach will then be filled by marine sand which will be spread out via a conveyor belt of a barge onto the filling area.  The dredging and sandfilling operations were defined as two scenarios and were simulated by Delft 3D.  The model results are presented in Table 6.9.  Note that both scenarios were taken as the worst case, in which it assumed no mitigation measures have been applied.  In reality a standing type silt curtain will enclose the sandfilling area whereas a cage type/ metal frame type silt curtain will enclose the dredging area, in order to reduce the maximum extent of visually intrusive sediment plume.  In addition, the sandfilling works were assumed to be undertaken without any groynes in place.  This is a highly conservative approach since in reality sandfilling will be conducted between the two groynes which will form a physical barrier to prohibit the transport of fine material to the surrounding from area.   It is assumed that one sandfilling barge will be used at a time during the sandfilling work.

 

The scenarios were simulated based on the assumptions as described in Appendix E.  The dredging was assumed to be operated at a maximum rate of 31 m³ hr^-1 for 8 working hours per day and 6 working days per week.  The filling was assumed to be operated at a maximum rate of 1,000 m³ day^-1 with continuous filling operations of 3 hours per day.

 

Modelling results indicate that SS elevations will be compliant with the assessment criterion for all WSRs in the dry and wet seasons under both two scenarios (Table 6.9). Table 6.10 shows the absolute values of the SS concentrations by adding the predicted SS elevations (from the model results) to the ambient level (the latest published EPD routine marine water monitoring data at the closest monitoring stations to the WSRs).

 

As mentioned in Section 6.3.1, AFCD’s guideline requires that the SS level remains below 50 mg L^-1 at the FCZs.  Table 6.10 shows that the SS concentrations at SR1 (Yim Tin Tsai West FCZ) and SR2 (Yim Tin Tsai East FCZ) are predicted to be lower than the assessment criterion of 50 mg L^-1 in both dry and wet seasons.  It is hence anticipated that the dredging and sandfilling operations are unlikely to impact the two FCZs.

 

Table 6.9: Predicted SS Elevations at WSRs due to Dredging or Sandfilling Operations

WSR	Depth (a)	Assessment Criterion (mg L-1)	Elevations in Suspended Solids (mg L-1)
			Dredging				Sandfilling
			Dry season		Wet season		Dry season		Wet season
			Average over spring-neap	Maximum during spring-neap	Average over spring-neap	Maximum during spring-neap	Average over spring-neap	Maximum during spring-neap	Average over spring-neap	Maximum during spring-neap
SR1 (Yim Tin Tsai West Fish Culture Zone)	S	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	M	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	B	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	DA	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
SR2 (Yim Tin Tsai East Fish Culture Zone)	S	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	M	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	B	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	DA	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
SR3 (Ting Kok SSSI, near Ting Kok) /SR5 (Ting Kok)	S	10	0.0	0.1	0.0	0.0	0.0	0.1	0.0	0.0
	M	10	0.0	0.1	0.0	0.0	0.0	0.1	0.0	0.0
	B	10	0.0	0.2	0.0	0.0	0.0	0.1	0.0	0.0
	DA	10	0.0	0.2	0.0	0.0	0.0	0.1	0.0	0.0
SR4 (Ting Kok SSSI, near Shuen Wan)	S	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	M	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	B	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	DA	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
SR6 (Yim Tin Tsai, next to Yim Tin Tsai West Fish Culture Zone)	S	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	M	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	B	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	DA	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
SR7 (Pak Sha Tau)	S	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	M	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	B	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	DA	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
SR13 (Sha Lan)	S	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	M	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	B	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	DA	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
SR14 (MSL of Chinese University)	S	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	M	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	B	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	DA	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
SR15 (WSD at Tai Po Industrial Estate)	S	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	M	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	B	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	DA	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
SR16 (Tai Mei Tuk Water Sports Centre)	S	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	M	10	0.0	0.1	0.0	0.2	0.0	0.1	0.0	0.1
	B	10	0.0	0.3	0.0	0.5	0.1	0.3	0.0	0.3
	DA	10	0.0	0.1	0.0	0.2	0.0	0.1	0.0	0.1
TM3 (Tolo Harbour & Channel WCZ)	S	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	M	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	B	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	DA	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
TM5 (Tolo Harbour & Channel WCZ)	S	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	M	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	B	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	DA	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
TM6 (Tolo Harbour & Channel WCZ)	S	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	M	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	B	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
	DA	10	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
Notes: (a)  For Depth, S = near water surface, M = mid-depth, B = near seabed, DA = depth-averaged

 

Table 6.10: Predicted Absolute SS Concentrations at WSRs due to Dredging or Sandfilling Operations

WSR	Depth (a)	Respect-ive EPD Monito-ring Station	Ambient Level (b) (mg L-1)	Suspended Solids Concentration (mg L-1)
				Dredging				Sandfilling
				Dry season		Wet season		Dry season		Wet season
				Average over spring-neap	Maximum during spring-neap	Average over spring-neap	Maximum during spring-neap	Average over spring-neap	Maximum during spring-neap	Average over spring-neap	Maximum during spring-neap
SR1 (Yim Tin Tsai West Fish Culture Zone)	S	TM5	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7
	M		nr	nd	nd	nd	nd	nd	nd	nd	nd
	B		2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8
	DA		2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8
SR2 (Yim Tin Tsai East Fish Culture Zone)	S	TM5	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7
	M		nr	nd	nd	nd	nd	nd	nd	nd	nd
	B		2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8
	DA		2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8
SR3 (Ting Kok SSSI, near Ting Kok) /SR5 (Ting Kok)	S	TM5	2.7	2.7	2.8	2.7	2.7	2.7	2.8	2.7	2.7
	M		nr	nd	nd	nd	nd	nd	nd	nd	nd
	B		2.8	2.8	3.0	2.8	2.8	2.8	2.9	2.8	2.8
	DA		2.8	2.8	3.0	2.8	2.8	2.8	2.9	2.8	2.8
SR4 (Ting Kok SSSI, near Shuen Wan)	S	TM5	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7
	M		nr	nd	nd	nd	nd	nd	nd	nd	nd
	B		2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8
	DA		2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8
SR6 (Yim Tin Tsai, next to Yim Tin Tsai West Fish Culture Zone)	S	TM5	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7
	M		nr	nd	nd	nd	nd	nd	nd	nd	nd
	B		2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8
	DA		2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8
SR7 (Pak Sha Tau)	S	TM6	2.2	2.2	2.2	2.2	2.2	2.2	2.2	2.2	2.2
	M		1.8	1.8	1.8	1.8	1.8	1.8	1.8	1.8	1.8
	B		2.3	2.3	2.3	2.3	2.3	2.3	2.3	2.3	2.3
	DA		2.1	2.1	2.1	2.1	2.1	2.1	2.1	2.1	2.1
SR13 (Sha Lan)	S	TM5	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7
	M		nr	nd	nd	nd	nd	nd	nd	nd	nd
	B		2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8
	DA		2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8
SR14 (MSL of Chinese University)	S	TM3	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7
	M		2.2	2.2	2.2	2.2	2.2	2.2	2.2	2.2	2.2
	B		2.5	2.5	2.5	2.5	2.5	2.5	2.5	2.5	2.5
	DA		2.5	2.5	2.5	2.5	2.5	2.5	2.5	2.5	2.5
SR15 (WSD at Tai Po Industrial Estate)	S	TM3	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7
	M		2.2	2.2	2.2	2.2	2.2	2.2	2.2	2.2	2.2
	B		2.5	2.5	2.5	2.5	2.5	2.5	2.5	2.5	2.5
	DA		2.5	2.5	2.5	2.5	2.5	2.5	2.5	2.5	2.5
SR16 (Tai Mei Tuk Water Sports Centre)	S	TM5	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7
	M		nr	nd	nd	nd	nd	nd	nd	nd	nd
	B		2.8	2.8	3.1	2.8	3.3	2.9	3.1	2.8	3.1
	DA		2.8	2.8	2.9	2.8	3.0	2.8	2.9	2.8	2.9
TM3 (Tolo Harbour & Channel WCZ)	S	-	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7
	M		2.2	2.2	2.2	2.2	2.2	2.2	2.2	2.2	2.2
	B		2.5	2.5	2.5	2.5	2.5	2.5	2.5	2.5	2.5
	DA		2.5	2.5	2.5	2.5	2.5	2.5	2.5	2.5	2.5
TM5 (Tolo Harbour & Channel WCZ)	S	-	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7	2.7
	M		nr	nd	nd	nd	nd	nd	nd	nd	nd
	B		2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8
	DA		2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8	2.8
TM6 (Tolo Harbour & Channel WCZ)	S	-	2.2	2.2	2.2	2.2	2.2	2.2	2.2	2.2	2.2
	M		1.8	1.8	1.8	1.8	1.8	1.8	1.8	1.8	1.8
	B		2.3	2.3	2.3	2.3	2.3	2.3	2.3	2.3	2.3
	DA		2.1	2.1	2.1	2.1	2.1	2.1	2.1	2.1	2.1
Notes: (a)     For Depth, S = near water surface, M = mid-depth, B = near seabed, DA = depth-averaged (b)     Ambient levels are determined based on the latest published EPD routine marine water monitoring data (1998-2005). (c)     “nr” denotes no record. (d)     “nd” denotes not determinable

The contour plots of the SS elevations during the dry and wet seasons are presented in Figures 6.4 to 6.7 respectively.  The mean contours over time for the surface layer and the bottom layer are shown on the top of the figures.  The maximum contours over time for the surface and the bottom layer are shown in the middle of Figure 6.4 to 6.7.  The surface layer represents the first tenth layer near to the water surface, whereas the bottom layer represents the last tenth layer near to the seabed. 

The maximum SS plots for both seasons suggest that plumes over 10 mg L^-1 (refers to the contour in green of <15 mg L^-1) are likely to be confined to the works area and will not reach the closest eastward WSR, ie SR16 – Tai Mei Tuk Water Sports Centre, and the nearby identified coral colonies.  The estimated maximum extension of the sediment plume (maximum values over 10 mg L^-1) is summarised in Table 6.11.

 

Table 6.11: Predicted Maximum Extension of the Sediment Plume

Season	Depth	Plume Size (m)
		Dredging		Sandfilling
		NW-to-SE Direction	NE-to-SW Direction	NW-to-SE Direction	NE-to-SW Direction
Dry	Surface	154	385	146	354
	Bottom	169	570	162	554
Wet	Surface	146	454	146	423
	Bottom	192	500	177	562

 

Due to the relatively limited spread of SS and no exceedances of the WQOs or tolerance criterion at sensitive receivers, no unacceptable elevations of SS would be expected to occur.

 

Sediment Deposition due to Dredging and Sandfilling Operations

 

Contour plots, which were illustrated in Figures 6.4 to 6.7, of sediment deposition as a result of dredging operations indicate that the majority of sediment settles either within or within relatively close proximity to the Project Site.  This is expected given the very low current velocities present at the site.  Table 6.12 summarises the predicted sediment deposition rate (less than 0.00001 g m^-2 day^-1) at a coral site at Pak Sha Tau (SR7) due to the grab dredging and sandfilling operations.  Based on the results, the predicted sedimentation rate will be compliant with the assessment criterion of 100 g m^-2 day^-1.  This indicates the marine works is unlikely to cause any unacceptable impacts to the WSRs, especially the coral site at Pak Sha Tau.

 

For the coral colonies identified in the dive survey, the sedimentation rate would be much less than the assessment criterion (refer to Figures 6.4 to 6.7).  Therefore it is expected that the dredging and sandfilling works would not adversely affect those corals.

 

Table 6.12: Predicted Sediment Deposition Rate at WSRs due to Dredging or Sandfilling Operations

WSR	Assessment Criterion (g m-2 day-1)	Sedimentation rate (g m-2 day-1)
		Dredging		Sandfilling
		Dry season	Wet season	Dry season	Wet season
		Average over spring-neap	Average over spring-neap	Average over spring-neap	Average over spring-neap
SR7 (Pak Sha Tau)	100	< 0.00001	< 0.00001	< 0.00001	< 0.00001

       

Dissolved Oxygen Depletion due to Dredging and Sandfilling Operations

 

The degree of oxygen 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 (SOD) on dissolved oxygen concentrations has been calculated based on the following equation ^([19]):

DO_Dep = C * SOD * K * 10^-6

where    DO_Dep           = Dissolved oxygen depletion (mg L^-1)

                                           C        = Suspended solids concentration (mg L^-1)

                                           SOD  = Sediment oxygen demand (mg kg^-1)

                                  K     = Daily oxygen uptake factor (set as 1 ⁽[20]⁾)

 

K was set to be 1, which means instantaneous oxidation of the sediment oxygen demand.  This was a more conservative prediction of DO depletion than this study since oxygen depletion is not instantaneous and will depend on tidally averaged SS 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 the DO concentrations in the water column.  The proposed analysis, which is on the conservative side, will not, therefore, underestimate the DO depletion.

SOD values of the sediment within the dredging area were obtained from the sediment testing for this Proposed Beach Development.  These values range between <100 and 600 mg kg^-1.  The maximum value 600 mg kg^-1 is used for the calculation in order to investigate the worst case. 

Based on the above, the calculated DO depletion at the WSRs will be less than 0.01 mg L^-1.  With respect to the ambient DO concentrations of approximately 6.5 mg L^-1 (annual mean recorded at EPD monitoring station TM5), it is expected that the dredging and sandfilling works are unlikely to attribute to the non-compliance of DO at the WSRs.

As discussed in Section 6.3.1, there is no specific DO level established for FCZs for the Tolo Harbour and Channel WCZ.  DO level of 5 mg L^-1 is a WQO parameter generally applied for FCZs at other WCZs.  Since the DO depletion is predicted to be less than 0.01 mg L^-1 with the ambient DO concentrations of approximately 6.5 mg L^-1, the DO concentrations at SR1 (Yim Tin Tsai West FCZ) and SR2 (Yim Tin Tsai East FCZ) are predicted to be in compliance with the assessment criterion.  It is hence no adverse impacts on the DO levels at two FCZs are expected.

 

            Release of Contaminants due to Dredging and Sandfilling Operations

Elutriate tests were undertaken to investigate whether dredging would cause the release of contaminants contained in the sediment.  Samples were taken from the sampling locations as shown in Figure 7.2.  

The elutriate test results indicate that concentrations of PAHs, total PCBs, TBT and all chlorinated pesticides were found below the reporting limits for all sampling locations.   This suggests that the leaching potential of these contaminants would be low.

The elutriate test results of the dissolved metals are presented in Table 6.13.  The results show that the concentrations of dissolved metals in most of the samples are below the reporting limits and all of them are compliant with the assessment criteria.  This indicates that dredging the sediments is unlikely to cause a detectable increase in contaminant levels in the surrounding water.  Note that the values shown in the table include the background concentrations.

Therefore, it is expected that dredging at the proposed Project Site is unlikely to cause unacceptable levels of contaminant release from the dredged sediment to the surrounding water. 

The potential source of sand will be imported from Mainland China (see details in Section 3.8).  Consideration of the source will make reference to the past completed projects and concurrent projects, for example, Peng Chau Sand Replenishment Project which imported the sand from Shajiao, Zhujiang estuary.  The imported sand shall comply with the requirements as promulgated in WBTC No. 10/1995.  In addition, the sand quality will be based on similar requirements in respect to sand for a recreational beach as set out in Section 3.6 of the Port Works Design Manual: Part 5 – Guide to Design of Beaches ^([21]).  It specifies that there should be no organic content and contaminated materials.  It is hence anticipated that the sand used for filling will not impact surrounding water.  

 

Table 6.13: Summary of Elutriate Test Results (Dissolved Metals)

Sample Reference		Heavy Metals
Sampling Locations	Depth	Cadmium (Cd)	Chromium (Cr)	Copper (Cu)	Nickel (Ni)	Lead (Pb)	Zinc (Zn)	Mercury (Hg)	Arsenic (As)	Silver (Ag)
	From    To	ug/L	ug/L	ug/L	ug/L	ug/L	ug/L	ug/L	ug/L	ug/L
Report Limit		0.2	1.0	1.0	1.0	1.0	10	0.1	1.0	1.0
Assessment Criterion		2.5	15	4.8	30	25	40	0.3	25	1.9
SS1	0.2-0.9m	<0.2	<1	<1	1.7	<1	<10	<0.1	<1	<1
SS1	0.9-1.2m	<0.2	<1	2.2	<1	<1	<10	<0.1	<1	<1
SS2	0.5-0.9m	<0.2	<1	<1	2.2	<1	<10	<0.1	<1	<1
SS2	0.9-1.9m	<0.2	<1	<1	2.3	<1	<10	<0.1	<1	<1
SS2	1.9-2.5m	<0.2	<1	<1	1.4	<1	<10	<0.1	<1	<1
SS3	0.0-0.9m	<0.2	<1	1.1	1.6	<1	<10	<0.1	4.5	<1
SS3	0.9-1.9m	<0.2	<1	1.3	1.8	<1	10	<0.1	<1	<1
SS3	1.9-2.8m	<0.2	<1	2.1	1.5	<1	<10	<0.1	<1	<1
SS4	0.0-0.9m	<0.2	<1	<1	1.1	<1	<10	<0.1	<1	<1
SS4	0.9-1.3m	<0.2	<1	<1	<1	<1	<10	<0.1	<1	<1
SS5	0.0-0.9m	<0.2	<1	<1	<1	1.4	<10	<0.1	<1	<1
SS6	0.0-0.9m	<0.2	<1	<1	<1	<1	<10	<0.1	1.9	<1
SS6	0.9-1.6m	0.3	<1	1.2	1.3	1.4	10	<0.1	5.2	<1
SS7	0.0-0.9m	<0.2	<1	<1	1.5	1.6	<10	<0.1	<1	<1
SS7	0.9-1.3m	<0.2	<1	<1	<1	<1	<10	<0.1	<1	<1
SS8	0.0-0.9m	<0.2	<1	<1	1.4	<1	<10	<0.1	<1	<1
SS8	0.9-1.7m	<0.2	<1	<1	<1	<1	<10	<0.1	1.8	<1
SS9	0.0-0.9m	<0.2	<1	<1	<1	<1	<10	<0.1	<1	<1
SS9	0.9-1.9m	0.3	<1	2.2	<1	<1	23	<0.1	<1	<1

 

Release of Nutrients due to Dredging Operations

 

By reviewing the sediment sampling results, the concentrations of nitrite and nitrate in the sampled sediments were below the reporting limits and the ammoniacal nitrogen levels were below 1.8 mg kg^-1.  Total Kjeldahl Nitrogen (TKN) concentrations in the sediment ranged between <50 and 210 mg kg^-1.   The maximum value of TKN in the sample sediment is about one-third of the ambient concentrations (annul mean recorded at EPD monitoring station TS4).  This indicates that the nutrient level in the sediment within the dredging area is relatively low compared to other areas in the WCZ.  

 

As discussed previously, the SS elevations at the WSRs would be less than 0.5 mg L^-1 and the sediment plume is predicted to be confined to the dredging site.   It is hence expected that the nutrient elevation in the water as a result of the sediment plume due to unmitigated dredging works would be minimal.   As a consequence of this prediction it is not expected that algal blooms are likely to occur.

 

Effect on Chlorophyll-a Level due to Dredging Operations

 

During dredging operations, the suspension of sediment and subsequent release of nutrients may affect the chlorophyll-a level in the vicinity of the Project Area.  Chlorophyll-a was hence modelled directly and the model results are presented in Tables 6.14 and 6.15 as well as in Figures 6.8 and 6.9.

 

Model results show that during the dredging operations the relative change in chlorophyll-a levels (pre-development phase minus dredging phase) is very small and ranges between -0.092 µg L^-1 (reduction) and 0.004 µg L^-1 (increment) -2.786% to 0.046% (Table 6.14).  The modeling also indicates that the relative percentage change (increase in chlorophyll-a) is less than 0.07%.  This demonstrates that the dredging works are unlikely to significantly elevate chlorophyll-a levels at WSRs or to cause any adverse impacts.  Chlorophyll-a concentrations may decrease as a result of the dredging activity during which the light availability will be reduced.  In other words, less light will be available for primary production and hence chlorophyll-a concentrations will tend to decrease.  It is anticipated that the very small change in chlorophyll-a concentrations will be transient and the chlorophyll-a level will return to pre-dredging conditions after the completion of dredging works. 

 

The predicted chlorophyll-a levels at the WSRs within or in the vicinity of the Project Site area, are checked for compliance with the assessment criterion (the WQO of chlorophyll-a for the buffer subzone of Tolo Harbour and Channel WCZ) ^([22]). The model results are determined in the same way as the WQO.  Table 6.15 presents the maximum 5-day arithmetic mean of chlorophyll-a concentrations at each water depth of the near-field WSRs. 

 

Based on the model results, it is therefore concluded that the dredging works would not cause any WQO non-compliances for chlorophyll-a.