7.1
With reference to Clause
7.2 The Human Health Risk Assessment (HHRA), which covers the assessment of risk to human health, is presented in this section of the EIA Report. Risk assessment for ecological resources is presented in Section 8 of the EIA Report.
7.3 The objective of the HHRA is to assess the potential human health risks/impacts associated with exposure to toxic substances from effluent discharges of the Project, due to ingestion of and contact with contaminated seawater during swimming or engaging in other water related activities and with the consumption of potentially contaminated seafood.
7.4
The study area of this
assessment is in line with the one for water quality assessment according to
Clause
7.5 The risk assessment will focus on assessing the potential risks/impacts to human health due to chronic exposure to the contaminants present in the HATS effluent discharge including potential contaminants produced in the disinfection process.
7.6 Under Agreement No. CE 7/2005 (EP) “Harbour Area Treatment Scheme Environmental Impact Assessment Study for the Provision of Disinfection Facilities at Stonecutters Island” (ADF EIA), a multi-tiered, multi-criteria evaluation exercise was conducted to select the disinfection option for the Stonecutters Island (SCISTW). Chlorination with dechlorination was selected to be the disinfection method. Therefore, the risk due to potential by-products produced in the chlorination / dechlorination process, together with the contaminants present in CEPT / secondary treated effluent will be assessed.
7.7 The approach and methodology of this HHRA will follow those adopted in the ADF EIA.
7.8 Three project scenarios were considered in the assessments:
·
Late Stage
·
Before commissioning of HATS
Stage 2B with disinfection, HATS discharges
·
HATS Stage 2B with disinfection
(ultimate year), HATS discharges
7.9 It should be noted that the year 2020 completion date for Stage 2B is an assumption made for the purpose of risk assessment in the current EIA Study. Also note that scenarios 2 and 3 would use design flows whereas scenario 1 would use a lower flow rate based on calculated effluent flow generated in 2020.
7.10 The detailed risk assessment methodology for HHRA is presented in Appendix 7.1. The framework of the risk assessment is as follows:
- Contaminant of Potential Concern (COPC) Identification and Contaminant of Concern (COC) Selection
- Potential Human Receptors Identification
· Risk/hazard Characterization
7.11 A brief overview of the risk assessment methodology is presented below:
7.12 This stage of the risk assessment establishes objective, scope and focus of the assessment, constructs the Site Conceptual Model (SCM) and defines assessment endpoint. SCM presents an overview of the chemical sources, exposure pathways and receptors of the risk assessment. SCM adopted in the HHRA is presented graphically in Figure 7.1. More detailed discussion is presented in Appendix 7.1.
7.13 A total number of 35 chemicals are identified as COPCs from the chlorination/dechlorination process. The COPCs included 9 chlorination by-products (CBPs) regulated by USEPA National Primary Drinking Water Standards; 25 priority pollutants[1] (which may contain potential CBPs) regulated by the USA National Pollutant Discharge Elimination System (NPDES)[2]; and total residual chlorine (as disinfectant residue). Chemical analysis is conducted to determine the COPC concentrations in chlorinated/dechlorinated (C/D) CEPT effluent and ambient seawater for the subsequent tasks of the risk assessment.
7.14 Unlike other conventional human health risk assessments for air pollution source (e.g. incinerator) and contaminated land/groundwater, a look-up table of contaminants/list of possible COPC for CBPs risk assessment in effluent is not identified from local and overseas authorities. Moreover, according to the review of local and overseas practice, list of “regulated CBPs in sewage effluent” is not identified.
7.15 Hence, a conservative approach is adopted in this Study to include all the regulated CBPs in drinking water plus the 25 priority pollutants (may contain potential CBPs) regulated by NPDES as COPCs, although these pollutants are not regulated due to the concern of generation during chlorination process.
7.16
The NPDES practice is adopted
because it contains the most comprehensive list of regulated pollutants for
effluent discharge, based on the review of practice in the
7.17 Therefore, the 35 COPCs identified from the chlorination/dechlorination process include all documented potential CBPs/disinfectant residue which are regulated due to their potential to cause impact to human health and/or ecological resources. The list of identified COPCs (which the COCs for risk calculation are selected from the list) is considered sufficiently comprehensive to assess the potential risk to human health due to chronic exposure to the contaminants produced in the disinfection process in the effluent discharges.
7.18
A comprehensive chemical
analysis was conducted under the Environmental and Engineering Feasibility Assessment
Studies in relation to the Way Forward of the HATS (HATS EEFS) (2004) to
determine the pollutant concentrations in HATS CEPT effluent (Stage 1 and Stage
7.19 The COCs are selected from the identified COPCs based on a number of selection rules and their risks are determined in the risk assessment.
7.20 Based on the SCM for HHRA, the completed and significant COC exposure pathways are incidental ingestion and dermal contact of seawater, and ingestion of contaminated seafood. Therefore, the potential human receptors (children and adult) are:
· People who swim or engage in other water related activities in the sea area which is contaminated by the selected COCs discharged from the outfall of SCISTW
· People who consume seafood which is contaminated by the selected COCs discharged from the outfall of SCISTW
7.22 This stage of HHRA involved determination of the relationship between the COC doses from exposure and corresponding response in humans (risk of cancer development, in terms of cancer slope factor and/or non-cancer health impact, in terms of reference dose).
7.23 This stage of the assessment characterizes the cancer risk (due to carcinogenic COCs) and health hazard (due to COCs inducing non-carcinogenic health impact) to the receptors associated with exposure of COCs.
7.24 The assessment results are needed to compare against the established assessment criteria to evaluate the environmental acceptability of the chlorination disinfection technology option, which are presented below.
7.25
At present USEPA has taken
cancer risk in the range of
7.26 Hazard Quotient (HQ) and Hazard Index (HI)[3] are used as the measure for the non-carcinogenic health hazards for both children and adult human receptor. At present USEPA has taken 1.0 as the screening value for HQ and HI. A HQ and/or HI below the screening value (i.e. 1) would indicate that the risk of the proposed action does not present an unacceptable risk and no further investigation would be required.
7.27 When the calculated HQ and HI are above the screening value, it does not immediately indicate that the proposed action would present an unacceptable risk. Rather, it triggers further investigation to examine whether the assumptions for the concerned chemicals are too conservative and whether the severities of the effect of the chemicals are of great concern.
7.28 The adoption of 1 as the screening value is consistent with the interpretation of HQ and HI in the approved “EIA for New Contaminated Mud Marine Disposal Facility at Airport East / East Sha Chau Area”.
7.29 In the ADF EIA, chemical analysis and whole effluent toxicity tests (WETT) were conducted for the C/D CEPT effluent from SCISTW and secondary treated effluent from Tai Po/Shatin Sewage Treatment Works to obtain the data for the risk assessments. The chemical analysis aimed to determine the concentrations of identified 35 COPCs (from chlorination/dechlorination process) in the C/D effluents and ambient seawater for COC selection and calculation of human health and ecological risk; whereas the WETT aimed to determine the toxicity of C/D effluent in order to assess the compliance of acute and chronic toxicity criteria (which was detailed in Section 8 of the EIA Report).
7.30
A comprehensive chemical
analysis was conducted under the HATS EEFS (2004) to determine the pollutant
concentrations in HATS CEPT effluent (Stage 1 and Stage
7.31
From the identified COPCs, COCs
are selected and their effluent concentrations are determined for calculation
of risks. The COC selection and
effluent concentrations are based on the chemical analysis results and a number
of established rules. The detailed
COC selection and effluent concentration determination process are presented in
Appendix 7.1; the selected COCs in HHRA and their determined effluent and ambient
seawater concentrations are summarized in Table
Table
|
Effluent Conc. (mg/L) |
||
|
From
chlorination/dechlorination process |
||
|
Total residual chlorine |
100 |
0 |
|
Chloroform |
7 |
0 |
|
Bromodichloromethane |
2.5b |
0 |
|
Dibromochloromethane |
2.5b |
0 |
|
Chloroacetic acid |
4 |
0 |
|
Dibromoacetic acid |
4 |
0 |
|
Dichloroacetic acid |
45.9 |
0 |
|
Trichloroacetic acid |
22 |
0 |
|
Tetrachloroethylene |
1.3 |
0 |
|
Trichloroethylene |
2 |
0 |
|
Pentachlorophenol |
1.25b |
0 |
|
2,4,6-trichlorophenol |
2 |
0 |
|
Alpha-benzene hexachloride |
0.25b |
0 |
|
Beta-benzene hexachloride |
0.5b |
0 |
|
Gamma-benzene hexachloride |
0.5b |
0 |
|
From CEPT
effluent |
||
|
Antimonyc |
0.804 |
0.21 |
|
Arsenicc |
1.49 |
1.48 |
|
Bariumc |
25.5 |
7.19 |
|
Chromium IIIc |
18 |
0.43 |
|
Leadc |
1.21 |
0.723 |
|
Mercuryc |
29.4ng/L |
0.06ng/L |
|
Nickelc |
28.5 |
1.02 |
|
Seleniumc |
0.4 |
0.05 |
|
Silverc |
3.83 |
0.058 |
|
Vanadiumc |
29.1 |
2.15 |
|
Zincc |
44.1 |
3.54 |
|
TCDD (I-TEQ) |
0.1pg/L |
0.039pg/L |
|
Toluene |
12 |
0 |
|
Malathion |
0.031 |
0 |
bSelected
COCs with concentration below detection limit in C/D effluent, their effluent
concentrations were assumed to be one-half of the detection limit. This is a standard approach accepted by
USEPA.
c Total
concentration for metals was adopted for human health risk assessment
Table 7.1b Selected COCs and Effluent Concentrations (Project Scenario 3)
|
COC |
Effluent Conc. (mg/L) |
Ambient Seawater Conca. (mg/L) |
|
From
chlorination/dechlorination process |
||
|
Bromoform |
49 |
0 |
|
Chloroform |
2.5b |
0 |
|
Bromodichloromethane |
2.5b |
0 |
|
Dibromochloromethane |
8 |
0 |
|
Dibromoacetic acid |
10 |
0 |
|
Dichloroacetic acid |
3 |
0 |
|
Trichloroacetic acid |
7 |
0 |
|
Pentachlorophenol |
1.25b |
0 |
|
Hexachlorobenzene |
0.25b |
0 |
|
Alpha-benzene hexachloride |
0.25b |
0 |
|
Beta-benzene hexachloride |
0.5b |
0 |
|
Gamma-benzene hexachloride |
0.5b |
0 |
|
From Secondary
Treated Effluent |
||
|
Antimonyc |
0.631 |
0.21 |
|
Bariumc |
24.5 |
7.19 |
|
Chromium IIIc |
8.38 |
0.43 |
|
Copperc |
9.98 |
2.25 |
|
Nickelc |
22.3 |
1.02 |
|
Seleniumc |
0.14 |
0.05 |
|
Silverc |
0.387 |
0.058 |
|
Vanadiumc |
30.5 |
2.15 |
|
Zinc |
11.8 |
3.54 |
|
TCDD (I-TEQ) |
0.062pg/L |
0.039pg/L |
|
Malathion |
0.015 |
0 |
Note: a For COCs that
are not detected in the ambient seawater samples, the ambient seawater
concentration is set as zero.
bSelected COCs with concentration below detection limit
in C/D effluent, their effluent concentrations were assumed to be one-half of
the detection limit. This is a
standard approach accepted by USEPA.
c Total concentration for metals was adopted for human
health risk assessment
7.32 As discussed in Section 6 and shown in Figure 6.43 and Figure 6.44, the effluent plume from SCISTW, Tai Po/Shatin STW and Pillar Point STW would not overlap each other. This means the contaminants discharged from Tai Po/Shatin STW and Pillar Point STW would not significantly contribute to the contaminant concentrations at the edge of Zone of Initial Dilution (ZID), edge of mixing zone (of the effluent plume from SCISTW) and the Tsuen Wan beaches, which would be due to the C/D effluent from SCISTW.
7.33
It is therefore appropriate
to apply the dilution factors calculated by water quality modelling at
different exposure points (i.e. edge of ZID, edge of mixing zone and the
nearest beach from SCISTW outfall) to calculate the contamination
concentration. Table
Table
|
38 |
|||||
|
68 |
|||||
|
60 |
|||||
|
Edge of Mixing Zone* |
Dry and Wet Season Combined |
54 |
|
148 |
172 |
|
Dry Season |
63 |
80 |
125 |
172 |
|
|
Wet Season |
54 |
89 |
172 |
209 |
|
|
Nearest Beach from SCISTW
Outfall |
Dry and Wet Season Combined |
135 |
163d |
Not calculated – not used for risk assessments |
|
|
Dry Season |
135 |
155 |
|||
|
Wet Season |
167 |
224 |
|||
Note: * The edge
of mixing zone of dichloroacetic acid (the COC with the largest mixing zone)
a Dilution factor
exceeded 90% of the time (i.e. 10% of values are below this value)
b Applied to determine COC conc. at edge of ZID
c Applied to COC conc. at edge of mixing zone
d Applied to determine COC conc. at the nearest beach
Table 7.2b Estimated Dilution Factors (Project Scenarios 2 and 3)
|
Exposure Point |
Season |
Min. Dilution Factor |
10 %tile Dilution Factora |
Average Dilution Factor |
Min. 4-day Average Dilution
Factor |
|
Edge of ZID |
Dry and Wet Season Combined |
35 |
49b |
60 |
Cannot be determined by near field model |
|
Dry Season |
Not calculated – not used for risk assessments |
63 |
|||
|
Wet Season |
57 |
||||
|
Edge of Mixing Zone* |
Dry and Wet Season Combined |
Cannot be determined as no mixing zone was determined for dry season |
|||
|
Dry Season |
No mixing zone determined |
||||
|
Wet Season |
43 |
|
113 |
128 |
|
|
Nearest Beach from SCISTW
Outfall |
Dry and Wet Season Combined |
119 |
146d |
Not calculated – not used for risk assessments |
|
|
Dry Season |
119 |
137 |
|||
|
Wet Season |
149 |
199 |
|||
Note: * The edge of
mixing zone of dichloroacetic acid (the COC with the largest mixing zone)
a Dilution factor
exceeded 90% of the time (i.e. 10% of values are below this value)
b Applied to
determine COC conc. at edge of ZID
c Applied to COC
conc. at edge of mixing zone
d Applied to
determine COC conc. at the nearest beach
7.34 In the HHRA, there are two main categories of human receptors, namely general public and fisherman (the more sensitive receptor since they consume more seafood in their diet). The following COC exposure scenarios were considered and evaluated:
· Accidentally drop into the harbour (at edge of ZID) and consumption of contaminated seafood
· Frequent swimming at the edge of mixing zone[4] and consumption of contaminated seafood
· Frequent swimming at Tsuen Wan beaches and consumption of contaminated seafood
7.35
Lifetime incremental cancer
risk and non-cancer health hazard quotient/hazard index are calculated to
determine the health impact due to exposure of carcinogenic COCs and COCs would
pose non-carcinogenic health effect respectively. For non-cancer health hazard, the effect
on adult and child of the 2 categories of human receptors are determined. The detailed assessment results are
presented in Appendix 7.2. Tables
Table
|
|
Incremental Lifetime Cancer Risk |
|||||
|
Project Scenario |
General Public |
Fishermen |
||||
|
Drop
at edge of ZID + seafood consumption |
Freq.
swim at edge of mixing zone + seafood consumption |
Freq.
swim at Tusen Wan beaches + seafood consumption |
Drop
at edge of ZID + seafood consumption |
Freq.
swim at edge of mixing zone + seafood consumption |
Freq.
swim at Tusen Wan beaches + seafood consumption |
|
|
Cancer Risk
due to Potential CBPs |
||||||
|
Scenario 1 |
0.00000006 |
0.00000328 |
0.00000165 |
0.00000006 |
0.00000328 |
0.00000165 |
|
Scenario 2 |
0.00000006 |
0.00000349 |
0.00000184 |
0.00000006 |
0.00000349 |
0.00000184 |
|
Scenario 3 |
0.00000006 |
0.00000349 |
0.00000184 |
0.00000006 |
0.00000349 |
0.00000184 |
|
Cancer Risk
due to Contaminants in CEPT / Secondary Treated Effluent |
||||||
|
Scenario 1 |
0.00000060 |
0.0000254 |
0.0000252 |
0.00000090 |
0.0000257 |
0.0000255 |
|
Scenario 2 |
0.00000062 |
0.0000254 |
0.0000252 |
0.00000094 |
0.0000257 |
0.0000255 |
|
Scenario 3 |
0.00000054 |
0.0000218 |
0.0000217 |
0.00000084 |
0.0000221 |
0.0000220 |
|
Cancer Risk
due to Potential CBPs and Contaminants in CEPT / Secondary Treated Effluent |
||||||
|
Scenario 1 |
0.00000065 |
0.0000287 |
0.0000268 |
0.00000096 |
0.0000290 |
0.0000271 |
|
Scenario 2 |
0.00000068 |
0.0000289 |
0.0000271 |
0.00000101 |
0.0000292 |
0.0000274 |
|
Scenario 3 |
0.00000060 |
0.0000253 |
0.0000236 |
0.00000090 |
0.0000256 |
0.0000236 |
Table 7.3b Estimated Non-cancer Health Hazard Index (Adult Receptor)
|
|
Hazard Index |
||||||||||
|
Project Scenario |
General Public |
Fishermen |
|||||||||
|
Drop at edge of ZID + seafood
consumption |
Freq. swim at edge of mixing zone +
seafood consumption |
Freq. swim at Tusen Wan beaches +
seafood consumption |
Drop at edge of ZID + seafood
consumption |
Freq. swim at edge of mixing zone +
seafood consumption |
Freq. swim at Tusen Wan beaches +
seafood consumption |
||||||
|
Hazard
Index due to Potential CBPs |
|||||||||||
|
Scenario 1 |
0.0000360 |
0.00212 |
0.00107 |
0.0000364 |
0.00212 |
0.00107 |
|||||
|
Scenario 2 |
0.0000389 |
0.00226 |
0.00119 |
0.0000394 |
0.00226 |
0.00119 |
|||||
|
Scenario 3 |
0.0000281 |
0.00154 |
0.000813 |
0.0000286 |
0.00154 |
0.000813 |
|||||
|
Hazard
Index due to Contaminants in CEPT / Secondary Treated Effluent |
|||||||||||
|
Scenario 1 |
0.00500 |
0.186 |
0.184 |
0.00783 |
0.188 |
0.186 |
|||||
|
Scenario 2 |
0.00519 |
0.186 |
0.184 |
0.00820 |
0.189 |
0.187 |
|||||
|
Scenario 3 |
0.00497 |
0.177 |
0.176 |
0.00791 |
0.180 |
0.179 |
|||||
|
Hazard Index
due to Potential CBPs and Contaminants in CEPT / Secondary Treated Effluent |
|||||||||||
|
Scenario 1 |
0.00504 |
0.188 |
0.185 |
0.00786 |
0.190 |
0.188 |
|||||
|
Scenario 2 |
0.00523 |
0.188 |
0.185 |
0.00824 |
0.191 |
0.188 |
|||||
|
Scenario 3 |
0.00500 |
0.179 |
0.177 |
0.00794 |
0.182 |
0.180 |
|||||
Table
|
|
Hazard Index |
|||||
|
Project Scenario |
General Public |
Fishermen |
||||
|
Drop
at edge of ZID + seafood consumption |
Freq.
swim at edge of mixing zone + seafood consumption |
Freq.
swim at Tusen Wan beaches + seafood consumption |
Drop
at edge of ZID + seafood consumption |
Freq.
swim at edge of mixing zone + seafood consumption |
Freq.
swim at Tusen Wan beaches + seafood consumption |
|
|
Hazard
Index due to Potential CBPs |
||||||
|
Scenario 1 |
0.0000422 |
0.00244 |
0.00123 |
0.0000426 |
0.00244 |
0.00123 |
|
Scenario 2 |
0.0000456 |
0.00260 |
0.00132 |
0.0000461 |
0.00260 |
0.00131 |
|
Scenario 3 |
0.0000317 |
0.00172 |
0.000909 |
0.0000322 |
0.00172 |
0.000910 |
|
Hazard
Index due to Contaminants in CEPT / Secondary Treated Effluent |
||||||
|
Scenario 1 |
0.00548 |
0.220 |
0.217 |
0.00831 |
0.222 |
0.220 |
|
Scenario 2 |
0.00567 |
0.220 |
0.218 |
0.00869 |
0.223 |
0.221 |
|
Scenario 3 |
0.00540 |
0.207 |
0.206 |
0.00834 |
0.210 |
0.209 |
|
Hazard
Index due to Potential CBPs and Contaminants in CEPT / Secondary Treated
Effluent |
||||||
|
Scenario 1 |
0.00553 |
0.222 |
0.218 |
0.00836 |
0.225 |
0.221 |
|
Scenario 2 |
0.00572 |
0.223 |
0.219 |
0.00873 |
0.226 |
0.222 |
|
Scenario 3 |
0.00543 |
0.209 |
0.207 |
0.00837 |
0.212 |
0.210 |
7.36
As seen in Tables
7.37 Moreover, it can be observed that in Scenarios 1 to 3, cancer risk and hazard index due to CBPs is at least 6 and 82 times lower than that due to pollutants present in CEPT/secondary treated effluent. The results showed that the chlorination/dechlorination process for CEPT/secondary treated effluent would only induce a very low level of incremental human health risk, which is very small when compared to the health risk due to the pollutants existed in CEPT effluent.
7.38 The health risk due to the extremely conservative (and not realistic) exposure scenario “Accidentally drop into the harbour at edge of ZID + Frequent swimming at the edge of mixing zone + Consumption of contaminated seafood (fishermen diet)” is calculated and the results are presented in Table 7.4.
Table 7.4 Estimated Lifetime Incremental Cancer Risk for Extremely Conservative Exposure Scenario
|
|
Exposure Scenario :Drop at edge of ZID + Freq. swim at edge of mixing zone + seafood consumption |
||
|
Project Scenario |
Incremental Lifetime Cancer Risk –
Fishermen Lifetime |
Hazard Index - Fishermen adult |
Hazard Index – Fishermen child |
|
Cancer Risk /
Hazard Index due to Potential CBPs |
|||
|
Scenario 1 |
0.00000334 |
0.00216 |
0.00248 |
|
Scenario 2 |
0.00000356 |
0.00230 |
0.00264 |
|
Scenario 3 |
0.00000355 |
0.00160 |
0.00176 |
|
Cancer Risk / Hazard Index due to
Contaminants in CEPT / Secondary Treated Effluent |
|||
|
Scenario 1 |
0.0000260 |
0.191 |
0.225 |
|
Scenario 2 |
0.0000260 |
0.191 |
0.226 |
|
Scenario 3 |
0.0000223 |
0.182 |
0.213 |
|
Cancer Risk
/ Hazard Index due to Potential CBPs and Contaminants in CEPT / Secondary
Treated Effluent |
|||
|
Scenario 1 |
0.0000293 |
0.193 |
0.228 |
|
Scenario 2 |
0.0000296 |
0.194 |
0.228 |
|
Scenario 3 |
0.0000259 |
0.184 |
0.215 |
7.39 As seen in Table 7.4, the health risk due to the extremely conservative exposure scenario is also found to be acceptable under the established assessment criteria.
7.40 While the assessment focused on assessing the potential risks/impacts to human health due to chronic exposure to the contaminants in the HATS effluent discharge, cumulative impact of the effluent discharge from Tai Po/Shatin Sewage Treatment Works (TP/ST STW) and Pillar Point Sewage Treatment Works (PPSTW) are considered and evaluated.
7.41 Results of water quality modelling showed that the effluent plume from SCISTW, TP/ST STW and PPSTW would not overlap each other. The results indicated that contaminants discharged from TP/ST STW and PPSTW would not significantly contribute to the concentration of pollutants at the edge of ZID, edge of mixing zone (of the effluent plume from SCISTW) and the Tsuen Wan beaches. Therefore, effluent discharged from the TP/ST STW and PPSTW would not induce cumulative impact with the C/D effluent from SCISTW.
7.42 The evaluation above is further supported by the findings of approved EIA Study for Tai Po Sewage Treatment Works - Stage V. The EIA Study for TPSTW Stage V indicated that the impact from the TPSTW and STSTW effluent would be very localized and confined within the Kai Tak Approach Channel and the existing Kwun Tong Typhoon Shelter. The effluent plume from SCISTW and TPSTW/STSTW would not overlap with each other.
7.43 A description of the assumptions associated with the HHRA is presented below.
· For each COC, the maximum effluent concentration and the maximum background concentration are used to calculate the COC concentration at exposure point. This conservative approach yields higher estimated risk than using mean concentrations.
· It is assumed that after the effluent is discharged, COCs in the effluent would only have their concentrations decrease as a result of dilution and dispersion. It is a conservative approach because COCs concentrations in the water column would also decrease because of different mechanisms such as degradation and / or volatilization.
·
The assumption made for the
incidental water ingestion rate – 50 ml/hour with an exposure duration 2.6 hours/day
appeared to be fairly conservative, especially for seawater which is less
palatable to ingest than fresh water.
While the seafood consumption rate of
· The inclusion of exposure scenarios involving dropping into the harbour and frequent swimming at the edge of mixing zone is to provide a worst case scenario in the risk assessment. The unrealistic assumption of dropping into the harbour once every year is made for the purpose of assessment. While the edge of mixing zone would be located well offshore and would be very difficult for swimmers to reach there frequently.
· Since a list of “regulated CBPs in sewage effluent” is not available locally or overseas, the COPC identification is based on literature search of documented potential CBPs[5] and regulatory practice of chlorinated organic substances in drinking water/sewage effluent[6]. This approach is conservative, as it may include chemicals that actually are not produced as COPC by chlorination of HATS effluent, as reflected by the fact that most of the identified COPC are not detected in C/D effluent. Nevertheless, for the purposes of the risk assessment, a concentration equivalent to half of the analytical detection limit of each undetected COC is adopted in the risk calculation. This conservative approach serves to counter the possibility that some chemicals from chlorination of HATS effluent may be present but are not included as a COPC in the risk calculation. Overall, in line with common practice, this approach to COPC identification is considered sufficiently comprehensive to assess the potential risk to human health.
· The COC concentrations of chlorinated/dechlorinated effluent are obtained from a number of bench scale process simulating the C/D treatment using a higher hypochlorite dosage (higher than the dosage in actual full-scale process implementation) to provide conservatism.
· Dilution factor estimated by water quality modelling is used to predict the COC dispersion in water and the COC concentrations at exposure points. Computer models are sophisticated tools used to simulate mother-nature, and uncertainties inherent in these models have been minimized by vigorous model calibration and verification. The conservative 10% dilution factor is used to estimate COC concentrations exposure points, in turn provided a more conservative risk estimate.
· The characteristic parameter values for human receptors are point estimates adopted from literatures, which may not precisely reflect the conditions of a range of potential human receptors. However, inclusion of fishermen receptors in the HHRA could provide risk estimate for more sensitive population.
· The health benchmarks[7] adopted from agencies would introduce uncertainty to the HHRA. These health benchmarks are used as single-point estimates throughout the analysis with uncertainty and variability associated with them. However, it should be noted that much of the uncertainty and variability associated with the health benchmarks shall be accounted for in the process that the agencies setting verified benchmarks, especially the more stringent values from agencies were adopted.
· Health hazard index is calculated by summation of all hazard quotients due to various COCs. This approach assumes that the health effects of the various COCs are additive, which does not consider the possible synergistic or antagonistic interaction of various COCs. However, several studies have demonstrated that the additive approach often predicts reasonably well the toxicities of mixtures composed of a substantial variety of both similar and dissimilar compounds (USEPA 1986).
7.44 In summary, the health risk assessment by design is very protective of human health by overstating potential exposures and risks. Conservative assumptions made in the risk assessment include (i) adopting maximum effluent concentration and background seawater concentration for risk calculation; (ii) assuming COCs in effluent would only have their concentrations decrease as a result of dilution and dispersion; (iii) applying conservative exposure parameters; (iv) inclusion of exposure scenarios involving dropping into the harbour and frequent swimming at the edge of mixing zone to provide a worst case scenario in the risk assessment. Despite uncertainties involved in some aspects of the risk assessment, conservative treatments (e.g. adopted the more stringent health benchmark values from agencies) are applied where appropriate. The health risk assessment represents the most useful tool that can be used to determine and protectively manage the risk to human health. It is considered that the human health risk assessment overall provided a conservative estimate of risk level and would not underestimate the risk.
7.45 The above health risk assessment indicates that calculated risks of all scenarios are found to be acceptable under the established assessment criteria. In view that the inherent conservative health risk assessment indicates acceptable health risk levels, no residual impact from the Project on human health is anticipated.
7.46 Dechlorination process is incorporated into the effluent disinfection process to remove TRC and reduce formation of CBPs. As discussed above, there would be no unacceptable human health risk induced by the Project and therefore no mitigation measures would be required.
7.47 It is recommended to establish a monitoring programme to determine whether the Project would induce increase of the concentration of potential CBPs and other contaminants in seawater and to verify the predictions of the risk assessment. Details of the programme are provided in a stand-alone EM&A Manual.
7.48 A detailed Human Health Risk Assessment has been conducted to assess the potential adverse human health effects that may result from exposure of toxic substances due to HATS effluent discharge.
7.49 The results of Human Health Risk Assessment revealed that potential risk/hazard impact due to potential chlorination by-products and other contaminants present in the chlorinated/dechlorinated HATS effluent would be acceptable under established assessment criteria in all the 3 Project Scenarios.
7.50 According to the risk assessment results, the Project would not cause unacceptable risk to human health. Therefore, the Project would be considered to be environmentally acceptable in terms of risks/impacts to human health.
1.
ALS Technichem (HK) Pty Ltd
(2005). Testing of
Chlorinated/Dechlorinated CEPT Effluent from
2. CDM (2004). Environmental and Engineering Feasibility Assessment Studies in Relation to the Way Forward of the Harbour Area Treatment Scheme – Working Paper No. 8 Ecological and Human Health Risk Assessment (Final).
3.
CityU Professional Services
Limited (2005). Testing of
Chlorinated/Dechlorinated Sewage Effluent from Tai
4. ERM (2005). Detailed Site Selection Study for a Proposed Contaminated Mud Disposal Facility within the Airport East/East of Sha Chau Area. Environmental Impact Assessment (EIA) and Final Site Selection Report.
5.
MCAL (2004). Tai
6.
7. USEPA (1986). Guidelines for the Health Risk Assessment of Chemical Mixtures.
[1] The 25 pollutants are
regulated in NPDES due to their presence in industrial effluent but not their
possible generation in chlorination process. However, a conservative approach is
adopted to study all these regulated chlorinated organic substances, which are
documented as potential CBPs, in
[2] The NPDES permit program controls water pollution by
regulating point sources that discharge pollutants into water of the
[3] HQ is the measure of health hazard due to exposure of a COC whereas HI
is the measure of health hazard due to exposure of all identified COCs, which
is calculated by summing the HQs of all identified COCs.
[4] The edge of mixing zone would be located well offshore and would be
difficult by swimmers. This
exposure scenario was included as a worst case scenario.
[5] Some of the documented potential
CBPs were generated by applying very high chlorine dose (in the order of
hundreds or thousands mg/L) to sewage effluent, which would not occur in the
HATS scenario.
[6] Regulation of chlorinated organic
substances was due to their presence in industrial effluent but not their possible
generation in chlorination process.
[7] Cancer slope factor and reference
dose of COCs.