This
Section addresses the potential impacts on water quality from the construction
and operation of the Designated and Potentially Designated Elements of the
Project.
5.2
Relevant Legislation and Guidelines
The
following relevant pieces of legislation and associated guidance are applicable
to the evaluation of water quality impacts associated with the construction and
operation of the Designated and Potentially Designated Elements of the Project.
· Water Pollution Control Ordinance (WPCO);
· Technical Memorandum for Effluents Discharged into Drainage and Sewerage
Systems Inland and Coastal Waters; and
· Environmental Impact Assessment Ordinance (Cap. 499. S.16), Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM), Annexes 6 and 14.
Apart from the above statutory requirements,
the Practice Note for Professional Persons, Construction
Site Drainage (ProPECC PN 1/94), issued by ProPECC in 1994, also provides
useful guidelines on the management of construction site drainage and
prevention of water pollution associated with construction activities.
5.2.1
Water
Pollution Control Ordinance
The WPCO
is the legislation for the control of water pollution and water quality in Hong
Kong. Under the WPCO, Hong Kong waters are divided into
10 Water Control Zones (WCZs).
Each WCZ has a designated set of statutory Water Quality Objectives
(WQOs). The WQOs set limits for
different parameters that should be achieved in order to maintain the water
quality within the WCZs. The
Project is wholly located within the Deep Bay WCZ, the boundaries of which are
shown on Figure
5.2a. The WQOs for the
Deep Bay WCZ, which are presented in Table
5.2a, are applicable as evaluation criteria for assessing compliance of any
effects from the construction and operation of the Project.
Table 5.2a Water
Quality Objectives for the Deep Bay Water Control Zone
|
Water Quality Objectives |
Part or parts of Zone |
|
A.
AESTHETIC APPEARANCE a)
Waste discharges shall cause no objectionable odours or
discolouration of the water. |
Whole zone |
|
b)
Tarry residues, floating wood, articles made of glass, plastic,
rubber or of any other substances should be absent. |
Whole zone |
|
c)
Mineral oil should not be visible on the surface. Surfactants should not give rise to a
lasting foam. |
Whole zone |
|
d)
There should be no recognisable sewage-derived debris. |
Whole zone |
|
e)
Floating, submerged and semi-submerged objects of a size likely to
interfere with the free movement of vessels, or cause damage to vessels,
should be absent. |
Whole zone |
|
f)
Waste discharges shall not cause the water to contain substances
which settle to form objectionable deposits. |
Whole zone |
|
B.
BACTERIA |
|
|
a)
The level of Escherichia coli should not exceed 610 per 100 mL,
calculated as the geometric mean of all samples collected in one calendar
year. b)
The level of Escherichia coli should be zero per 100 mL, calculated
as the running median of the most recent 5 consecutive samples taken at
intervals of between 7 and 21 days. c)
The level of Escherichia coli should not exceed 1,000 per 100 mL,
calculated as the running median of the most recent 5 consecutive samples
taken at intervals of between 7 and 21 days. d)
The level of Escherichia coli should not exceed 180 per 100 mL,
calculated as the geometric mean of all samples collected from March to
October inclusive in one calendar year.
Samples should be taken at least 3 times in a calendar month at
intervals of between 3 and 14 days. |
Secondary Contact Recreation Subzone and Mariculture Subzone. Yuen Long & Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone,
Ganges Subzone and Water Gathering Ground Subzones. Yuen Long & Kam Tin (Lower) Subzone and other inland waters. Yung Long Bathing Beach Subzone. |
|
C.
COLOUR |
|
|
a)
Waste discharges shall not cause the colour of water to exceed 30
Hazen units. b)
Waste discharges shall not cause the colour of water to exceed 50
Hazen units. |
Yuen Long & Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone,
Ganges Subzone and Water Gathering Ground Subzones. Yuen Long and Kam Tin (Lower) Subzone and other inland waters. |
|
D.
DISSOLVED OXYGEN |
|
|
a)
Waste discharges shall not cause the level of dissolved oxygen to
fall below 4 mg per litre for 90% of the sampling occasions during the year;
values should be taken at 1 metre below surface. b)
Waste discharges shall not cause the level of dissolved oxygen to
fall below 4 mg per litre for 90% of the sampling occasions during the year;
values should be calculated as water column average (arithmetic mean of at
least 2 measurements at 1 metre below surface and 1 metre above seabed). In addition, the concentration of
dissolved oxygen should not be less than 2 mg per litre within 2 metres of
the seabed for 90% of the sampling occasions during the year. c)
The dissolved oxygen level should not be less than 5 mg per litre for
90% of the sampling occasions during the year; values should be taken at 1
metre below surface. d)
Waste discharges shall not cause the level of dissolved oxygen to be
less than 4 mg per litre. |
Inner Marine Subzone excepting Mariculture Subzone. Outer Marine Subzone excepting Mariculture Subzone. Mariculture Subzone. Yuen Long & Kam Tin (Upper and Lower) Subzones, Beas Subzone,
Indus Subzone, Ganges Subzone, Water Gathering Ground Subzones and other
inland waters of the Zone. |
|
E.
PH |
|
|
a)
The pH of the water should be within the range of 6.5 - 8.5
units. In addition, waste
discharges shall not cause the natural pH range to be extended by more than
0.2 units. b)
Waste discharges shall not cause the pH of the water to exceed the
range of 6.5 - 8.5 units. c)
The pH of the water should be within the range of 6.0 - 9.0 units. d)
The pH of the water should be within the range of 6.0 - 9.0 units for
95% of samples. In addition,
waste discharges shall not cause the natural pH range to be extended by more
than 0.5 units. |
Marine waters excepting Yung Long Bathing Beach Subzone. Yuen Long & Kam Tin (Upper and Lower) Subzones, Beas Subzone,
Indus Subzone, Ganges Subzone and Water Gathering Ground Subzones. Other inland waters. Yung Long Bathing Beach
Subzone. |
|
F.
TEMPERATURE |
|
|
Waste discharges shall not cause the natural daily temperature range
to change by more than 2.0 oC. |
Whole zone |
|
G.
SALINITY |
|
|
Waste discharges shall not cause the natural ambient salinity level
to change by more than 10%. |
Whole zone |
|
H.
SUSPENDED SOLIDS |
|
|
a)
Waste discharges shall neither cause the natural ambient level to be
raised by 30% nor give rise to accumulation of suspended solids which may
adversely affect aquatic communities. b)
Waste discharges shall not cause the annual median of suspended
solids to exceed 20 mg per litre. |
Marine waters. Yuen Long & Kam Tin (Upper and Lower) Subzones, Beas Subzone,
Indus Subzone, Ganges Subzone, Water Gathering Ground Subzones and other inland
waters. |
|
I.
AMMONIA |
|
|
The un-ionized ammoniacal nitrogen level should not be more than
0.021 mg per litre, calculated as the annual average (arithmetic mean). |
Whole zone |
|
J.
NUTRIENTS |
|
|
a)
Nutrients shall not be present in quantities sufficient to cause
excessive or nuisance growth of algae or other aquatic plants. b)
Without limiting the generality of objective (a) above, the level of
inorganic nitrogen should not exceed 0.7 mg per litre, expressed as annual
mean. c)
Without limiting the generality of objective (a) above, the level of
inorganic nitrogen should not exceed 0.5 mg per litre, expressed as annual
water column average (arithmetic mean of at least 2 measurements at 1 metre
below surface and 1 metre above seabed). |
Inner and Outer marine Subzones. Inner Marine Subzone. Outer Marine Subzone. |
|
K.
5-DAY BIOCHEMICAL OXYGEN DEMAND |
|
|
a)
Waste discharges shall not cause the 5-day biochemical oxygen demand
to exceed 3 mg per litre. b)
Waste discharges shall not cause the 5-day biochemical oxygen demand
to exceed 5 mg per litre. |
Yuen Long & Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone,
Ganges Subzone and Water Gathering Ground Subzones. Yuen Long and Kam Tin (Lower) Subzone and other inland waters. |
|
L.
CHEMICAL OXYGEN DEMAND |
|
|
a)
Waste discharges shall not cause the chemical oxygen demand to exceed
15 mg per litre. b)
Waste discharges shall not cause the 5-day chemical oxygen demand to
exceed 30 mg per litre. |
Yuen Long & Kam Tin (Upper) Subzone, Beas Subzone, Indus Subzone,
Ganges Subzone and Water Gathering Ground Subzones. Yuen Long and Kam Tin (Lower) Subzone and other inland waters. |
|
M.
TOXINS |
|
|
a)
Waste discharges shall not cause the toxins in water to attain such levels
as to produce significant toxic, carcinogenic, mutagenic or teratogenic
effects in humans, fish or any other aquatic organisms, with due regard to
biologically cumulative effects in food chains and to interactions of toxic
substances with each other. b)
Waste discharges shall not cause a risk to any beneficial uses of the
aquatic environment. |
Whole zone Whole zone |
|
N.
PHENOLS |
|
|
Phenols shall not be present in such quantities as to produce a specific
odour, or in concentration greater than 0.05 mg per litre as C6H5OH. |
Yung Long Bathing Beach
Subzone. |
|
O.
TURBIDITY |
|
|
Waste discharges shall not reduce light transmission substantially
from the normal level. |
Yung Long Bathing Beach
Subzone. |
5.2.2
Technical
Memorandum for Effluent Discharges
All discharges during both the construction the
operational phases of the Project are required to comply with the Technical Memorandum for Effluents Discharged
into Drainage and Sewerage Systems, Inland and Coastal Waters (TM) issued
under Section 21 of the WPCO. The TM defines discharge limits to
different types of receiving waters.
Under the TM, effluents discharged into the drainage and sewerage systems,
inshore and coastal waters of the WCZs are subject to pollutant concentration
standards for particular discharge volumes. Any new discharges within a WCZ are subject to licence
conditions and the TM acts as a guideline for setting discharge standards for
the licence. The current discharge
licence standards for the Yuen Long STW are shown in Table 5.2b.
Table 5.2b Current
Discharge Licence Standards for the Yuen Long STW
|
Licence No. |
Licence Expiry Date |
Flow Rate (m3 day-1) |
SS Concentration (mg L-1) |
BOD Concentration (mg L-1) |
|
3L1/4 |
30 November 2004 |
210,000 |
30 |
20 |
In
addition to the provisions of the TM, the ‘Deep Bay Zero Discharge Policy’ aims
to provide protection to the inland and marine water quality of the Deep Bay
WCZ. The policy requires that
developments within the Deep Bay catchment areas do not result in an increase
in pollution loads to the inland and marine waters.
Annexes 6 and 14 of the EIAO-TM
provide general guidelines and criteria to be used in assessing water quality
issues.
5.3
Existing Environment/Sensitive Receivers
5.3.1
Inland
Waters
The construction and operation of the proposed works
for the Project have the potential to affect a number of inland watercourses
within the Deep Bay WCZ. There are
two EPD routine water quality monitoring stations along these inland
watercourses, one along the Kam Tin River (Station KT2) and one in the Tin Shui
Wai Nullah (TSR1), the locations of which are shown in Figure 5.3a. A summary of water
quality data for these stations is presented in Table 5.3a. These data
were measured in 1999 and are the most recently published data ([1]).
Table 5.3a EPD
Routine River Water Quality Monitoring Data in the Vicinity of the Project Area
|
Parameter |
Station KT2 |
Station TSR1 |
|
Dissolved Oxygen (mg L-1) |
5.5 (1.9 - 14.4) |
7.9 (1.4 - 11.6) |
|
PH |
7.3 (6.9 - 8.2) |
7.7 (7.3 - 8.9) |
|
Suspended Solids (mg L-1) |
65 (21 – 100) |
29 (6 – 72) |
|
5-day Biochemical Oxygen
Demand (mg L-1) |
22 (9 - 190) |
19 (5 – 41) |
|
Chemical Oxygen Demand (mg
L-1) |
57 (31 -240) |
35 (8 – 56) |
|
Oil & Grease (mg L-1) |
1.2 (0.5 -3.0) |
0.8 (0.5 - 4.6) |
|
E.
coli (cfu 100mL-1) |
9,680 (900 - 260,000) |
3,590,000 (310,000 -
67,000,000) |
|
Ammonia-nitrogen (mg L-1) |
14.00 (2.30 – 21.00) |
4.50 (1.40 - 25.00) |
|
Nitrate-nitrogen (mg L-1) |
0.27 (0.01 - 2.00) |
0.79 (0.01 - 3.00) |
|
Total Kjeldahl Nitrogen (mg
L-1) |
17.00 (4.50 – 27.00) |
7.05 (2.20 – 30.00) |
|
Ortho-phosphate (mg L-1) |
0.67 (0.06 - 5.20) |
0.63 (0.21 - 3.20) |
|
Total Phosphorous (mg L-1) |
1.80 (0.96 -6.50) |
1.15 (0.29 - 4.00) |
|
Notes: 1.
Data presented are annual medians of monthly samples, except for E. coli which are in annual geometric
means. 2.
Figures in brackets are annual ranges. 3. Shaded cells
indicate exceedance of the WQOs. |
||
The above data show that there are a number of
breaches of the WQOs for dissolved oxygen, chemical oxygen demand, 5-day
biochemical oxygen demand, suspended solids and E. coli for both
monitoring stations. The main
factor contributing to the poor water quality at these two stations is
uncontrolled discharges of livestock wastes. Improvements may be expected in the future due to greater
enforcement of livestock waste control measures.
In
addition to determining compliance with the WQOs, EPD also compiles a Water
Quality Index (WQI). The WQI is
based on the key parameters of dissolved oxygen saturation, 5-day biochemical
oxygen demand and ammonia-nitrogen.
The river water quality at KT2 was found to be in the WQI category of
‘bad’ for 1999, which is an improvement from 1998, while at Station TSR1 the
quality was defined as ‘bad’, which is consistent with earlier years.
5.3.2
Marine
Waters
The streams within the Study Area discharge
into inner Deep Bay, along the shallow southern shoreline of Deep Bay where
there are extensive mud flats.
Tidal current speeds in the area are extremely low, resulting in long
residence times for the marine waters.
There is one routine EPD water quality monitoring station located in the
vicinity of the discharge locations for the streams which may be affected by
the works and the location of the station is shown in Figure 5.3a. A summary of water quality data for
this station is presented in Table 5.3b. These data were measured in 1999 and
are the most recently published data ([2]).
Table 5.3b EPD
Routine Marine Water Quality Monitoring Data in the Vicinity of the Project
Area
|
Water Quality Parameter |
Monitoring Station DM1 |
|
Temperature (°C) |
24.7 (16.1 – 31.7) |
|
Salinity (ppt) |
18.0 (5.5 - 26.4) |
|
Dissolved Oxygen (mg L-1) |
4.6 (1.8 - 7.7) |
|
5-day Biochemical Oxygen
Demand (mg L-1) |
3.8 (1.9 - 6.6) |
|
Suspended Solids (mg L-1) |
70.8 (19.0 - 180.0) |
|
Total Inorganic Nitrogen
(mg L-1) |
4.12 (1.28 - 7.95) |
|
Unionised Ammonia (mg L-1) |
0.040 (0.01 - 0.071) |
|
Chlorophyll-a (mg L-1) |
7.0 (0.2 – 31.0) |
|
E. coli (cfu 100mL-1) |
700 (16 - 14,000) |
|
Notes: 1. Data presented
are depth averaged, except as specified. 2. Data presented
are annual arithmetic mean except for E.
coli, which are geometric mean values. 3. Data enclosed in
brackets indicate the ranges. 4. Shaded cells
indicate non-compliance with the WQOs for a parameter. |
|
In 1999 there were exceedances of the WQOs for
dissolved oxygen, total inorganic nitrogen and unionised ammonia. These measurements reflect the poor
state of the water quality in the inner part of Deep Bay, which prompted the
implementation of the ‘Deep Bay Zero Discharge Policy’. A significant trend of increasing
nutrient (total inorganic nitrogen and ammonia) and decreasing dissolved oxygen
levels has been observed over the last ten years.
The marine waters of Deep Bay are heavily
affected by discharges from rivers in both Hong Kong and Shenzhen. The values of suspended sediment
concentrations are high compared to other parts of Hong Kong, which may be
attributed to the presence of shallow areas with mud flats and the discharges
from the rivers. The relatively
high E. coli values (up to 14,000 cfu
100mL-1) show that there is a strong influence from sewage discharges. Despite the nutrient enrichment of the
marine waters at Station DM1, as shown by the high total inorganic nitrogen
levels, the concentrations of chlorophyll-a
are not shown to be particularly high.
The data for salinity show that during the wet summer months the marine
waters are dominated by freshwater discharges and the salinity declines
rapidly.
5.3.3
Sensitive
Receivers
Inland Waters
There are no specific water quality sensitive
receivers identified for the inland waters. However, the poor quality of these waters, as discussed
above, means that the majority of the stream courses would be termed as
‘stressed’ and as such measures should be taken to prevent any impacts from the
construction and operation of the Project. In particular, the sewerage pipes will cross streams or
pumping stations and will be built in the immediate vicinity of streams at the
following locations, which are shown on Figure 5.3b.
· Shui Tsan Tin;
· Kam Tsin Wai;
· Ng Ka Tsuen;
· Kam Hing Wai; and
· Nam Sang Wai.
It is to be expected that the operation of the
project will result in an improvement to the quality of the inland waters by
connecting unsewered areas to the Yuen Long STW, where the sewage effluent will
be treated prior to discharge.
Marine
Waters
The closest marine sensitive area which may be
affected by the construction and operation of the project works are the Mai Po
and Inner Deep Bay Ramsar Site and the Mai Po Marshes and Inner Deep Bay Sites
of Special Scientific Interest (SSSI), as shown on Figure 5.3b. It is to be expected that the Project
will result in improvements to the quality of the marine waters in these areas
by improving the quality of the inland watercourses which discharge into them.
5.4.1
Construction
Phase
The
assessment of the potential water quality impacts of the various construction
activities has been undertaken in a qualitative manner. Consideration has been given to
controlling potentially harmful impacts from the site works and to the use of
‘best practice’ measures to minimise the potential for discharges of pollutants
to water courses in the vicinity of the works areas. The construction activities which have been considered
include the construction of pumping stations (site formation and building
works), the laying of sewerage pipelines (site formation and trenching) and
pipeline crossings of streams.
5.4.2
Operation
Phase
The operation of the sewerage mains and pumping
stations will result in a reduction of pollution loads to the inland
watercourses, through the connection of unsewered areas to the sewerage system
and subsequent transport to the Yuen Long STW. The magnitude of the reduction has been estimated through an
analysis of the designs of the sewerage main and pumping stations, which
provided information on the quantities of sewage to be transported. This analysis then enabled a
qualitative assessment of the potential improvements to water quality in the affected
inland watercourses.
The connection of the unsewered areas will
result in an increase in the treated sewage effluent flows from the Yuen Long
STW to the Shan Pui River, which discharges into inner Deep Bay. There will be a net decease in the
total load to Deep Bay as the increase in treated effluent flows will be offset
by the reduction in untreated sewage effluent discharges to the inland
watercourses, which are in the upstream/catchment areas of the Shan Pui River.
The effects of the increased flows from the
Yuen Long STW and the overall reduction in pollutant load on water quality have
been quantified using water quality modelling. The modelling has been undertaken using the Delft 3D water
quality model, which was set up and calibrated for Deep Bay as part of the
study Deep Bay Water Quality Regional
Control Strategy Study. This
detailed model covers the whole of Deep Bay and extends beyond the mouth of the
bay, as shown in Figure 5.4a. Data on pollution loads into Deep Bay are contained within
the Pollution Load Inventory from the
study Update on Cumulative water Quality
and Hydrological Effect of Coastal Development and Upgrading Assessment Tool. The locations of the sources of
polluting discharges to Deep Bay and the surrounding area contained in the Pollution Load Inventory are shown in Figure 5.4b. The implementation of the Project will
result in changes in the loading input at source YL2 (due to connection of unsewered
areas to the Yuen Long STW and increasing discharges from the Yuen Long STW).
The water quality model has been used to
simulate two scenarios:
·
A Commissioning Scenario, which simulated the
pollution loads following implementation of the Stage 1 works in 2007; and
·
A Non-Commissioning Scenario, which
simulated the pollution loads in 2007 if the Stage 1 works were not
implemented.
These two scenarios allowed the water quality
conditions after completion of the works and the differences in water quality
prior to the construction of the works to be quantified. Both of these scenarios have been
simulated for representative wet and dry season conditions, in order to
simulate the seasonal variation in the hydrodynamic/oceanographic conditions in
Deep Bay.
The loadings for source YL2 (see Figure 5.4b)
in the Commissioning Scenario have
been derived as part of this Study.
In order to define the future loads the areas to be connected to the
Yuen Long STW have been represented by the pumping stations that will transport
the flows, the locations of which are shown on Figure 5.4c. Sources of wastewater discharges
include domestic, commercial and industrial effluents. Data on the future flows and loads
transported by the pumping stations to the Yuen Long STW have been
provided by DSD. The data also include a breakdown of the future domestic,
commercial and industrial flows that will not be connected to the STW. It has been assumed that these flows
will be discharged to inland waterways.
In the case of the commercial and industrial flows, it has been assumed
that those flows not connected to the STW will undergo treatment prior to
discharge to inland waterways.
The loadings for source YL2 (see Figure 5.4b)
in the Non-Commissioning Scenario
have been derived as part of this Study.
In this scenario only the domestic, commercial and industrial effluent
discharges from the existing Yuen Long Town cathchment will be connected to the
STW. All other wastewaters will be
discharges to inland waterways.
Domestic sewage effluents are assumed to be untreated, while commercial
and industrial effluents will receive treatment prior to discharge.
There may be a concern arising from the
potential for a shut-down of the pumping stations, either through mechanical
breakdown or failure of the power supply, to result in a short term discharge
of untreated sewage effluent to surface waters. The designs of the pumping stations, particularly of
backup/redundant systems, will be reviewed to determine the potential for
failures and, if necessary, additional design features defined to minimise the
risk. In addition, a series of
measures will be defined to minimise the duration of untreated sewage effluent
discharges, in the unlikely event that a failure were to occur and hence reduce
the potential impacts to water quality.
5.5
Identification of Potential Impacts
5.5.1
Construction
Phase
During construction of the sewers and pumping
stations the primary sources of potential impacts to water quality will be from
pollutants in site run-off, which may enter surface waters directly or enter
storm drains discharging into these waters. Pollutants, mainly suspended sediments, may also enter these
waters if pumped groundwater is not adequately controlled.
Wastewater from temporary site facilities
should be controlled to prevent direct discharge to surface waters. Such wastewater may include sewage
effluent from toilets. Water from
plant servicing facilities may be contaminated with oil and other petroleum
products and would have the potential to discharge to surface waters if
spillages are not contained.
For the sewer crossings of the Yuen Long and
Kam Tin Main Drainage Channels, the sewers will be laid using the trenchless
pipe jacking method. This method
avoids direct disturbance of any sediments on the bed of the channels. However, in the event of malfunction of
the tunnelling machine, a rescue pit would be sunk, which would require
dredging works. This could
potentially cause localised impacts to water quality, particularly if the
sediments are contaminated.
Mitigation measures may be necessary to prevent the transport of sediment
in suspension away from the works area.
In addition, embankments along the sides of the channels will need to be
removed and reinstated once the sewers have been laid, which could result in
spillage of material to the channels.
There may also be the potential for adverse impacts to surface water
quality if pumped groundwater is not controlled.
At smaller, natural streams (see Figure 5.3b)
the sewers will be laid using the open trench method. In order to allow this work to be undertaken in dry
conditions two possible methodologies will be used, as follows:
·
For
low flows, submersible pumps will be used to bypass the works area; and
·
For
higher flows, temporary diversion of the stream will be required, either along a
temporary channel or through pipes.
These
methods of construction will result in minimal impacts to water quality in the
streams. However, there may be the
potential for adverse impacts to surface water quality if pumped groundwater is
not controlled.
5.5.2
Operation
Phase
The operation of the works will result in an
improvement to the quality of inland waters through the connection of unsewered
areas to sewer mains, thereby removing sources of polluting discharges to
streams/watercourses. The sewer
mains will then be connected to the Yuen Long STW, where the effluents will be
treated and discharged to the Shan Pui River. This will result in an increase in treated sewage effluent
discharges from the Yuen Long STW, but will not increase the total pollutant load
to Deep Bay. The potential
improvements to water quality in Deep Bay, where the affected streams
discharge, was demonstrated through the use of water quality modelling.
In order to prevent the uncontrolled discharge
of untreated sewage effluent to surface waters there will be a need to minimise
the risk of failure of the pumping stations. Measures to minimise the risk could include the provision of
standby/redundant pumps and the provision of a back-up power supply. Despite the provision of such measures
the potential for failure may still exist and procedures should be put in place
to reduce the duration of the discharge of untreated sewage effluent in order
to prevent adverse impacts to surface waters. Such procedures could include the setting up of a system for
early reporting of failures, possibly through the use of telemetry systems to
monitor the pumps, and the actioning of timely repairs.
5.6
Assessment of Environmental Impacts
5.6.1
Construction
Phase
The potential impacts of land based
construction activities on water quality, as described in Section 5.5.1 may be readily controlled by appropriate on-site
measures. These measures are
described in Section 5.7.1 and, as
such, no further assessment of impacts has been carried out. The measures described in Section 5.7.1 are not only sufficient to
control/prevent impacts to the water sensitive receivers in the vicinity of the
works area, but such controls will also prevent adverse downsteam impacts to
both the fresh and marine waters of the Deep Bay WCZ.
The methods of construction for the crossings
of the main drainage channels and smaller streams will prevent the majority of
potential impacts to water quality, as discussed in Section 5.5.1.
However, specific mitigation measures may be required to minimise
smaller scale impacts, which are discussed in Section 5.7.1.
5.6.2
Operation
Phase
The detailed derivation of the input parameters
for the water quality modelling was presented in a Technical Note on Water
Quality Modelling Methodology (1), which is contained in Annex
B1.
The
modelling results have been produced as a series of contour plots and
statistical analyses of key parameters at selected points. The contour plots and statistical
analyses have been presented in the following formats:
· dissolved oxygen – depth averaged
values which are exceeded for 90% of the simulation time;
· dissolved oxygen – bottom values
which are exceeded for 90% of the simulation time;
· 5-day biochemical oxygen demand –
depth averaged mean values;
· total inorganic nitrogen – depth
averaged mean values;
· unionised ammonia – depth averaged
mean values;
· chlorophyll-a – depth averaged mean
values;
· E. coli –
depth averaged geometric mean values; and
· suspended sediment – depth averaged
mean values.
The above presentation of modelling results
will allow a comparison to be made with the relevant Water Quality Objectives
for both the Commissioning and Non-Commissioning Scenarios. The selected output locations are shown
in Figure
5.6a and represent the marine waters of Deep Bay and the identified
sensitive receivers.
The results of the water quality modelling at
the selected points are contained in Tables
5.6a and 5.6b for the wet and dry
seasons respectively.
The data in Tables
5.6a and 5.6b illustrate that the
main effects of the commissioning of the works will be in the immediate
vicinity of the discharge point YL2 (Station 11). Further afield the effects of the works are predicted to be
minimal, as shown by the lack of difference between the Pre-Commissioning and
Commissioning scenarios. The
modelling predicts that there will be increases in dissolved oxygen
concentrations and decreases in BOD, E.
coli and SS concentrations in the vicinity of the discharge point YL2 at
Station 11 in the wet season. The
greatest improvement is in E. coli
concentrations, which decrease from a geometric mean of 159,000 to 143,000 cfu
100mL-1. In the dry
season similar improvements in water quality are predicted, although the
magnitude of the improvements is generally less, reflecting the lower loads in
the dry season.
.3b Table
5.6a Wet Season Water Quality
Modelling Results
|
Station |
DO (mg L-1) |
DO (Bottom) (mg L-1) |
BOD (mg L-1) |
TIN (mg L-1) |
NH3-N (mg L-1) |
Chlorophyll-a (µg L-1) |
E. coli (cfu 100mL-1) |
SS (mg L-1) |
||||||||
|
|
Base |
Comp |
Base |
Comp |
Base |
Comp |
Base |
Comp |
Base |
Comp |
Base |
Comp |
Base |
Comp |
Base |
Comp |
|
1 |
4.9 |
4.9 |
4.9 |
4.9 |
0.7 |
0.7 |
1.8 |
1.8 |
0.009 |
0.009 |
2.1 |
2.1 |
129 |
125 |
19.1 |
19.1 |
|
2 |
4.2 |
4.2 |
4.1 |
4.1 |
1.0 |
1.0 |
2.4 |
2.4 |
0.017 |
0.017 |
2.3 |
2.4 |
203 |
200 |
23.9 |
23.9 |
|
3 |
4.0 |
4.0 |
3.9 |
3.9 |
1.4 |
1.4 |
3.3 |
3.3 |
0.024 |
0.024 |
2.6 |
2.6 |
251 |
249 |
30.4 |
30.4 |
|
4 |
2.9 |
2.9 |
2.8 |
2.8 |
2.6 |
2.6 |
5.8 |
5.8 |
0.054 |
0.054 |
1.9 |
1.9 |
1,170 |
1,170 |
49.1 |
49.1 |
|
5 |
5.3 |
5.3 |
5.2 |
5.2 |
0.6 |
0.6 |
1.5 |
1.5 |
0.006 |
0.006 |
2.1 |
2.1 |
85 |
84 |
17.3 |
17.3 |
|
6 |
5.1 |
5.1 |
5.0 |
5.0 |
0.8 |
0.8 |
1.6 |
1.6 |
0.008 |
0.008 |
2.2 |
2.2 |
297 |
292 |
18.8 |
18.8 |
|
7 |
4.5 |
4.5 |
4.5 |
4.5 |
1.2 |
1.2 |
2.2 |
2.2 |
0.015 |
0.015 |
2.2 |
2.2 |
1,730 |
1,690 |
23.2 |
23.2 |
|
8 |
3.7 |
3.7 |
3.6 |
3.6 |
1.8 |
1.8 |
3.1 |
3.1 |
0.024 |
0.024 |
2.0 |
2.0 |
3,560 |
3,450 |
29.9 |
29.9 |
|
9 |
1.4 |
1.4 |
1.3 |
1.3 |
2.7 |
2.7 |
5.4 |
5.4 |
0.054 |
0.054 |
1.7 |
1.7 |
3,100 |
3,050 |
47.4 |
47.4 |
|
10 |
0.0 |
0.0 |
0.0 |
0.0 |
8.8 |
8.8 |
15.3 |
15.3 |
0.170 |
0.170 |
1.1 |
1.1 |
18,700 |
18,700 |
127.0 |
127.0 |
|
11 |
1.6 |
1.7 |
1.6 |
1.7 |
4.8 |
4.5 |
4.5 |
4.5 |
0.028 |
0.028 |
0.9 |
0.9 |
159,000 |
143,000 |
33.8 |
32.8 |
|
Notes: 1.
Base refers to the
Pre-Commissioning scenario, and Comp
refers to the Commissioning scenario. 2.
All values are depth averaged, unless otherwise specified. 3.
DO values are 10%ile. 4.
BOD, TIN, NH3-N, Chlorophyll-a and SS are arithmetic mean values. 5.
E. coli are geometric
mean values. |
||||||||||||||||
Table 5.6b Dry Season Water
Quality Modelling Results
|
Station |
DO (mg L-1) |
DO (Bottom) (mg L-1) |
BOD (mg L-1) |
TIN (mg L-1) |
NH3-N (mg L-1) |
Chlorophyll-a (µg L-1) |
E. coli (cfu 100mL-1) |
SS (mg L-1) |
||||||||
|
|
Base |
Comp |
Base |
Comp |
Base |
Comp |
Base |
Comp |
Base |
Comp |
Base |
Comp |
Base |
Comp |
Base |
Comp |
|
1 |
7.4 |
7.4 |
7.4 |
7.4 |
2.1 |
2.1 |
1.2 |
1.2 |
0.004 |
0.004 |
16.4 |
16.5 |
211 |
200 |
18.4 |
18.4 |
|
2 |
6.9 |
6.9 |
7.0 |
7.0 |
2.9 |
2.9 |
1.8 |
1.8 |
0.007 |
0.007 |
22.9 |
22.9 |
441 |
428 |
23.1 |
23.1 |
|
3 |
6.4 |
6.4 |
6.4 |
6.4 |
3.8 |
3.8 |
2.9 |
2.9 |
0.012 |
0.012 |
26.3 |
26.3 |
1,530 |
1,520 |
31.9 |
31.9 |
|
4 |
3.9 |
3.9 |
3.9 |
3.9 |
5.7 |
5.7 |
5.7 |
5.7 |
0.029 |
0.029 |
19.3 |
19.3 |
9,240 |
9,220 |
55.2 |
55.2 |
|
5 |
7.2 |
7.2 |
7.2 |
7.2 |
1.9 |
1.9 |
1.2 |
1.2 |
0.005 |
0.005 |
14.3 |
14.3 |
227 |
215 |
18.1 |
18.1 |
|
6 |
7.0 |
7.0 |
7.0 |
7.0 |
2.4 |
2.4 |
1.6 |
1.6 |
0.007 |
0.007 |
17.0 |
17.0 |
764 |
721 |
20.6 |
20.6 |
|
7 |
6.3 |
6.3 |
6.2 |
6.3 |
3.4 |
3.4 |
2.4 |
2.4 |
0.012 |
0.012 |
20.4 |
20.5 |
4,390 |
4,140 |
27.0 |
26.9 |
|
8 |
5.1 |
5.1 |
5.1 |
5.1 |
3.8 |
3.8 |
3.0 |
3.0 |
0.014 |
0.014 |
18.9 |
18.9 |
6,570 |
6,330 |
32.0 |
32.0 |
|
9 |
3.8 |
3.8 |
3.8 |
3.8 |
5.1 |
5.1 |
4.8 |
4.8 |
0.025 |
0.025 |
17.1 |
17.1 |
12,100 |
12,000 |
47.7 |
47.7 |
|
10 |
2.2 |
2.2 |
2.5 |
2.5 |
8.3 |
8.3 |
8.6 |
8.6 |
0.048 |
0.048 |
11.7 |
11.7 |
58,700 |
58,600 |
83.4 |
83.3 |
|
11 |
3.1 |
3.1 |
3.4 |
3.4 |
5.2 |
5.0 |
3.7 |
3.7 |
0.013 |
0.013 |
10.5 |
10.6 |
170,000 |
153,000 |
31.5 |
31.0 |
|
Notes: 1.
Base refers to the
Pre-Commissioning scenario, and Comp
refers to the Commissioning scenario. 2.
All values are depth averaged, unless otherwise specified. 3.
DO values are 10%ile. 4.
BOD, TIN, NH3-N, Chlorophyll-a and SS are arithmetic mean values. 5.
E. coli are geometric
mean values. |
||||||||||||||||
Contour plots of the water quality parameters
are contained in Annex B2. The plots show that the major
contributor to the water quality conditions in the inner part of Deep Bay is
the outflow from the Shenzhen River.
In general, water quality conditions within Deep Bay are worse in the
wet season, compared to the dry season.
The contour plots show that there are minimal differences between the
Pre-Commissioning and Commissioning scenarios, which reflects the results shown
in Tables 5.6a and 5.6b.
Overall, the results of the water quality
modelling predict that there will be small improvements in the water quality of
Deep Bay in the vicinity of the discharge point YL2, which is at the mouth of
the Shan Pui River.
The operation of the Stage 1 works will result
in an improvement in the water quality within the streams and watercourses
adjacent to the areas to be connected to the new sewerage system. This is because currently untreated sewage
effluent discharges will be connected to the Yuen Long STW. Based on the data contained in the Technical Note on Water Quality Modelling Methodology, contained in Annex B1, the reductions
in pollutant loads due to the connection of currently unsewered domestic
premises are estimated to be as follows:
·
Suspended
solids – 320 kg day-1;
·
BOD –
340 kg day-1;
·
COD –
730 kg day-1;
·
TKN –
70 kg day-1;
·
Ammonia
– 40 kg day-1; and
·
E. coli – 3.5x1014 cfu day-1.
These loads represent approximately 10% of the
total pollutant loads to the inland waterways. It may thus be expected that there will be at least a 10%
improvement in water quality within the affected waterways.
Impacts to water quality may occur due to the
uncontrolled discharge of untreated effluent to inland waterways in the event
of failure of the pumping stations.
As discussed in Section 5.5.2,
the risk of potential impacts may be minimised through the implementation of
mitigation measures in the design and operation of the pumping stations. These measures, which would serve to
minimise the risk of failure and facilitate a rapid response in the event of
failure, are described in Section 5.7.2.
5.7
Mitigation of Adverse Environmental Impacts
5.7.1
Construction
Phase
Construction phase mitigation
measures to prevent the uncontrolled discharge of wastewater from the
construction site, in accordance with Practice
Note for Professional Persons on Construction Site Drainage, Professional Persons
Environmental Protection Department, 1994 (ProPECC PN 1/94) include the use
of sediment traps, wheel washing facilities for vehicles leaving the site,
adequate maintenance of drainage systems to prevent flooding and overflow,
sewage collection and treatment, and comprehensive waste management
(collection, handling, transportation, disposal) procedures.
Construction
Runoff and Drainage
The following measures are
recommended for reducing the potential for impacts to water quality from
construction run off and site drainage:
·
At the
start of site establishment, perimeter cut-off drains to direct off-site water
around the site should be constructed and internal drainage works and erosion
and sedimentation control facilities implemented. Channels, earth bunds or sand bag barriers should be
provided on site to direct stormwater to silt removal facilities. The design of the temporary on-site
drainage system will be undertaken by the contractor prior to the commencement
of construction.
·
The
design of efficient silt removal facilities should be based on the guidelines
in Appendix A1 of ProPECC PN 1/94,
which states that the retention time for silt/sand traps should be 5 minutes
under maximum flow conditions.
Sizes may vary depending upon the flow rate, but for a flow rate of 0.1
m3 s-1 a sedimentation basin of 30m3 would be
required and for a flow rate of 0.5 m3 s-1 the basin
would be 150 m3. The
detailed design of the sand/silt traps will be undertaken by the contractor
prior to the commencement of construction.
·
Ideally,
construction works should be programmed to minimise surface excavation works
during the rainy season (April to September). All exposed earth areas should be completed and vegetated as
soon as possible after earthworks have been completed, or alternatively, within
14 days of the cessation of earthworks where practicable. If excavation of soil cannot be avoided
during the rainy season, or at any time of year when rainstorms are likely,
exposed slope surfaces should be covered by tarpaulin or other means.
·
The
overall slope of the site should be kept to a minimum to reduce the erosive
potential of surface water flows, and all trafficked areas and access roads
protected by coarse stone ballast.
An additional advantage accruing from the use of crushed stone is the
positive traction gained during prolonged periods of inclement weather and the
reduction of surface sheet flows.
·
All
drainage facilities and erosion and sediment control structures should be
regularly inspected and maintained to ensure proper and efficient operation at
all times and particularly following rainstorms. Deposited silt and grit should be removed regularly and
disposed of by spreading evenly over stable, vegetated areas.
·
Measures
should be taken to minimise the ingress of site drainage into excavations. If the excavation of trenches in wet
periods is necessary, they should be dug and backfilled in short sections
wherever practicable. Water pumped
out from trenches or foundation excavations should be discharged into storm
drains via silt removal facilities.
·
Open
stockpiles of construction materials (for example, aggregates, sand and fill
material) of more than 50 m3 should be covered with tarpaulin or
similar fabric during rainstorms.
Measures should be taken to prevent the washing away of construction
materials, soil, silt or debris into any drainage system.
·
Manholes
(including newly constructed ones) should always be adequately covered and
temporarily sealed so as to prevent silt, construction materials or debris
being washed into the drainage system and storm runoff being directed into foul
sewers.
·
Precautions
to be taken at any time of year when rainstorms are likely, actions to be taken
when a rainstorm is imminent or forecasted, and actions to be taken during or
after rainstorms are summarised in Appendix
A2 of ProPECC PN 1/94.
Particular attention should be paid to the control of silty surface
runoff during storms events, especially for areas located near steep slopes.
·
All
vehicles and plant should be cleaned before leaving a construction site to
ensure no earth, mud, debris and the like is deposited by them on roads. An adequately designed and sited wheel
washing bay should be provided at every site exits and wash-water should have
sand and silt settled out and removed at least on a weekly basis to ensure the
continued efficiency of the process.
The section of access road leading to, and exiting from, the wheel-wash
bay to the public road should be paved with sufficient backfall toward the
wheel-wash bay to prevent vehicle tracking of soil and silty water to public
roads and drains.
·
On-site
drainage system should be equipped with oil interceptors to separate oil / fuel
from contaminated storm water.
General
Construction Activities
The following measures are
recommended for reducing the potential for general construction waste impacts
to water quality:
·
Construction
solid waste, debris and rubbish on site should be collected, handled and
disposed of properly to avoid water quality impacts. Requirements for solid waste management are detailed in Section 6.7 of this Report.
·
All
fuel tanks and storage areas should be provided with locks and sited on sealed
areas, within bunds of a capacity equal to 110% of the storage capacity of the
largest tank to prevent spilled fuel oils from reaching water sensitive
receivers nearby.
Sewage
Effluent from Construction Work Force
The following measures are recommend
for reducing the potential for impacts to water quality from sewage effluent
from construction work force.
Construction work force sewage should be
handled by portable chemical toilets or sewage holding tanks with the sewage
regularly collected by a reputable sewage collector for disposal at for example,
Shek Wu Hui STW.
MDC and Stream Crossings
The methods of construction of the crossings of
the main drainage channels and small streams will prevent adverse impacts to
water quality, as discussed in Section
5.5.1. However, if there are
equipment malfunction with the tunnelling machine when crossings of the main
drainage channels then a rescue pit may need to be excavated, which could have
potential impacts to water quality.
If a rescue pit is necessary then the following mitigation measures should
be implemented during excavation.
·
Excavation
should be undertaken during periods of low tide, when the water level within
the main drainage channel is at it lowest; and
·
If
excavation during periods of high rainfall and/or at times other than low tide
is unavoidable then silt curtains should be deployed around the excavation
machine to prevent the transport of sediment in suspension beyond the works
area.
At smaller stream crossings where diversion of
the existing flows is required through a temporary channel it is recommended
that the temporary channel be lined with shotcrete in order prevent erosion of
the channel introducing increased suspended solids into downstream stretches of
the stream. Diversion through
temporary pipes would prevent potential impacts of this form. Diversion of low flows by submersible
pumps would not cause adverse impacts as it would simply mean bypassing of the
existing stream water around the works area, without introducing additional
pollutants. It is recommended, on
a precautionary principal, that silt traps be installed either at the inlets or
outlets of the submersible pumps to prevent any suspended solids generated in
the vicinity of the works area being introduced into the downstream stretches
of the stream.
Material excavated from the open trench
crossings of streams and from any rescue pits is likely to be stockpiled for
disposal. This material should be
covered with impermeable material and placed on an impermeable liner in order
to prevent rainfall eroding the material leading to stormwater runoff with high
suspended sediment concentrations.
The disposal of excavated materials are discussed in Section 6.5.2.
5.7.2
Operation
Phase
The normal operation of the Stage 1 works has
been shown to result in an improvement in water quality in both the inland and
marine waters of the Deep Bay WCZ and thus no mitigation measures will be
required.
The following measures should be implemented to
reduce the risk of failure of the pumping stations which would result in an emergency
discharge of untreated sewage effluent.
Should a failure occur the intention is to minimise the duration.
·
The
overflow bypass should be operated only in an emergency, such as prolonged
power failure. Overflow must not
occur on a regular basis such as to facilitate routine maintenance.
·
Standby
pump should be provided to facilitate maintenance and repairing of equipment;
·
Dual
(back-up) power supply should be provided. Dual power supply could be in the format of ring main, or an
automatic-operated emergency generator with sufficient capacity to cope with
the demand loading of the essential plant equipment;
·
If the
pumping station is unmanned, a telemetry system should be provided to the
nearest manned station/plant so that swift actions could be taken in the case
of malfunction of the unmanned facilities;
·
Hand-cleaned
screens should be provided at the overflow bypass to prevent the discharge of
floating solids into receiving water bodies. The clear spacing of the bar screen should normally be about
25mm;
·
The
discharge point of the overflow bypass should be below the low water mark; and
·
The
discharge point of the overflow bypass should be away from sensitive receivers
such as gazetted beaches, mariculture zones, seawater intakes, water gathering
grounds, country parks, marinas, boat parks, nature reserves, sites of special
scientific interest, marine parks/marine reserves, streams with water for human
consumption…etc, and water with low assimilative capacity such as typhoon
shelter or embayed water. In this
connection, the location of the overflow bypass should be provided by the
project proponent at detail design stage for DEP approval.
5.8
Residual Environmental Impacts
5.8.1
Construction
Phase
No
residual water quality impacts were predicted to occur due to construction of
the Stage 1 works provided the above described mitigation measures are
implemented.
5.8.2
Operation
Phase
The operation of the Stage 1 works was
predicted to result in improvements to the water quality within the Deep Bay
WCZ. Residual water quality
impacts due to uncontrolled discharge of effluents from pumping station failure
were not predicted, provided the mitigation measures described in Section 5.7.2 are implemented.
5.9
Environmental Monitoring and Audit
5.9.1
Construction
Phase
The majority of construction of sewers/rising mains and pumping stations will be land based activities. Any construction activities across natural streams will be several km away from Deep Bay. Construction activities across the MDC will use the pipe jacking method ie boring a tunnel for the sewers underneath the river bed of the MDC. Unlike the conventional dredging method for laying the sewers, this method will avoid the release of river sediments under normal operating conditions. It is not envisaged that adverse water quality impacts will arise, especially with the implementation of the recommended mitigation measures. No monitoring of water quality would be required during the construction phase. It is recommended that regular audits of the implementation of the specified mitigation measures, as described in Section 5.7.1, be carried out during the construction phase.
Any wastewater
discharges from the construction sites will require a WPCO discharge licence to be issued. It may be that there will be a requirement for monitoring
the quality/quantity of the discharges to show compliance with the conditions
of the licence. Such monitoring,
however, would not form part of the EM&A programme.
5.9.2
Operation
Phase
No monitoring of water quality would be required
during the operational phase of the Designated and Potentially Designated
Elements.
Routine monitoring of the quality of the
treated sewage effluent from the Yuen Long STW is currently being carried out
by the Drainage Services Department (DSD) in order to satisfy the conditions of
the WPCO discharge licence. Such monitoring would continue
following the upgrade/expansion of the STW. It should be noted that prior to the operation of the Stage
1 works a new discharge licence will be required. Such monitoring, however, would not form part of the
EM&A programme for the Stage 1 works.
5.10
Summary and Conclusions
This Section has addressed the potential
impacts on water quality from the construction and operation of the Designated
and Potentially Designated Elements of the Project.
The construction phase assessment considered
the following aspects.
·
the
potential impacts to inland water quality from the land based construction
activities; and
·
the
potential impacts to inland water quality from crossings of the main drainage
channels and streams.
Potential impacts to surface water quality due
to land based construction activities would primarily occur due to surface
run-off and wastewater generation from within the construction sites, including
sewage effluent from the workforce.
The potential impacts may be readily controlled by on-site mitigation
measures, which were specified in detail.
The proposed construction methods for the
crossings of the main drainage channels and streams would be sufficient to
avoid adverse impacts to water quality.
However, in case of the possibility of excavation works in the main
drainage channel mitigation measures were specified to prevent adverse impacts
to water quality.
The
operation phase assessment considered the following aspects.
·
The
potential impacts to inland and marine water quality due to the connection of
currently unsewered areas to the Yuen Long STW; and
·
The
potential impacts to inland and marine water quality due to uncontrolled effluent
discharges due to failure of pumping stations.
Detailed
water quality modelling was undertaken to determine the effects on marine water
quality of decreasing the discharge of untreated sewage effluent to inland
waterways through connection to the Yuen Long STW and the subsequent increases
in the treated effluent flows from the Yuen Long STW. The results of the water quality modelling determined that
there would be improvements in marine water quality in the vicinity of the
mouth of the Shan Pui River, where the Yuen Long STW discharges. An assessment of the impacts of the
Stage 1 works on inland water quality determined that there would be a 10%
reduction in polluting discharges to the inland waterways and that there would
thus be an improvement in water quality.
Mitigation measures were defined to reduce the
risks of failure of pumping stations and to ensure that timely responses are
initiated in cases of failure in order to prevent adverse impacts to water
quality due to the discharge of untreated sewage effluent.
No monitoring of water quality would be
required during either the construction or operation phases. It was recommended that regular audits
of the implementation of the specified mitigation measures be carried out
during the construction of the Project.
