6. SEDIMENT CONTAMINATION

6.1. The Project would involve dredging of sediment for the construction of the proposed submarine outfall. This section evaluated the potential environmental impacts that would likely result from the dredged marine sediment. Since the proposed STW would be located at a disused quarry, there would be no need for dredging for reclamation. An assessment of environmental implications associated with other waste arisings that are likely to be generated during the construction and operation phases of the proposed sewerage scheme is presented in Section 7.

Environmental Legislation, Standards and Guidelines

6.2. Environment, Transport and Works Bureau Technical Circular (Works) (ETWB TCW) No. 34/2002 on the Management of Dredged/Excavated Sediment was applicable. This Technical Circular outlines the new procedures to be followed in assessing and classifying the sediment and explains the marine disposal arrangement for the classified material. A Sediment Quality Report is required to be completed and submitted to EPD at least 3 months prior to tendering of the dredging contract being tendered. Dumping permits from EPD are required for marine disposal of dredged materials.

Description of the Environment

6.3. The baseline sediment quality of the Study Area was assessed based on the best available information obtained from the Initial Environmental Impact Assessment of the Consultancy Study for Outlying Island Refuse Transfer Facilities Studies (Agreement No. CE35/91) which included EPD’s marine sediment monitoring at Sok Kwu Wan in 1991 and the sediment quality data from the Lantau Port & Western Harbour Development Study Environmental Survey (Table 6.1). Reference was made to the sediment quality classification criteria from ETWB TCW No. 34/2002 (Tables 6.2 and 6.3) to assess the baseline sediment quality of the Study Area.

Table 6.1 Baseline Sediment Quality at Sok Kwu Wan

Contaminants (mg kg^-1)	EPD Marine Sediment Monitoring Data in 1991	Sediment Quality Data from the Lantau Port & Western Harbour Development Study^a	Lower Chemical Exceedance Level (LCEL)	Upper Chemical Exceedance Level (UCEL)
Cadmium (Cd)	9.1^b	< 0.5	1.5	4
Chromium (Cr)	42	27	80	160
Copper (Cu)	56	72	65	110
Mercury (Hg)	0.06	0.57^c	0.5	1
Nickel (Ni)	20	24	40	40
Lead (Pb)	73	59	75	110
Zinc (Zn)	120	136	200	270

a The data presented are averages of the marine sediment data taken in the period of November 1991 and December 1992

b> UCEL

c> LCEL & < UCEL

6.1. The monitoring locations are shown in Figure 6.1.

Table 6.2 Sediment Classification Criteria

Category	Description
L	Sediment with all contaminant levels not exceeding the Lower Chemical Exceedance Level (LCEL). The material must be dredged, transported and disposed of in a manner which minimises the loss of contaminants either into solution or by resuspension.
M	Sediment with any one or more contaminant levels exceeding the Lower Chemical Exceedance Level (LCEL) and none exceeding the Upper Chemical Exceedance Level (UCEL). The material must be dredged and transported with care, and must be effectively isolated from the environment upon final disposal unless appropriate biological tests demonstrate that the material will not adversely affect the marine environment.
H	Sediment with any one or more contaminant levels exceeding the Upper Chemical Exceedance Level (UCEL). The material must be dredged and transported with great care, and must be effectively isolated from the environment upon final disposal.

Table 6.3 Sediment Quality Criteria for the Classification of Sediment

Contaminants	Lower Chemical Exceedance Level (LCEL)	Upper Chemical Exceedance Level (UCEL)
Metals (mg kg^-1 dry weight)
Cadmium (Cd)	1.5	4
Chromium (Cr)	80	160
Copper (Cu)	65	110
Mercury (Hg)	0.5	1
Nickel (Ni)	40	40
Lead (Pb)	75	110
Silver (Ag)	1	2
Zinc (Zn)	200	270
Metalloid (mg kg^-1 dry weight)
Arsenic (As)	12	42
Organic – PAHs (µg kg^-1 dry weight)
Lower Molecular Weight PAHs	550	3160
High Molecular Weight PAHs	1700	9600
Organic-non-PAHs (µg kg^-1 dry weight)
Total PCBs	23	180
Organometallics (µg TBT L^-1 in interstitial water)
Tributyltin (TBT)	0.15	0.15

6.4. According to ETWB TCW No. 34/2002, Category L sediment would be disposed at a gazetted open sea disposal site. For Category M sediment, they could be disposed at dedicated marine disposal sites if the biological screening test is passed. If the biological screening test is failed, the dredged Category M sediment would need to be disposed at confined marine disposal facility (e.g. East Sha Chau contaminated mud disposal pits). For Category H sediment, if none of the contaminant levels exceed 10 times of the LCEL, or one or more contaminant levels exceed 10 times of the LCEL but with the biological screening test passed, the sediment would be disposed at confined marine disposal facility. In case the biological screening test is failed, the sediment would need special treatment and disposal. Figure 6.3 shows various disposal routes for the three categories of sediment.

6.5. The monitoring results indicate that the sediment at Sok Kwu Wan was relatively uncontaminated with respect to metals, in which all tested metals fall below the LCEL except cadmium which exceeds the UCEL, and mercury which slightly exceeds the LCEL but fell below the UCEL. According to the WBTC, the dredged sediment from the Study Area would likely be classified as Category H, which requires confined marine disposal (e.g. the contained disposal facility at East Sha Chau). However, in view of the fact that the available sediment data are of limited extent and collected about eight years ago, it was considered that it could not fully represent the existing sediment quality of the proposed dredged areas of the present study. Therefore, a preliminary sediment contamination survey was conducted under the Project to provide more accurate and updated information on the baseline sediment quality of the proposed sites of the dredged areas such that recommendation on the appropriate disposal requirements of the dredged material could be made.

6.6. A sediment contamination survey was carried out in July and August 2001 to provide information on the quantity and extent of contaminated dredged material to be disposed of. The sediment contamination survey involved sediment and marine water sampling and testing for metals and organics. The proposal for the sediment contamination survey has been submitted to and accepted by EPD.

Objectives of the Survey

6.7. The sediment contamination survey was designed to satisfy the requirements for an EIA study under the Environmental Impact Assessment Ordinance (EIAO), and the Study Brief:

Ÿ To predict and quantify the water quality impacts caused by sediment re-suspension and contaminant release (Study Brief Section 6.3.4 (a) (6) (ix)).

Ÿ To recommend suitable handling and disposal measures for sludge, contaminated spoils generated during construction stage (Study Brief Section 6.3.4 (a) (7) (ii)).

Specification of Survey Design

6.8. The sediment contamination survey design was made in accordance with WBTC No. 3/2000 and the Engineer’s Geotechnical Site Investigation Specification.

6.9. Sediment samples were collected at every 200 m along the proposed dredged trench of the submarine outfall, which would have a total length of about 765 m. About 480 m of the outfall pipeline would be constructed by Horizontal Directional Drilling (HDD) technique, which is a no-dig method. Hence, dredging for the pipeline section using the HDD technique would not be required. Figure 6.2 shows the vibrocoring locations. Sampling locations of marine water are also shown in Figure 6.2. Water samples were collected to provide information on the background concentrations of heavy metal and micro-organic pollutants in marine water (within the mariculture zone for W4) for the assessment of contaminant release from sediment during dredging and for the sample preparation of the elutriate tests.

6.10. Each sediment sub-sample recovered from the vibrocoring was tested in the laboratory for the following parameters:

(a) Metals concentrations including chromium (Cr), copper (Cu), mercury (Hg), lead (Pb), silver (Ag) , cadium (Cd), nickel (Ni), zinc (Zn)

(b) Metalloid concentrations including arsenic (As)

(c) Concentrations of organic compounds including total polychlorinated biphenyls (PCBs), total polyaromatic hydrocarbons (PAHs), and tributyltin (TBT) for two composite samples from each vibrocoring location. The composite sample is a mix of the sub-samples at all specified sample depths.

(d) Ammoniacal Nitrogen (NH₄-N)

(e) Total Kjeldhal Nitrogen (TKN)

(f) Elutration tests (including Cd, Cr, Cu, Hg, Ni, Pb, Zn, As, total PAHs, total PCBs, TBT, NH₄-N, and TKN) for two composite samples from each vibrocoring location. The composite sample is a mix of the sub-samples at all specified sample depths.

(g) Concentration of TBT in interstitial water of the sediment for two composite samples from each vibrocoring location. The composite sample is a mix of the sub-samples at all specified sample depths.

6.11. Elutriate tests were also conducted to assess the likelihood of release of contaminants from sediment to the water, when the seabed is disturbed during dredging. If the contaminant levels are higher in the elutriates in comparison with the sample of marine water from the same site, it may be concluded that the contaminants would likely be released into the open waters during dredging activities. The potential water quality impacts associated with the dredging of sediment were discussed in Section 5 of this Report.

Baseline Conditions

6.12. The results of the marine sediment quality analysis as compared with the sediment quality criteria for the classification of sediment are presented in Table 6.4.

Table 6.4 Classification of Contaminated Sediments Analysis Results

Vibrocore	Sampling Depth (m)⁽⁴⁾	Metals and Metalloid Content (mg kg^-1)									(mg kg^-1)					(mg-TBT L^-1)	Overall Classification
		Cu	Cd	Cr	Pb	Ni	Zn	Hg	Ag	As		Total PCBs	PAHs Low⁽¹⁾	PAHs High⁽²⁾	TBT in Sediment	TBT in Interstitial water
VC1	0.0 – 0.9	9	<0.2	34	25	25	83	<0.05	0.07	5	VC1 Comp	<3	<55	<170	<0.5	<0.015	L
	1.0 – 1.9	10	<0.2	35	25	24	83	<0.05	0.08	4	VC1 Comp DUP	<3	<55	<170	<0.5	<0.015	L
	2.0 – 2.9	11	<0.2	38	25	26	86	<0.05	0.07	5							L
VC2	0.0 – 0.9	9	<0.2	36	25	25	82	<0.05	0.07	4	VC2 Comp	<3	<55	<170	<0.5	<0.015	L
	1.0 – 1.9	9	<0.2	35	25	25	83	<0.05	0.05	5	VC2 Comp DUP	<3	<55	<170	<0.5		L
	2.0 – 2.9	10	<0.2	36	25	25	82	<0.05	0.07	5							L
VC3	0.0 – 0.9	8	<0.2	32	23	23	77	<0.05	0.05	5	VC3 Comp	<3	<55	<170	<0.5	<0.015	L
	1.0 – 1.9	9	<0.2	35	26	26	84	<0.05	0.07	5	VC3 Comp DUP	<3	<55	<170	<0.5	<0.015	L
	2.0 – 2.9	10	<0.2	36	25	25	82	<0.05	0.07	6							L

Notes:

1. Low molecular weight PAHs, that is, acenaphthene, acenaphthylene, anthracene, fluorene, naphthalene and phenanthrene.

2. High molecular weight PAHs, that is, 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.

3. There was insufficient interstitial water to carry out duplicate sample for VC2 for testing of TBT in interstitial water.

6.13. The sediment testing results indicate that the sediment at all three vibrocore locations was classified as Category L.

Identification, Prediction and Evaluation of Environmental Impacts

6.14. Dredging works for laying of approximately 240m pipeline and the diffuser zone of the submarine outfall would have some impact on the marine environment. Dredging of seabed can release sediments, hence increasing suspended solids concentrations. Contaminants originally trapped in the sediments, if any, would also be released into the water column during the dredging process.

6.15. Based on the preliminary design, the total volume of dredged material is estimated to be approximately 26,000 m³. A review of the sediment quality data from the sediment contamination survey indicates that the marine sediments to be dredged for the submarine outfall were classified as Category L. The dredged sediment would therefore be suitable for open sea disposal. The potential environmental effects of the removal of these sediments on water quality were assessed and presented in Section 5 of this Report.

6.16. In order to minimise any potential adverse impacts arising from the dredged marine sediment, the sediment should be dredged, transported and disposed of in a manner that minimises the loss of contaminants either into solution or by resuspension. Mitigation measures to minimise potential environmental impacts were recommended below. The proposed dredging work is considered small scale (< 30,000 m³). It was anticipated that, with the implementation of mitigation measures, no unacceptable impacts would result from the transportation and disposal of the dredged sediment.

Mitigation of Adverse Environmental Impacts

6.17. The basic requirements and procedures for dredged mud disposal are specified under the ETWB TCW No. 34/2002. The management of the dredging, use and disposal of marine mud is monitored by the MFC, while the licensing of marine dumping is the responsibility of the Director of Environmental Protection (DEP).

6.18. The uncontaminated dredged sediment would be loaded onto barges and transported to the designated marine disposal site. Appropriate dredging methods have been incorporated into the recommended water quality mitigation measures in Chapter 5 (Water Quality Impact), which would include the use of closed-grab dredgers and silt curtains. Category L sediment would be suitable for disposal at a gazetted open sea disposal ground.

6.19. During transportation and disposal of the dredged sediment, the following measures should be taken to minimize potential impacts on water quality:

· Bottom opening of barges should be fitted with tight fitting seals to prevent leakage of material. Excess material should be cleaned from the decks and exposed fittings of barges and hopper dredgers before the vessel is moved.

· Monitoring of the barge loading should be conducted to ensure that loss of material does not take place during transportation. Transport barges or vessels should be equipped with automatic self-monitoring devices as specified by the DEP.

Definition and Evaluation of Residual Environmental Impacts

6.20. With the implementation of the recommended mitigation measures for the handling, transportation and disposal of the dredged marine sediment, no residual impact would be anticipated during the construction of the Project.