5 Water Quality Impact Assessment
5.2 Environmental Legislation, Standards and Guidelines
5.3 Description of the Environment
5.4 Identification of Water Quality Impact
5.7 Impact Prediction and Assessment
5.8 Water Quality Mitigation Measures
5.9 Evaluation of Residual Impacts
5.10 Environmental Monitoring and Audit Requirements
5
WATER QUALITY IMPACT ASSESSMENT
5.1.1
This
Chapter presents an assessment of the potential water quality impact arising from
the construction and operation of the Project. Mitigation measures have been
identified to alleviate the impact and their effectiveness has been
evaluated.
5.2
Environmental Legislation, Standards
and Guidelines
Water Pollution Control
Ordinance (Cap. 358):
5.2.1
The Water Pollution Control Ordinance (Cap. 358) is the major
legislation relating to the protection and control of water quality in Hong
Kong. According to the Ordinance and its subsidiary legislation, Hong Kong
waters are divided into ten water control zones (WCZ). Corresponding statements of Water Quality
Objectives (WQO) are stipulated for different water regimes (marine waters,
inland waters, bathing beaches subzones, secondary contact recreation subzones
and fish culture subzones) in each of the WCZ based on their beneficial uses.
The assessment area in this Project covers the Port Shelter WCZ. The corresponding WQOs are
listed in Table 5-1.
Table 5-1: Summary
of Water Quality Objectives for Port Shelter WCZ
|
Parameters |
Objectives |
Sub-Zone |
|
Offensive odour, tints |
Not to be present |
Whole zone |
|
Colour |
Not to exceed 50 Hazen units, due to human activity |
Inland waters |
|
Visible foam, oil scum, litter |
Not to be present |
Whole zone |
|
E coli |
Not to exceed
610 per 100mL, calculated as the geometric mean of all samples collected in
one calendar year |
Secondary
Contact Recreation Subzone and Fish Culture Subzones |
|
Not to exceed
180 per 100mL, calculated as the geometric mean of all samples collected from
March to October inclusive in one calendar year. |
Bathing Beach
Subzones |
|
|
Dissolved oxygen (DO) within 2 m of the seabed |
Not less than 2
mg L-1 for 90%
of samples |
Marine waters
including Fish Culture Subzones |
|
Depth-averaged DO |
Not less than 4
mg L-1 for 90%
of samples |
Marine waters
except Fish Culture Subzones |
|
Not less than 5
mg L-1 for 90%
of samples |
Fish Culture
Subzones |
|
|
Dissolved Oxygen (DO) |
Not less than 4.0 mg/L |
Inland waters |
|
pH |
To be in the range of 6.5 - 8.5, change due to human activity not to
exceed 0.2 |
Inland water in
Ho Chung (A) Sub-zone. Marine waters
except Bathing Beach Subzones |
|
Not to exceed the range of 6.0 - 9.0 due to human activity |
Bathing Beach
Subzones |
|
|
Salinity |
Change due to human activity not to exceed 10% of ambient |
Whole zone |
|
Temperature |
Change due to human activity not to exceed 2oC |
Whole zone |
|
Suspended solids (SS) |
Not to raise the ambient level by 30% caused by human activity |
Marine waters |
|
Annual median not to exceed 25 mg/L due to human activity |
Inland waters |
|
|
Ammonia (NH3-N) |
Annual mean not to exceed 0.021 mg(N)/l as unionized form |
Whole zone |
|
Nutrients |
Shall not cause excessive algal growth or other aquatic plants. |
Marine waters |
|
Annual mean depth-averaged inorganic nitrogen not to exceed 0.1 mg/L |
||
|
Total inorganic nitrogen (TIN) |
Annual mean depth-averaged inorganic nitrogen not to exceed 0.1
mg(N)/l |
Marine waters |
|
5-Day biochemical oxygen demand (BOD5) |
Not to exceed 5 mg/L |
Inland waters |
|
Chemical Oxygen Demand (COD) |
Not to exceed 30 mg/L |
Inland waters |
|
Phenol |
Shall not be
present in such quantities as to produce a specific odour, or in
concentrations greater than 0.05 mg per litre as C6H5OH |
Bathing Beach
Subzones |
|
Dangerous
substances |
Should not attain such levels as to produce significant toxic,
carcinogenic, mutagenic or teratogenic effects in humans, fish or any other
aquatic organisms. |
Whole zone |
|
Human activity should not cause a risk to any beneficial use of the
aquatic environment. |
Whole zone |
|
|
Turbidity |
No changes in
turbidity or other factors arising from waste discharges shall reduce light
transmission substantially from the normal level. |
Bathing Beach
Subzones |
Source: Statement of Water Quality Objectives (Port Shelter Water Control
Zone)
Technical Memorandum on Effluent Discharge
Standard
5.2.2 Beside setting the WQOs, the WPCO controls effluent discharging into the WCZs through a licensing system. Guidance on the permissible effluent discharges based on the type of receiving waters (foul sewers, stormwater drains, inland and coastal waters) is provided in the Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-DSS). The limits given in the TM cover the physical, chemical and microbial quality of effluents. Any effluent discharge during the construction and operational stages should comply with the standards for effluents discharged into the inshore waters or marine waters. The standards of effluents for this project is summarized in Table 5-2.
Table 5-2: Standards
for Effluents Discharged into the Coastal Waters of Tolo and Port Shelter Water
Control Zones
|
Flow rate (m3/day) |
≦10 |
>10 and ≦200 |
>200 and ≦400 |
>400 and ≦600 |
>600 and ≦800 |
>800 and ≦1000 |
>1000 and ≦1500 |
>1500 and ≦2000 |
>2000 and ≦3000 |
>3000 and ≦4000 |
>4000 and ≦5000 |
>5000 and ≦6000 |
|
Determined |
||||||||||||
|
pH (pH units) |
6-9 |
6-9 |
6-9 |
6-9 |
6-9 |
6-9 |
6-9 |
6-9 |
6-9 |
6-9 |
6-9 |
6-9 |
|
Temperature (∘C) |
45 |
45 |
45 |
45 |
45 |
45 |
45 |
45 |
45 |
45 |
45 |
45 |
|
Colour (lovibond
units) (25mm cell length) |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
|
Suspended solids |
30 |
30 |
30 |
30 |
30 |
30 |
15 |
15 |
15 |
15 |
15 |
15 |
|
BOD |
20 |
20 |
20 |
20 |
20 |
20 |
10 |
10 |
10 |
10 |
10 |
10 |
|
COD |
80 |
80 |
80 |
80 |
80 |
80 |
50 |
50 |
50 |
50 |
50 |
50 |
|
Oil & Grease |
20 |
20 |
20 |
20 |
20 |
20 |
10 |
10 |
10 |
10 |
10 |
10 |
|
Iron |
10 |
10 |
10 |
7 |
5 |
4 |
2.7 |
2 |
1.3 |
1 |
0.8 |
0.6 |
|
Boron |
5 |
4 |
3 |
2.5 |
2 |
1.6 |
1.1 |
0.8 |
0.5 |
0.4 |
0.3 |
0.2 |
|
Barium |
5 |
4 |
3 |
2.5 |
2 |
1.6 |
1.1 |
0.8 |
0.5 |
0.4 |
0.3 |
0.2 |
|
Mercury |
0.1 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
|
Cadmium |
0.1 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
|
Other toxic metals
individually |
1 |
1 |
0.8 |
0.5 |
0.5 |
0.4 |
0.1 |
0.1 |
0.1 |
0.1 |
0.1 |
0.1 |
|
Total toxic metals |
2 |
2 |
1.6 |
1 |
1 |
0.8 |
0.2 |
0.2 |
0.2 |
0.2 |
0.14 |
0.1 |
|
Cyanide |
0.1 |
0.1 |
0.1 |
0.1 |
0.1 |
0.1 |
0.05 |
0.05 |
0.03 |
0.02 |
0.02 |
0.01 |
|
Phenols |
0.5 |
0.5 |
0.5 |
0.25 |
0.25 |
0.25 |
0.1 |
0.1 |
0.1 |
0.1 |
0.1 |
0.1 |
|
Sulphide |
5 |
5 |
5 |
5 |
5 |
5 |
2.5 |
2.5 |
2.5 |
1 |
1 |
0.5 |
|
Total residual
chlorine |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
|
Total nitrogen |
20 |
20 |
20 |
15 |
15 |
15 |
15 |
15 |
10 |
10 |
10 |
10 |
|
Total phosphorus |
8 |
8 |
5 |
5 |
5 |
5 |
5 |
5 |
5 |
5 |
5 |
5 |
|
Surfactants (total) |
15 |
15 |
15 |
15 |
15 |
15 |
10 |
10 |
10 |
10 |
10 |
10 |
|
E. coli (count/100ml) |
1000 |
1000 |
1000 |
1000 |
1000 |
1000 |
1000 |
1000 |
1000 |
1000 |
1000 |
1000 |
Notes:
1
All units in mg/L unless
otherwise stated;
2
All figures are upper limits unless otherwise indicated.
Practice Notes
5.2.3
A practice note (PN) for
professional persons was issued by the EPD to provide environmental guidelines
for the handling and disposal of construction site discharges. The Practice
Note (PN) for Professional Persons on “Construction Site Drainage” (ProPECC PN
1/94) issued by EPD provides good practice guidelines for dealing with ten
types of discharge from a construction site. These include surface runoff,
groundwater, boring and drilling water, bentonite slurry, water for testing and
sterilisation of water retaining structures and water pipes, wastewater from
building constructions, acid cleaning, etching and picking wastewater, and
waste water from site facilities. Practices outlined in the ProPECC PN 1/94
should be followed as far as possible during construction to minimize the water
quality impact due to construction site drainage.
Recommendations from Literature
5.2.4 Assessment criteria for corals were also reviewed through literature search. According to the studies by Pastororok & Bilyard[1], Hawker & Connel[2], Erftemeijer et. al.[3] and Golbuu et.al.[4] on coral reef, sedimentation rate and suspended solid concentration are important on coral survival and growth. The overall environmental criteria for coral from these papers have been included in the report: the sedimentation rate should be less than 0.1kg/m2/day. With reference to a past AFCD study, elevation in suspended solids concentration should be no more than 10 mg/L or smaller than 30% from baseline for avoiding unacceptable impact on coral[5].
5.3
Description of the Environment
5.3.1. The Study Area covers Po Toi O bay,
Clearwater Bay and waters surrounding Steep Island as shown in Figure 5-3.
5.3.2. The project site
(area covered within the proposed work boundary in Figure 1-1)
is located within Port Shelter WCZ where water quality was rated as “good” in
2011 for its high dissolved oxygen, low nutrients (total inorganic nitrogen)
and E. coli levels[6]. The
compliance rate of water quality objectives was reduced in 2012 due to
exceedance in total inorganic nitrogen[7].
This was probably caused by increased shipping and port activities in the
eastern waters7. Despite the water quality was rated as “good” again
in 2013, an increasing trend for TIN was recorded during the period from 2009
to 2013[8].
5.3.3. The nearest water
quality monitoring station maintained by EPD is MM19, which is located between
Steep Island and Ninepin Group (Kwo Chau Islands), and about 3 km from Po Toi O
mouth. The latest 5-year (2009-2013) water quality at MM19 is summarized in the
Table 5-3.
Table 5-3: Water
Quality Monitoring Results at MM19 from 2009 to 2013
|
Parameters |
2009 |
2010 |
2011 |
2012 |
2013 |
Mean |
|
pH |
8.1 |
8.0 |
8.0 |
7.9 |
8.1 |
8.0 |
|
(8 - 8.3) |
(7.7 - 8.2) |
(7.8 - 8.3) |
(7.5 - 8.1) |
(7.8 - 8.3) |
- |
|
|
Salinity (psu) |
33.3 |
32.9 |
33.1 |
32.5 |
32.4 |
32.9 |
|
(31.9 - 34) |
(31.5 - 33.9) |
(31.8 - 34) |
(31.5 - 33.6) |
(31.1 - 33.5) |
- |
|
|
Turbidity (NTU) |
4.5 |
4.5 |
2.4 |
2.2 |
10.1 |
4.2 |
|
(0.8 - 11.1) |
(1.1 - 20.4) |
(1 - 5.6) |
(1.2 - 3.9) |
(1.2 – 88.7) |
- |
|
|
Temperature (°C) |
23.4 |
22.3 |
22.2 |
22.6 |
22.7 |
22.6 |
|
(18 - 29.1) |
(17.1 - 27.5) |
(15 - 29.1) |
(15.6 - 27.5) |
(17.0 - 27.4) |
- |
|
|
Suspended Solids
(mg/L) |
3.1 |
2.3 |
2.7 |
2.5 |
2.5 |
2.6 |
|
(1.3 - 5.4) |
(0.8 - 5.4) |
(0.9 - 7.5) |
(0.9 - 4.7) |
(0.9 – 5.5) |
- |
|
|
DO (mg/L) Depth Average |
6.1 |
6.4 |
6.4 |
6.8 |
6.6 |
6.5 |
|
(5.0 – 7.0) |
(3.7 - 8.2) |
(3.6 - 7.9) |
(4.8 - 8.3) |
(5.2 - 8.0) |
- |
|
|
DO (mg/L) Bottom |
5.8 |
5.8 |
5.9 |
6.2 |
5.8 |
5.9 |
|
(4 – 7.4) |
(2.5 – 7.9) |
(2.7 - 7.9) |
(2.2 - 8.0) |
(3.2 - 8.0) |
- |
|
|
DO (%saturation)
Depth Average |
87.8 |
88.3 |
89.2 |
94.4 |
91 |
90.1 |
|
(74.3 - 102.3) |
(54 - 103) |
(51.3 - 106.3) |
(71 - 112) |
(79 - 101) |
- |
|
|
DO (%saturation) Bottom |
81 |
78 |
81 |
85 |
80 |
81 |
|
(56 - 100) |
(36 - 99) |
(37 - 96) |
(32 - 108) |
(47 – 101) |
- |
|
|
Total Inorganic
Nitrogen (mg/L) |
0.05 |
0.09 |
0.091 |
0.124 |
0.09 |
0.089 |
|
(0.01 - 0.11) |
(0.027 - 0.187) |
(0.033 - 0.21) |
(0.037 - 0.27) |
(0.01 - 0.15) |
- |
|
|
Ammonia Nitrogen
(mg/L) |
0.017 |
0.021 |
0.028 |
0.035 |
0.021 |
0.024 |
|
(0.005 - 0.034) |
(0.011 - 0.035) |
(0.009 - 0.069) |
(0.018 - 0.065) |
(0.006 - 0.037) |
- |
|
|
Unionised Ammonia
(mg/L) |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
0.001 |
|
(0.001 - 0.003) |
(0.001 - 0.002) |
(0.001 - 0.002) |
(0.001 - 0.002) |
(0.001 - 0.003) |
- |
|
|
Nitrite Nitrogen
(mg/L) |
0.007 |
0.012 |
0.009 |
0.013 |
0.01 |
0.01 |
|
(0.002 - 0.014) |
(0.002 - 0.019) |
(0.002 - 0.027) |
(0.003 - 0.04) |
(0.002 - 0.017) |
- |
|
|
Nitrate Nitrogen
(mg/L) |
0.026 |
0.057 |
0.055 |
0.076 |
0.056 |
0.054 |
|
(0.004 - 0.068) |
(0.007 - 0.15) |
(0.004 - 0.18) |
(0.007 - 0.227) |
(0.003 - 0.117) |
- |
|
|
Total Kjeldahl
Nitrogen (mg/L) |
0.114 |
0.132 |
0.123 |
0.162 |
0.16 |
0.138 |
|
(0.06 - 0.147) |
(0.09 - 0.193) |
(0.053 - 0.22) |
(0.08 - 0.277) |
(0.07 - 0.35) |
- |
|
|
Total Nitrogen
(mg/L) |
0.148 |
0.201 |
0.187 |
0.251 |
0.23 |
0.20 |
|
(0.067 - 0.22) |
(0.14 - 0.337) |
(0.07 - 0.323) |
(0.15 - 0.38) |
(0.15 - 0.41) |
- |
|
|
Orthophosphate
Phosphorus (mg/L) |
0.008 |
0.01 |
0.009 |
0.009 |
0.01 |
0.009 |
|
(0.005 - 0.012) |
(0.005 - 0.026) |
(0.003 - 0.016) |
(0.005 - 0.014) |
(0.006 - 0.017) |
- |
|
|
Total Phosphorus
(mg/L) |
0.02 |
0.024 |
0.023 |
0.025 |
0.03 |
0.024 |
|
(0.02 - 0.023) |
(0.02 - 0.043) |
(0.02 - 0.037) |
(0.02 - 0.03) |
(0.02 - 0.04) |
- |
|
|
Silica (mg/L) |
0.55 |
0.66 |
0.611 |
0.69 |
0.67 |
0.63 |
|
(0.24 - 1.07) |
(0.2 - 1.5) |
(0.16 - 1.3) |
(0.24 - 1.13) |
(0.22 - 1.40) |
- |
|
|
BOD5
(mg/L) |
0.55 |
0.66 |
0.69 |
0.44 |
0.60 |
0.59 |
|
(0.1 - 1.1) |
(0.4 - 1.2) |
(0.1 - 1) |
(0.2 - 0.9) |
(0.1 - 1.5) |
- |
|
|
Chlorophyll-a
(μg/L) |
2.2 |
1.8 |
1.8 |
1.6 |
1.7 |
1.8 |
|
(0.8 - 5) |
(0.8 - 4.3) |
(0.5 - 6) |
(0.9 - 2.6) |
(0.5 - 11.1) |
- |
|
|
E. coli (cfu/100mL) |
1.1 |
1.2 |
1 |
1 |
1 |
1.1 |
|
(1 - 2) |
(1 - 1.9) |
(1 - 1) |
(1 - 1) |
(1 - 2) |
- |
|
|
Faecal Coliforms
(cfu/100mL) |
1.2 |
1.2 |
1.1 |
1.1 |
1.1 |
1.3 |
|
(1 - 2.5) |
(1 - 8.7) |
(1 - 2.3) |
(1 - 1.8) |
(1 - 3.4) |
- |
5.3.4. Clear Water Bay
First Beach and Clear Water Bay Second Beach are located within the Study Area
(see Figure 5-2). The water
quality in these two beaches is regularly monitored by EPD. As the Second Beach
is one of the gazetted beaches that opens all year round, the monitoring
frequency is at least 3 times per month in whole year. The monitoring frequency
in the First Beach is at least 3 times per month for bathing season (March to
October) and at least once per month in non-bathing season. They were rated as
“good” in 2013. The latest 5-year (2009-2013) water quality of these two
beaches is shown in Table 5-4 and Table 5-5.
Table 5-4: Beach
Water Quality Monitoring Results at Clear Water Bay First Beach from 2009 to
2013
|
Parameter |
2009 |
2010 |
2011 |
2012 |
2013 |
|
pH |
8.4 |
8.4 |
8.5 |
8.4 |
8.4 |
|
(8.3 - 8.8) |
(8.3 - 8.6) |
(8.3 - 8.6) |
(8.2 - 8.6) |
(8.0 - 8.7) |
|
|
Salinity (psu) |
30.3 |
29.9 |
31.2 |
30.1 |
29 |
|
(26.2 - 33.4) |
(17.8 - 33.4) |
(26.5 - 32.9) |
(23.6 - 32.7) |
(22.4 - 32.3) |
|
|
Turbidity (NTU) |
7.6 |
4.8 |
4.6 |
4.4 |
4.9 |
|
(2.6 - 24.3) |
(1.5 - 11.3) |
(1.0 - 26.1) |
(1.1 - 11.7) |
(1.4 - 21.7) |
|
|
Temperature (oC) |
26.8 |
26.3 |
26.5 |
26.7 |
26.8 |
|
(18.4 - 31.5) |
(17.6 - 32.0) |
(17.9 - 31.4) |
(15.6 - 31.7) |
(18.7 - 31.5) |
|
|
Dissolved Oxygen |
6.9 |
7 |
6.8 |
6.7 |
6.8 |
|
(5.7 - 8.8) |
(5.8 - 8.6) |
(5.4 -8.4) |
(5.6 - 8.5) |
(5.2 - 8.4) |
|
|
E. coli (no./100 ml) |
15 |
10 |
7 |
8 |
24 |
Table 5-5: Beach
Water Quality Monitoring Results at Clear Water Bay Second Beach from 2009 to
2013
|
Parameter |
2009 |
2010 |
2011 |
2012 |
2013 |
|
pH |
8.5 |
8.4 |
8.4 |
8.4 |
8.4 |
|
(8.3 - 8.8) |
8.2 - 8.6) |
(8.3 - 8.6) |
(8.2 - 8.6) |
(7.9 - 8.7) |
|
|
Salinity (psu) |
30.4 |
30.6 |
31.7 |
30.5 |
29.9 |
|
(24.6 - 34.0) |
(23.2 - 33.5) |
(29.1 - 33.1) |
(20.2 - 32.9) |
(24.2-32.5) |
|
|
Turbidity (NTU) |
8.7 |
5.6 |
5.3 |
7.1 |
5.7 |
|
(2.3 - 26.5) |
(1.2 - 28.1) |
(1.1 - 20.5) |
(1.8 - 48.9) |
(1.9-20.6) |
|
|
Temperature (oC) |
27.2 |
26.3 |
26.4 |
26.9 |
27 |
|
(18.5 - 32.2) |
(16.9 - 32.0) |
(17.7 - 32.1) |
(15.2 -32.2) |
(18.8 - 31.4) |
|
|
Dissolved Oxygen |
6.9 |
6.9 |
6.8 |
6.6 |
6.6 |
|
(5.8 - 8.6) |
(5.9 - 8.6) |
(5.9 - 8.5) |
(4.9 - 8.6) |
(5.1 - 7.6) |
|
|
E. coli (no./100 ml) |
27 |
13 |
5 |
18 |
15 |
5.3.5.
Based on Table
5-3, the geometric mean of E. coli
at MM19 in the latest five years well complied with the WQO (180 cfu/100ml for
bathing beach subzone and 610 cfu/100ml for fish culture zone). Besides, the E. coli levels recorded in the two
beaches were also well below the criteria, indicating the baseline water
quality within the Study Area (Figure 5-3)
is generally good in terms of E. coli level.
5.3.6.
In terms of the DO, the annual averaged levels in
the two bathing beaches in Clear Water Bay were very close to each other. All
depth-average DO ranges (i.e. 100% of the samples) lied above 4mg/L, showing
compliance with the WQO. The monitoring results at the beaches and marine
monitoring stations showed a wide variation in DO for each year.
5.3.7.
For unionized ammonia, which is toxic to fishes,
the monitoring results at MM19 are much lower than the WQO (0.021 mg/L).
5.3.8.
TIN is a nutrient parameter which may potentially
cause algal bloom. Its level at MM19 has shown exceedance of the WQO. The overall 5-year
mean of TIN was within the WQO (0.1 mg/L) but the annual average in 2012
exceeded the WQO criteria. An increasing trend appeared in the past few years
and the maximum levels exceeded the WQO in every year, indicating a risk of
algae bloom in the surrounding waters.
5.3.9.
The pH level was in full compliance with the WQO at
both MM19 station and the two beaches.
On site
Monitoring
5.3.10. Even though MM19
is the nearest monitoring station, it is located in the open sea so that the
monitoring results may not represent the actual water quality in Po Toi O bay.
In order to determine the baseline water quality in the area for subsequent
analysis and water quality modelling, baseline water quality monitoring was
conducted at the locations listed in Table
5-6, and their locations are shown in Figure 5-1.
Table 5-6: Water
Quality Sampling Stations
|
Station |
Location |
|
WM1 |
Inner bay of Po Toi O |
|
WM2 |
Inner bay of Po Toi O |
|
WM3 |
Near Fish Culture Zone |
|
WM4 |
Near Po Toi O residence |
|
WM5 |
Near Tai Wong Kung residence |
|
WM6 |
At the mouth of Po Toi O Bay |
|
WM7 |
Outside of Po Toi O Bay |
Frequency and Duration of Monitoring
5.3.11. The water
qualities at these stations were monitored for 2 weeks at a frequency of 3
times per week at mid-ebb and mid-flood during dry and wet seasons. The dry
season monitoring was conducted from 24 March to 23 April 2014 while that for
wet season was from 26 May to 18 June 2014.
Water Quality Parameters and Laboratory
Testing
5.3.12. During the
monitoring, water samples were extracted at 1m below surface, 1m above seabed
and at the mid-depth level. However, if the water depth was less than 3m, water
samples were only extracted at the mid-depth level. The samples were sent to a
HOKLAS accredited laboratory for laboratory testing for the following
parameters:
·
pH
·
Salinity
·
Turbidity
·
Temperature
·
Suspended
Solids
·
Dissolved
oxygen
·
Total
Inorganic Nitrogen
·
Ammonium-nitrogen
·
Unionized
Ammonia
·
Nitrite-nitrogen
·
Nitrate-nitrogen
·
TKN
·
Total
Nitrogen
·
Orthophosphate
Phosphorous
·
Total
Phosphorus
·
Silica
·
BOD5
·
Chlorophyll-a
·
E-coli
Analysis of Results
5.3.13. The monitoring
results were analyzed for 2-week arithmetic means for different depths, except E. coli which was analyzed for 2-week
geometric means. The key water quality parameters (depth averaged) are
summarized in Table 5-7 and Table 5-8.
Table 5-7: Summary
of the Monitoring Data for Dry Season
|
Dry Season |
WM1 |
WM2 |
WM3 |
WM4 |
WM5 |
WM6 |
WM7 |
|
|
Salinity (ppt) |
mean |
26.1 |
26.2 |
26.3 |
26.3 |
26.3 |
26.3 |
26.3 |
|
range |
25.5-26.8 |
25.4 - 26.8 |
25.5 - 26.8 |
25.3 - 26.8 |
25.3 - 26.9 |
25.5 - 26.8 |
25.5 - 26.9 |
|
|
σ |
0.53 |
0.55 |
0.53 |
0.52 |
0.51 |
0.57 |
0.54 |
|
|
SS (mg/L) |
mean |
2.9 |
3.3 |
3.2 |
3.1 |
3.3 |
3.5 |
3.4 |
|
range |
2.5 - 5.1 |
2.5 - 9.3 |
2.5 - 5.3 |
2.5 - 6.6 |
2.5 - 6.2 |
2.5 - 8.2 |
2.5 - 8.2 |
|
|
σ |
0.78 |
1.91 |
1.03 |
1.16 |
1.13 |
1.56 |
1.94 |
|
|
UIA (mg/L) |
mean |
0.011 |
0.010 |
0.011 |
0.011 |
0.012 |
0.009 |
0.009 |
|
range |
0.001-0.022 |
0.002 - 0.022 |
0.003 - 0.022 |
0.003 - 0.02 |
0.004 - 0.021 |
0.003 - 0.019 |
0.002 - 0.018 |
|
|
σ |
0.006 |
0.006 |
0.005 |
0.01 |
0.005 |
0.005 |
0.005 |
|
|
TIN (mg/L) |
mean |
0.31 |
0.28 |
0.3 |
0.29 |
0.27 |
0.24 |
0.23 |
|
range |
0.15 - 0.51 |
0.12 - 0.48 |
0.19 - 0.52 |
0.14 - 0.50 |
0.14 - 0.48 |
0.11 - 0.43 |
0.13 - 0.49 |
|
|
σ |
0.115 |
0.117 |
0.099 |
0.12 |
0.09 |
0.1 |
0.1 |
|
|
E.
coli (no. / 100ml) |
mean |
3.1 |
3.4 |
3.1 |
2 |
1.4 |
2.9 |
2.4 |
|
range |
1 - 17 |
1 - 39 |
1 - 63 |
1 - 70 |
1 - 6 |
1 - 22.5 |
1 - 31 |
|
|
σ |
3 |
4 |
4 |
4 |
2 |
3 |
3.2 |
|
|
DO (mg/L) |
mean |
6.6 |
6.6 |
6.7 |
6.7 |
6.6 |
6.7 |
6.7 |
|
range |
4.7 - 8.7 |
4.7 - 8.4 |
4.8 - 8.6 |
4.9 - 8.8 |
4.7 - 8.4 |
4.7 - 8.9 |
4.3 - 8.6 |
|
|
σ |
1.2 |
1.2 |
1.3 |
1.3 |
1.2 |
1.4 |
1.4 |
|
|
DO (bottom) (mg/L) |
mean |
6.4 |
6.6 |
6.5 |
6.8 |
6.5 |
6.7 |
6.4 |
|
range |
4.7 - 8.5 |
4.8 - 8.6 |
4.9 - 8.8 |
4.9 - 8.7 |
4.6 - 8.5 |
4.6 - 9 |
4.2 - 8.4 |
|
|
σ |
1.1 |
1.2 |
1.2 |
1.3 |
1.2 |
1.4 |
1.4 |
|
Table 5-8: Summary
of the Monitoring Data for Wet Season
|
Wet Season |
WM1 |
WM2 |
WM3 |
WM4 |
WM5 |
WM6 |
WM7 |
|
|
Salinity (ppt) |
mean |
27.7 |
27.8 |
27.7 |
27.7 |
27.7 |
27.7 |
27.7 |
|
range |
25 - 30.8 |
24.9 - 31 |
24.9 - 30.8 |
24.9 - 30.8 |
25.1 - 30.7 |
24.8 - 30.9 |
24.7 - 30.8 |
|
|
σ |
2.1 |
2.2 |
2.2 |
2.2 |
2.1 |
2.2 |
2.2 |
|
|
SS (mg/L) |
mean |
3.5 |
3.5 |
3.2 |
3.1 |
3.5 |
3.3 |
3.2 |
|
range |
2.5 - 5.5 |
2.5 - 5.7 |
2.5 - 3.8 |
2.5 - 4.3 |
2.5 - 8.4 |
2.5 - 4.1 |
2.5 - 4.2 |
|
|
σ |
1 |
1 |
0.41 |
0.65 |
1.71 |
0.61 |
0.68 |
|
|
UIA (mg/L) |
mean |
0.010 |
0.009 |
0.009 |
0.008 |
0.004 |
0.008 |
0.008 |
|
range |
0.005 - 0.016 |
0.004 - 0.019 |
0.004 - 0.017 |
0.004 - 0.023 |
0.004 - 0.016 |
0.004 - 0.014 |
0.004 - 0.015 |
|
|
σ |
0.004 |
0.005 |
0.005 |
0.006 |
0.005 |
0.003 |
0.003 |
|
|
TIN (mg/L) |
mean |
0.43 |
0.41 |
0.42 |
0.41 |
0.43 |
0.4 |
0.4 |
|
range |
0.3 - 0.63 |
0.29 - 0.61 |
0.21 - 0.62 |
0.24 - 0.63 |
0.29 - 0.62 |
0.29 - 0.65 |
0.28 - 0.63 |
|
|
σ |
0.112 |
0.116 |
0.125 |
0.121 |
0.114 |
0.11 |
0.111 |
|
|
E. coli (no. / 100ml) |
mean |
14.5 |
13.7 |
10.4 |
18.7 |
12.4 |
18.5 |
14.7 |
|
range |
1 - 660 |
1 - 470 |
1 - 225.5 |
1 - 510 |
1 - 180 |
1 - 370 |
1 - 383.3 |
|
|
σ |
7 |
6 |
6 |
10 |
7 |
6 |
8 |
|
|
DO (mg/L) |
mean |
6.4 |
6.3 |
6.4 |
6.4 |
6.5 |
6.3 |
6.4 |
|
range |
5.7 - 7.2 |
5.7 - 7 |
5.8 - 7.2 |
5.7 - 7.2 |
6.1 - 7.1 |
5.8 - 7.1 |
6 - 7.1 |
|
|
σ |
0.5 |
0.5 |
0.4 |
0.5 |
0.4 |
0.4 |
0.5 |
|
|
DO (bottom) (mg/L) |
mean |
6.4 |
6.2 |
6.4 |
6.4 |
6.5 |
6.2 |
6.3 |
|
range |
5.6 - 7.3 |
5.7 - 7 |
5.7 - 7.2 |
5.6 - 7.4 |
6.1 - 7.2 |
5.5 - 6.9 |
5.8 - 7.1 |
|
|
σ |
0.6 |
0.5 |
0.5 |
0.6 |
0.4 |
0.5 |
0.5 |
|
5.3.14. Comparing results in Tables 5-7 and 5-8, the water quality in dry season was generally better than that
in wet season. For both dry and wet seasons, no significant change was found in
the concentrations of all parameters from inner Po Toi O bay to the outer bay.
In both seasons, the mean baseline concentrations in E. coli, UIA
and DO complied with the WQO. However, the TIN concentration exceeded the
criteria (0.1 mg/L) for several times. The exceedance may be caused or
contributed by the nearby fish culture zone.
5.4
Identification of Water Quality Impact
Land-based Construction Activities
5.4.1
During
the construction of the sewerage facilities, various construction activities such
as slope-cutting of the hill, excavation for underground storage tank, breaking
up of the road pavement and footpath for sewers and rising mains installation,
and stockpiling of construction materials would generate solid waste and fine
particles. Leakage of chemicals (e.g. lubricant oil, fuel and solvent, see Chapter 9 – Waste Management Implications
& Land Contamination for details) during the construction of the Po Toi
O Sewage Treatment Works (PTOSTW) is also a concern. These materials can be
carried down to the water receiving body by construction site runoff,
especially during the rainy season. Site runoff potentially deteriorates the
water quality, such as increasing the turbidity, reducing the dissolved oxygen
concentration, discolouring the water and reducing light penetration. Windblown
dust will be generated from exposed soil surface in this coastal site and may
fall onto the nearby water bodies.
5.4.2
Also,
various construction activities may generate wastewater directly. These include
general cleaning and polishing, wheel washing and dust suppression. These
types of wastewater would contain high concentration of suspended
solids. Uncontrolled sewage discharge from workers may also pollute the
water in the bay. Increased nutrient level from contaminated discharges
and sewage from workforce could also lead to a number of secondary water
quality impacts including localized increase in ammonia and nitrogen
concentrations.
5.4.3
Bentonite will be used as drilling liquid during horizontal directional
drilling for
the proposed submarine
outfall.
Reconditioned
bentonite free from cutting will be reused for drilling. Therefore no wastewater will be
discharged.
Marine-based
Construction Activities
5.4.4
Construction
of the submarine outfall will be by means of horizontal directional drilling
from the rising mains at the rocky shore through the seabed to minimize impact
on the rocky shore and the coastal shelf. A diffuser will be installed on top
of a riser shaft extending about 1m above the seabed at the end of the submarine
outfall.
5.4.5
An
area of 10m x 50m will be fully enclosed by sheet pile cofferdam at the
diffuser point. About 500m2 seabed will be dredged to remove the
sediments in the seabed temporarily in order to ensure the stability of the
seabed for the installation of the diffuser. Most of the area will be
backfilled with rockfill and the permanent area lost at the diffuser is about 5
m2. After the backfilling work is completed, the cofferdam will be
removed.
5.4.6
Installation
and extraction of sheet piles will be conducted by vibratory action. This will
cause minor displacement of marine sediment, which will quickly settle without
significant increase in suspended solids. Also, sediment confined within the
cofferdam would be dredged by closed-grab and stored in sealed compartment of
the barge anchored outside the cofferdam. In addition, and backfilling works
will be confined within the cofferdam. No opening of cofferdam is required and
thus there will be no release of sediment into water bodies. Increase in suspended solids is not likely to
happen and no significant water quality impact is expected.
5.4.7
The size
of dredging barge is about 12m x 25m. Given that the water depth is about 10m
near the proposed diffuser location, there is sufficient water depth for
maneuvering the barge.
Effluent Discharge during
PTOSTW Operation
5.4.8
During
operation of PTOSTW, effluent will be discharged via the submarine outfall and
diffuser into the Po Toi O Bay and dispersed by the ambient water current,
potentially causing water quality impacts on the bay, the adjacent water bodies
and sensitive receivers.
Emergency Plant
Breakdown during PTO STP Operation
5.4.9
The
Po Toi O Sewage Treatment Plant (PTO STP) cannot operate in case of power or
equipment failure. The Supervisory control and
data acquisition (SCADA) system in the
PTO STP will signal to the operation and maintenance personnel for emergency
attendance. Standby pump and screen will be provided at the PTO STP. According to the performance pledge of CLP,
electricity provision will be restored within 2 hours after fault outage. Also,
emergency generator will be delivered to PTO STP within 4 hours by future term contractor in case of plant failure where necessary.
5.4.10 As the average dry weather flow
(ADWF) of PTO STP would
be small (about 139m3/day),
it would be possible to deploy tankers to transport away the sewage to Tseung
Kwan O Preliminary Treatment Works (TKO PTW) (or
other nearby STW) in case the PTO STP cannot be recovered in a short period of
time.
5.4.11
Emergency storage of 4-hour ADWF (23.19m3) will be provided
in the PTO STP. In case of plant failure, three 12m3 sewage tankers
will be called in to
transport the sewage from PTO STP to TKO PTW. Each tanker will take different
travel routes to reduce the risk of delay due to traffic jam. The tanker
arrangement is as follows:
Table 5-9: Tanker Away Arrangement
|
Tanker |
Source |
Potential Route |
Arrival Time |
Duty |
|
A |
DSD’s tanker which stationed at Sai Kung Sewage Treatment Works |
Sai Kung STW |
1.5 hours from plant failure |
Transport sewage from PTO STP to TKO PTW |
|
B |
DSD Sewage Treatment Division Term Contractor, required to arrive in 2
hours in contract |
Shatin STW* |
2.5 hours from plant failure |
Transport sewage from PTO STP to TKO PTW |
|
C |
DSD District Term Contractors (any 1 of the 3 districts), required to arrive in 1.5 hours in contract |
Kwun Tong Preliminary
Treatment Works* |
2.5 hours from plant failure |
Standby at PTO STP, to be on duty if Tanker A/B breaks down or encounters
traffic delay |
* Typical location where term contractor’s tankers are stationed
5.4.12
The distance between PTO STP and TKO PTW is about 12.1km or 18-minute
travel distance. Including sewage loading and unloading time, each tanker is
assumed to take 2 hours round trip. Tankers A and B will work in shift to
continuously remove sewage from PTO STP. If one of the tankers fails to arrive
at PTO STP on time, Tanker C will come in to ensure that at least two tankers will be operating. An operation drill
prior to future operation will be conducted to confirm the time estimates
achievable at peak hours.
5.4.13 Appendix 5.5 shows the fluctuation of sewage volume in PTO STP in case plant failure occurs during peak sewage flow (6pm). With continuous removal of sewage by tankers in rotation, the highest quantity stored in the plant will be 17.24m3, which is well below the emergency storage capacity (23.19m3). With about additional 6 m3 storage buffer, the chance of having sewage volume exceeding the storage capacity is very low. No overflow of sewage from the PTO STP is anticipated.
5.4.14 Each tanker will deliver 12m3 sewage from PTO STP to TKO
PTW. Based on DSD’s past
experience, it takes 15 minutes to unload all sewage. The average flow
rate will be 12m3/15 minutes/60 seconds = 0.013m3/s,
which is far below the design capacity of TKO PTW (5.55m3/s [9]). No overloading of TKO PTW is anticipated.
5.4.15 Considering the
project scale, risk of emergency condition, construction difficulties and cost,
the following provisions are the most appropriate and practical mitigation
measures in case of plant/power failure:
·
Delivery
of an emergency generator to PTO STP within 4 hours from plant failure
·
Provision
of dual power by CLP;
·
Provision
of a supervisory control and data acquisition system (SCADA), which signals to
the operation and maintenance personnel for emergency attendance in case of
plant failure;
·
Provision
of a standby pump and screen at the PTO STP;
·
Provision
of emergency storage of 4-hr ADWF sewage retention time;
·
Arrangement
of tankers for continuous removal of incoming sewage to other sewage treatment
plants for treatment to ensure a sufficient buffer for emergency storage.
Sludge & Chemical
Waste Management during PTOSTW Operation
5.4.17
Sludge,
chemicals for cleaning and chemical waste generated will be transported on land
by sealed tanker. Therefore, no leakage and pollution of marine waters is
expected.
5.4.18
Regular chemical cleaning
of the MBR membranes will be required to prevent membrane
fouling and to maintain the membrane
performance. Sodium hypochlorite and oxalic acid will be used for removal of
organic and inorganic deposits respectively. The cleaning process will take
place in-situ on the permeate side without removal of membrane. The deposits
will dissolve in the sewage and be treated by the MBR system. The unused
chemical will be neutralized by hydrogen chloride and follow the sewage flow in
the MBR system and eventually be discharged at the diffuser. The water quality
impact due to discharge of cleaning chemical will not be significant.
5.5.1 Water sensitive receivers (WSR) likely to be affected by the Project were identified from on-site visits and surveys. The locations of the representative WSRs are shown in Figure 5-2.
·
Po
Toi O Fish Culture Zone (F1-F3)
·
Coral
communities at Po Toi O (Cor 1 - Cor7)
·
Major
Amphioxus habitat (Amph)
·
Clear
Water Bay First Beach (B1)
·
Clear
Water Bay Second Beach (B2)
·
Streams
(W1 – W3)
·
Secondary
contact recreation sub-zones
·
Spawning
grounds for commercial fisheries area
·
Shorelines
at the Coastal Protection Area Zone at Po Toi O
5.5.2
Artificial
reefs (ARs) are placed in the outer Port Shelter between Steep Island, Tiu
Chung Chau and Basalt Island. Part of the ARs falls into the Study Area far
away from the diffuser location (see Figure 8-1).
Therefore, the pollutants from construction and operation of this Project are
not expected to reach this area. Also, as the ARs are placed in the open water
with deep water and high current flow, any pollutants reaching this area would
have been well dispersed and diluted. No assessment on this WSR is necessary.
5.5.3
The proposed STP is
sandwiched by W1 and W2, which both appeared dry during surveys in wet and dry
seasons. Part of W3 was natural with stony bottom while the water upstream and
downstream was channelled through pipes. The outfall is located near the rocky
and sandy shores and the water eventually entered the sea. The water
occasionally appeared milky and gave off odour. The bottom was lined with
orange, slimy substance which was possibly produced from iron bacteria. These
observations indicate that the water is iron rich and polluted. The condition
of W3 has been reported to EPD and DSD for investigation.
Construction
Phase Impact Assessment
5.6.1
The water
quality impact due to land-based construction works would
be minimized by following the criteria and guidelines for water pollution evaluation
as stated in Annexes 6 and 14 of the EIAO-TM. Wastewater generated during
construction phase shall be treated up to the effluent standard under the WPCO
before discharge. With implementation of mitigation measures and the proposed
good site practices in Section 5.8,
the land-based water quality impact would be minimized to acceptable level.
5.6.2
A fully enclosed cofferdam will be erected around the proposed diffuser
with the water inside emptied. Silty water removed from
the cofferdam should be de-silted in a sedimentation tank before discharge back
into the sea. The cofferdam will be formed by steel sheetpiles to surround the works
area. Lateral loading from wave and water pressure will be resisted by struts
and walings system. Preloaded struts will be adopted to ensure sufficient
resistant for 10m water depth and wave actions. The dredger barge will be anchored outside
the cofferdam and move the closed grab into the cofferdam for dredging. No
opening of cofferdam would be required and therefore no release of suspended
solids into water column is expected.
5.6.3
As there will be no
significant sediment release to the water column due to sheet piling and
confined dredging, no significant marine-based water quality impact is
expected. No computational modelling of sediment dispersion is required.
Operational Phase
Impact Assessment
Normal Operation
5.6.4 Under normal operation of the sewage treatment plant, the treated effluent from the proposed diffuser should follow the effluent discharge standard so that the potential impact is likely to be minor. Water quality impacts arising from effluent discharge from the diffuser under normal operation on WSRs were assessed by a set of hydrodynamic and water quality models.
Computational Modeling
5.6.5 The Delft3D suite of computer models was adopted for the assessment of the water quality during the operation phase. The Delft3D-FLOW was used to simulate the water currents and water levels in Po Toi O and the adjacent waters while the Delft3D-WAQ was used to simulate the water quality.
5.6.6 A local fine grid model (Po Toi O computer model, hereafter called “PTO Model”) was set up to cover the entire Po Toi O and Clear Water Bay, the eastern part of Tai Hang Tun, eastern part of Shek Mei Tau, southern part of Trio Island and the periphery of Steep Island. The PTO model was nested with the Update Model which has been well calibrated. A description of the hydrodynamic and water quality model and the calibration of this model are given in Appendix 5.1.
Scenario Runs
5.6.7
2 scenarios of Delft3D-WAQ
were run to simulate the water quality in both dry season and wet season,
covering one full spring neap cycle. They are listed below:
·
Scenario
1- Baseline Scenario without
project
·
Scenario
2- Normal Operation of PTOSTW,
24-hour continuous and uniform discharge of treated effluent at the proposed
discharge point
Scenario 1 (Baseline Scenario)
5.6.8
The existing water quality should be mainly affected
by the stormwater and sewage discharge including stormwater
surface runoff, open drainage channel flows, the
existing effluent discharge from the sewage treatment plant at Clear Water Bay
Second Beach, fish culture zone, Yacht Club and
sewage discharge from septic tanks
surrounding the village houses. Estimations of the
pollution inventory are detailed in Appendix 5.2.
Scenario 2 (Normal Discharge in Operational Phase)
5.6.9
During the operation of the
proposed sewage treatment plant, the sewage from Po Toi O Village, Seacrest
Villa and Fairway Vista will be collected and treated at the plant. As a
result, individual discharges from septic tanks and soakaway of these residences
will be replaced by treated effluent
discharge at the diffuser after the completion of the
sewerage works. However, the sewage generated from Tai Wong Kung
which is located opposite of Po Toi O bay will remain to be treated by existing
septic tanks/soakaway system.
5.6.10 Sewage in Po Toi O comes from two sources: domestic activities (residential)
and restaurant activities (commercial). Restaurant activities are the highest
on weekends. Therefore, the average dry weather flow of sewage is calculated by
averaging sewage production on weekdays and weekends. Given that the daily
flow rate of the discharge from the proposed PTOSTW is about 139 m3/day and
assuming a scenario with 24-hour continuous release, the
instantaneous flow rate is approximately 0.00161 m3/s. The
peak flow which is about 0.00644 m3/s would
be considered as the worst case scenario. The detailed calculation can be found
in Appendix 5.3.
5.6.11
In assessing the
potential water quality impact due to the discharge of treated effluent from the submarine outfall (the diffuser), the VISJET model was employed to characterize the initial dilution of the effluent discharge from the diffuser in
the Zone of Initial Dilution (ZID) and the
plume size, e.g. the plume depth and plume thickness in the near field. A detailed description of VISJET model is provided
in Appendix 5.4.
5.6.12
The effluent loadings from the VISJET calculations were fed to the Delft3D –WAQ in order to
compute the water quality in the
far field of Po Toi O and the adjacent
waters.
Sensitivity Test
5.6.13
The design of sewer alignment has taken into account
technical feasibility. However, the actual sewer connection rate may not reach 100%
due to a number of reasons, such as complicated house ownership, unsuccessful
attempt to contact owner, owner with plan to re-build the village house and
thus postpone the sewer connection[10]. Upon
completion of public sewer construction, the sewer connection rate generally
can reach 80% to 90% in long run. As at end 2014, the territorial connection
rate is 85%10.
5.6.14
A sensitivity test was conducted to compare the
difference in water quality at WSRs in wet and dry seasons under two sewer
connection rates: 100% (full connection) and 77% (interim connection, with
reference to North District and Yuen Long 10). The detailed results
were tabulated in Appendix 5.7. The
results show no significant difference in water quality change for interim or
full sewer connection rates. In the following assessment, the water quality
assessment will be based on 100% sewer connection rate.
Other
Operational Impacts
5.6.15
Marine organisms may grow on the diffuser and cause
blockage of the ports (dispersion holes). Self-cleaning flow velocity at the
ports can prevent marine growth inside the ports. In addition, the diffuser
risers and ports will be made up of material (e.g. high-density polyethylene
(HDPE)) that allows cleaning of marine growth off the diffuser surface by
divers easily. No anti-fouling agent will be applied so that there will be no
risk of biocide leakage.
Cumulative Impact
5.6.16
There
may be two potential concurrent projects in the area, i.e.
·
Roundabout
near the minibus stop
·
Fish
culture zone (FCZ) dredging and relocation
5.6.17
CEDD
has confirmed that they have no programme for the fish culture zone dredging
and relocation during the course of the EIA study. There is also no information
on the status and programme of the roundabout project from HyD. Nevertheless,
the land-based construction works are unlikely to have any major water impact
during the construction and operation of the PTOSTW. Therefore, no cumulative water quality
impact is anticipated.
5.7
Impact Prediction and Assessment
5.7.1
The key water quality
parameters studied includes depth-averaged and bottom suspended solids (SS),
depth-averaged unionized ammonia (UIA), depth-averaged total inorganic nitrogen
(TIN), depth-average E. coli (E. coli), depth-averaged dissolved oxygen
(DO), dissolved oxygen in bottom layer (Bottom DO) and sedimentation rate. The
concentration of SS, UIA and TIN are averaged over a spring-neap cycle, E. coli is geometric mean, DO and Bottom
DO are 10th percentile over the period, and sedimentation rate is
the maximum value over the period. Salinity was not assessed as no significant
change due to effluent discharge is expected. The proposed diffuser location
has more than 10 m water depth and is far from the major Amphioxus habitat in
the area and rocky shore where coral grows (>100m). As the effluent
discharge rate during normal operation is only about 7 L/s, the change in
salinity at these WSRs due to the proposed operation of the PTOSTW is expected
to be negligible.
5.7.2
All criteria are based on the WQO, except in assessing impact on corals where specific suspended solids and sedimentation
rate criteria have been
adopted, which represent the limits that coral can withstand based on past studies[11],[12],[13],[14]. For
E. coli, the WQO criteria is 180 no./100ml
at gazetted beaches and 610 no./100ml at FCZ.
5.7.3
Detailed modelled results
and the criteria for each parameter in baseline and normal discharge scenarios
are available in Appendix 5.8.
Scenario 1 (Baseline
Scenario)
5.7.4
The modelled baseline water
quality in the Study Area (as described in
Section 5.3.1) is in line with the descriptions in Section 5.3, which was good and complied with the WQO criteria
except TIN.
Scenario 2 (Normal Discharge in Operational Phase)
5.7.5
The contour plots of the
modelled results during normal operation for dry season are presented in Figure A5.6-1 to Figure A5.6-8 in Appendix 5.6 while the contour plots for wet season are presented in Figure A5.6-9 to Figure A5.6-16 in Appendix 5.6. These plots were generated to show the worst concentration at each
grid over the 15-day spring neap cycle.
5.7.6
For the normal operation of
PTOSTW, the concentration of depth averaged SS are in the range of 2.14 mg/L to
2.46 mg/L in dry season while the concentrations are between 1.63 mg/L and 2.61
mg/L in wet season. The bottom SS levels in dry and wet seasons are in the
range of 2.48 mg/L to 2.65 mg/L and 1.96 mg/L to 2.51 mg/L respectively. No
significant change is found between the baseline and normal operation scenarios
for depth averaged and bottom SS in both dry and wet seasons. All values comply
with the WQO (<30% increment) and criteria for coral.
5.7.7
The sedimentation rates
under normal operation in dry and wet seasons have no significant change from
those in the baseline. The rates are predicted to be lower than 1.8 g/m2/day
which are far below the unacceptable value of sedimentation rate (100 g/m2/day)
for normal coral growth.
5.7.8
No significant change in DO
and Bottom DO concentrations is expected under normal operation when compared
with the baseline scenario. Both of these scenarios are in range from 7.0 to
7.2 mg/L for dry season and 5.0 to 5.6 mg/L for wet season and they all comply
with the WQO criteria.
5.7.9
The depth-averaged
concentrations of UIA are in the range of 0.0025 mg/L to 0.0043 mg/L in dry season, while the concentrations are between
0.0027 mg/L to 0.0063 mg/L in wet season. No significant change is found
between the baseline and normal operation scenarios in both dry and wet
seasons. All values comply with the WQO (<0.021 mg/L).
5.7.10
The baseline TIN
concentrations range from 0.16 to 0.21 mg/L for dry season and 0.09 to 0.17
mg/L for wet season. Both are already above the WQO criteria. Under normal
operation of PTOSTW, the TIN concentrations are predicted to stay in the same
range as in baseline case. As the major source of TIN (fish rafts) remains as before,
the TIN concentrations would still exceed the WQO criteria under normal
operation.
5.7.11
The geometric mean of
depth-averaged E. coli in baseline
scenario are much lower than the WQO criteria (180 no./100ml for bathing beach
subzone and 610 no./100ml for fish culture zone or secondary contact recreation
subzone). With the normal operation of PTOSTW, the concentration of E. coli in wet and dry seasons is
predicted to be similar to the baseline scenario.
5.7.12
Overall, it is predicted
that following the normal operation of the PTOSTW, no substantial change in the
water quality in Po Toi O is expected. Therefore, no significant water quality
impact on the surrounding water is anticipated.
5.8
Water Quality Mitigation Measures
Construction Phase
Works near Waterbodies
5.8.1
The STP is sandwiched by two streams which may have water flow under
heavy rain. There is a u-channel at the foot of the manmade slope where the STP
will be constructed. It brings the water seeped from the surrounding slope to
the culverts downstream of the above streams and eventually flow into the sea.
Protection to this u-channel shall be provided (e.g. covered by pore-less metal
plate) to prevent materials dropping into it.
5.8.2
Place sandbag along the
upstream section of the stream near Fairway Vista and along rocky shore during
open cut excavation for laying of gravity sewers/rising mains nearby to prevent
the excavated materials from falling into the water and being carried into the
sea. The downstream water from the outfall at the rocky shore shall be temporarily
diverted away from the work boundary for excavation works into sea directly,
e.g. through a pipe without the need of construction work. The diversion shall
be removed to revert the flow to normal after completion of the excavation
works.
5.8.3
A drip tray/container
should be provided underneath the bentonite recycling system to prevent any
leakage from entering the watercourse or sea.
Works in the Sea
5.8.4
Sheet piles in marine
waters should be installed by vibratory action. Single-layer silt curtain should
be equipped during the installation and extraction works.
5.8.5
Marine works (dredging, construction and installation works at diffuser
location, backfilling) shall be carried out inside fully enclosed, watertight
cofferdam. The cofferdam can only be removed after completion of work during
ebb tide to avoid water quality impact on the Po Toi O Bay.
5.8.6
Dredging should be carried
out by closed-grab dredger. The marine sediment should be placed in sealed
compartment of the marine barge.
5.8.7
Silty water removed from the
cofferdam should be de-silted in a sedimentation tank before discharge back
into the sea.
Good Site
Practices
5.8.8
To minimize the potential
impact from the land-based construction works, good practices outlined in
ProPECC PN1/94 should be followed as far as possible. Mitigation measures
should include, but not limited to, the following:
·
Perimeter
channels and catchpits shall be constructed prior to commencement of site
formation works and earthworks;
·
Removal
facilities (sand traps, silt traps and sediment basins) should be provided to
collect the surface run-off from construction sites. Channel or earth bund or
sand bag barriers should be provided to direct the stormwater to removal
facilities;
·
Silt
removal facilities, channels and manholes should be maintained regularly;
·
Works
program should be designed to minimize the scale of soil excavation during the
rainy season (April to September) as far as possible;
·
Works
program should be well designed to minimize work areas to reduce the soil
exposure and site runoff;
·
To
avoid the surface runoff from the earthwork, the exposed soil area should be
installed with surface protection measure such as covering by tarpaulin before
arrival of rainstorm;
·
Minimize
exposed earth after completion of work in a certain area by hydroseeding,
vegetating, soil compacting or covering with bitumen;
·
Protect
temporary access roads by crushed stone or gravel;
·
Prevent
rainwater from entering trenches. Excavation of trenches should be dug and
backfilled in short sections during rainy seasons. Remove silt in rainwater
collected from trenches or foundation excavations prior to discharge to storm
drains.
·
Open
stockpiles of construction materials (e.g. aggregates, sand and fill material)
onsite should be covered with tarpaulin or similar fabric during rainstorm;
·
All
materials stored on the marine barge should be covered with tarpaulin or
similar fabric. If any equipment or material that contain oil/chemicals would
be stored on the marine barge, a robust drip tray/container should be provided
underneath to prevent any leakage from entering the sea.
·
All
plant and vehicles should be washed before they leave the construction site.
The wash-water should have sand and silt settled out or removed before
discharging into storm drain;
·
Any
wastewater generated from construction works should undergo removal of
settleable solids in a silt removal facility;
·
Remove
waste from the construction site regularly to prevent waste accumulation and
chance of wash-off.
·
Provide
sufficient chemical toilets with regular maintenance by licensed chemical waste
collector where necessary
Prevention of
Pollution from Chemicals
·
Register
as chemical waste producer if chemical waste will be generated.
·
Perform
maintenance of vehicles and equipment that have oil leakage and spillage
potential on hard standings within a bunded area with sumps and oil
interceptors.
·
Dispose
chemical waste in accordance to Waste Disposal Ordinance. Follow the Code of
Practice on the Packaging, Labelling and Storage of Chemical Wastes, examples
as follows:
i.
Store chemical wastes with suitable containers to
avoid leakage or spillage during storage, handling and transport
ii.
Label chemical waste containers according to the
CoP to notify and warn the waste handlers
iii.
Store chemical wastes at designated safe location
with adequate space
5.8.9
As there should be no
immerse need for chemical or oil in this small scale project, the quantity
stored or used onsite should be limited to exert a significant water quality
impact in case of leakage. The largest amount of chemical involved should be
bentonite used in HDD, which should be stored in enclosed system with low
chance of leakage. Nevertheless, given the proximity to sensitive receivers
(e.g. coral, amphioxus, fish culture zone), the Contractor shall devise an
emergency contingency plan as part of the environmental monitoring and audit
programme for accidental leakage or spillage of motor oil, bentonite, chemicals
(e.g. paint) and etc. during construction phase. It should details the
communication line between Contractor, relevant government and stakeholders,
remediation plan for containing and cleaning of the leakage, evaluation and
improvement of work and determine follow-up action (e.g. monitoring).
Operational Phase
Normal Operation
5.8.10 Considering the problem of insufficient water depth and proximity to the
rocky shore for the diffuser location proposed in the project profile, the
diffuser in this EIA has been relocated to the current position which is far
away from the rocky shore and has over 10m water depth. The model results show
no water quality impact from the normal operation when the effluent discharge
standard in Table 5-2 is strictly
followed. Chemicals will be transferred and stored within the sewage treatment
plant and thus no spillage outside the premise is expected. No further
mitigation is needed for normal operational phase.
5.8.11 Arrangement in case of emergency plant failure summarized from Section 5.4.9 - 5.4.16 is listed below. Considering the project scale, risk of emergency
condition, construction difficulties and cost, the following provisions are the
most appropriate and practical mitigation measures in case of plant/power
failure. No emergency discharge of untreated sewage is expected.
·
Delivery
of an emergency generator to PTO STP within 4 hours from plant failure
·
Provision
of dual power by CLP;
·
Provision
of a supervisory control and data acquisition system (SCADA), which signals to
the operation and maintenance personnel for emergency attendance in case of
plant failure;
·
Provision
of a standby pump and screen at the PTO STP;
·
Provision
of emergency storage of 4-hr ADWF sewage retention time;
·
Arrangement
of tankers for continuous removal of incoming sewage to other sewage treatment
plants for treatment to ensure a sufficient buffer for emergency storage.
5.9
Evaluation of Residual Impacts
5.9.1
With proper implementation of mitigation measures, residual impact is
anticipated to be acceptable during construction and
operational phases.
5.10
Environmental Monitoring and Audit
Requirements
5.10.1 Regular monitoring
of water quality should be carried out at water quality monitoring stations
near the dredging point (impact station), upstream of the impact (control
stations), and near representative sensitive receivers (e.g. Fish Culture Zone,
coral and Amphioxus) before and during cofferdam installation works, throughout
dredging works and during cofferdam extraction works.
5.10.2 No substantial
change in water quality in the Po Toi O Bay is expected during normal operation
of the PTOSTW. However, in view of the sensitivity of fish culture zone, corals
and amphioxus to water quality change, marine water quality monitoring is
proposed for the first commencement year to ensure that no deterioration of
water quality arises in the semi-enclosed bay due to effluent discharge.
5.10.3 Practical and well
sufficient preventative and mitigation measures have been proposed to protect
water quality during marine construction works, such as no open dredging for
submarine outfall alignment construction by adopting HDD, dredging and filling
within fully enclosed cofferdam and use of closed grab dredger for the
construction of the submarine outfall diffuser. During operation of the
proposed STP, effluent that meets the water quality requirements under WPCO
will be discharged in the outer Po Toi O Bay. As no adverse water deterioration
is expected during construction and operational phases given the sufficient
design and mitigation measures, real-time reporting of monitoring data for the
Project through a dedicated internet website is considered not necessary.
Detailed EM&A requirement are provided in EM&A Manual.
5.11.1 During normal operation of the PTOSTW, all water quality parameters
would comply with the WQO except TIN, which is attributed to the background
level.
5.11.2 Considering the project scale, risk of emergency condition, construction
difficulties and cost, the following provisions are the most appropriate and
practical mitigation measures in case of plant/power failure:
·
Delivery
of an emergency generator to PTO STP within 4 hours from plant failure
·
Provision
of dual power by CLP;
·
Provision
of a supervisory control and data acquisition system (SCADA), which signals to
the operation and maintenance personnel for emergency attendance in case of
plant failure;
·
Provision
of a standby pump and screen at the PTO STP;
·
Provision
of emergency storage of 4-hr ADWF sewage retention time;
·
Arrangement
of tankers for continuous removal of incoming sewage to other sewage treatment
plants for treatment to ensure a sufficient buffer for emergency storage.
5.11.3 Based on these provisions, emergency discharge is not expected, and thus
no water quality impact in case of
plant/equipment failure is anticipated.
5.11.4
Regarding construction
phase impact from the submarine outfall, no open dredging and backfilling will be required.
The submarine outfall will be constructed below seabed by HDD. At the emerging
point for diffuser, a fully enclosed cofferdam shall be erected to surround the
dredging site and the dredger barge shall be anchored outside the cofferdam. No
leakage of suspended solids is expected.
5.11.5
Regarding construction
phase impact from land-based works, standard water protection measures shall be
implemented to minimize water quality impact.
5.11.6 With proper implementation of
mitigation measures, no insurmountable water quality impact due to this Project
is expected.
[1] Pastorok, R.A. and Bilyard, G.R. (1985). “Effects of sewage pollution on
coral-reef communities.” Marine
Ecology Progress Series 21: 175-189.
[2] Hawker, D. W. and Connell, D. W. (1992). “Standards and Criteria for Pollution Control in Coral Reef Areas” in Connell, D. W and Hawker, D. W. (eds.), Pollution in Tropical Aquatic Systems, CRC Press, Inc.
[3] Erftemeijer P. L.A.,
Riegl B., Hoeksema B. W. & Todd P. A.. (2012). “Environmental impacts of
dredging and other sediment disturbances on corals: A review”. Marine Pollution
Bulletin Volume 64, Issue 9, September 2012, Pages 1737–1765
[4] Golbuu Y., van Woesik
R., Richmond R. H., Harrison P. & Fabricius K. E.. (2011). “River discharge
reduces reef coral diversity in Palau”. Marine Pollution Bulletin Volume 62,
Issue 4, April 2011, Pages 824–831
[5] AFCD. 2005. Tender
No. AFC/SQ/58/03 Establishing threshold tolerance of local corals to
sedimentation, Final Report.
[6] EPD. 2012. Marine
Water Quality in Hong Kong in 2011. [online] http://wqrc.epd.gov.hk/pdf/water-quality/annual-report/Marine2011EN.pdf.
Accessed: 26 August 2015.
[7] EPD. 2013. Marine Water Quality in Hong Kong in 2012.
[online]
http://wqrc.epd.gov.hk/pdf/water-quality/annual-report/Report2012eng.pdf. Accessed: 26 August 2015.
[8] EPD. 2014. Marine Water Quality
in Hong Kong in 2013.
[online] http://wqrc.epd.gov.hk/pdf/water-quality/annual-report/MarineReport2013eng.pdf. Accessed: 26 August
2015.
[9] Maunsell Consultants
Asia Ltd. 2005. Further Development of Tseung Kwan O Feasibility Study
(Application No. EIA-111/2015) Ch. 6 Sewerage and Sewage Treatment. [online]
Available at: http://www.epd.gov.hk/eia/register/report/eiareport/eia_1112005/HTML/EIA/HTML/Volume%201/Sec_6%20Sewerage.htm.
Last Accessed: 23 March 2016.
[10] EPD. 2015. Supplementary
information pertaining to item PWSC(2014-15)49. [online]
Available
at http://www.legco.gov.hk/yr14-15/english/fc/pwsc/papers/pwsc20150210pwsc-120-1-e.pdf. Accessed:
02 February 2016.
[11] Pastorok
R.A. & Bilyard G.R. (1985). Effects of sewage pollution on coral-reef
communities. Marine Ecology Progress
Series 21: 175-189.
[12] Hawker D.W. & Connell D.W. (1992). Standards
and Criteria for Pollution Control in Coral Reef Areas. In Connell D.W. &
Hawker D.W. (eds.), Pollution in Tropical
Aquatic Systems. CRC Press, Boca Raton.
[13] Erftemeijer P. L.A.,
Riegl B., Hoeksema B. W. & Todd P. A.. (2012). “Environmental impacts of
dredging and other sediment disturbances on corals: A review”. Marine Pollution
Bulletin Volume 64, Issue 9, September 2012, Pages 1737–1765
[14] AFCD. 2005. Tender
No. AFC/SQ/58/03 Establishing threshold tolerance of local corals to
sedimentation, Final Report.