5.1.1
This section presents an assessment of
the potential water quality impacts associated with the construction and
operation phases of the proposed drainage improvement works. The key environmental issue in respect of
water quality impacts associated with the Project would be the excavation works
required for the drainage improvement to Pak Ngan Heung River, Tai Tei Tong
River, Luk Tei Tong River and Luk Tei Tong By-pass Channel . Recommendations for mitigation measures have
been made, where necessary, to reduce the identified water quality impacts to
an acceptable level.
Water
Pollution Control Ordinance (WPCO)
5.2.1
The Water Pollution Control Ordinance
(Cap. 358), in existence since 1980, 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 the WCZ based on their beneficial uses. The study area is located within the
Southern WCZ.
5.2.2
River water quality is ranked
according to its compliance with the WQOs.
The annual compliance of the monthly water quality data with each of the
water quality parameters is indicated by a percentage, with 100% compliance indicating
that all 12 months’ data are within the WQO.
Details of the WQOs statement for the inland waters of the Southern WCZ
are listed in Table 5.1.
Table 5.1 Water Quality Objectives for Inland
Waters
|
Parameters
|
Water Quality Objectives
(WQOs)
|
Water Control Zones
(WCZs) or sub-zones to which the WQOs apply
|
|
PH
|
6.5 – 8.5
|
Mui Wo (C), (E) sub-zones
|
|
6.0 – 9.0
|
Other inland waters except Mui Wo (A), (B), (D), (F) sub-zones
|
|
Suspended solids (SS)
|
Annual median £ 20 mg/L
|
Mui Wo (C), (E) sub-zones
|
|
Annual median £ 25 mg/L
|
Other inland waters except Mui Wo (A), (B), (D), (F) sub-zones
|
|
Dissolved oxygen (DO)
|
³ 4 mg/L
|
All parts of WCZ
|
|
Chemical oxygen demand (COD)
|
£ 30 mg/L
|
All parts of WCZ
|
|
5-day biochemical oxygen demand (BOD)
|
£ 5 mg/L
|
All parts of WCZ
|
Technical
Memorandum
5.2.3
Besides setting the WQOs, the WPCO
controls effluent discharging into the WCZs through a licensing system. A Technical
Memorandum (TM) on Standards for Effluents Discharged into Drainage and
Sewerage Systems, Inland and Coastal Waters was issued under the WPCO which
gives guidance on the permissible effluent discharges based on the type of
receiving waters (foul sewers, storm water drains, inland and coastal waters).
The limits control the physical, chemical and microbial quality of
effluents. Under the TM, effluents
discharged into inland waters are subject to standards for particular volumes
of discharge. The discharge standard
will vary with the use of downstream water.
The majority of the inland water bodies within the study area are used
for agricultural purposes, which are classified as Group B inland waters. Any new effluent discharges during the
construction phase would be required to comply with the terms and conditions of
a discharge licence, issued by EPD, under the WPCO.
Practice
Notes
5.2.4
A practice note (PN) for professional
persons was issued by the EPD to provide environmental guidelines for handling and
disposal of construction site discharges.
The ProPECC PN 1/94 “Construction Site Drainage” provides good practice
guidelines for dealing with various types of discharge from a construction
site. Practices outlined in the PN
should be followed as far as possible during construction to minimize the water
quality impact due to construction site drainage.
5.3.1
The water quality in the Pak Ngan
Heung River, Tai Tei Tong River, Luk Tei Tong River and Luk Tei Tong (2) River
is monitored under the EPD routine river water quality monitoring programme (Figure 5.1). The three rivers are tributaries of the
Silver River (also named the Mui Wo River).
A summary of the published EPD monitoring data collected in these three
rivers is presented in Table 5.2.
River water quality monitoring data for the year 2002 at stations MW1,
MW2 and MW4 showed full compliance with the WQOs of pH, suspended solids,
dissolved oxygen, BOD5 and COD.
A fairly high E. coli level was reported at MW4 and high levels
at stations MW1 and MW2. It was
reported that the high bacterial levels could be attributed to sullage and
occasional overflow from septic tanks as well as surface runoff from nearby
villages.
5.3.2
A description of the physical characteristics
of Pak Ngan Heung River, Tai Tei Tong River, Luk Tei Tong River and Luk Tei
Tong (2) River as observed during the ecological surveys conducted from July
2003 to March 2004 is presented in Section 7.
Table 5.2 Summary of Water Quality Monitoring
Data for Tributaries of Mui Wo River in 2002
|
Parameter
|
Unit
|
Sampling Station
|
|
MW1
|
MW2
|
MW4
|
|
Dissolved
oxygen
|
mg/L
|
8.5
(7.6 – 11.4)
|
9.2
(6.8 – 10.9)
|
8.2
(6.0 – 10.6)
|
|
pH
|
|
7.1
(6.7 – 7.9)
|
7.6
(7.2 – 8.0)
|
7.2
(6.7 – 8.0)
|
|
Suspended
solids
|
mg/L
|
2
(1 – 4)
|
5
(4 – 13)
|
9
(3 – 15)
|
|
5-day Biochemical Oxygen Demand
|
mg/L
|
1
(1 – 3)
|
3
(1 – 5)
|
1
(1 – 3)
|
|
Chemical
Oxygen Demand
|
mg/L
|
3
(2 – 12)
|
11
(2 – 15)
|
10
(2 – 14)
|
|
Oil &
grease
|
mg/L
|
0.5
(0.5 – 0.6)
|
0.5
(0.5 – 0.6)
|
0.5
(0.5 – 0.5)
|
|
Faecal
coliforms
|
cfu/100mL
|
6,300
(500 –
39,000)
|
24,000
(2,300 –
440,000)
|
3,700
(490 –
16,000)
|
|
E.coli
|
cfu/100mL
|
1,600
(180 –
22,000)
|
4,800
(520 –
87,000)
|
980
(160 –
6,300)
|
|
Ammonia-nitrogen
|
mg/L
|
0.04
(0.01 –
0.54)
|
0.48
(0.03 –
1.80)
|
0.15
(0.08 –
0.26)
|
|
Nitrate-nitrogen
|
mg/L
|
0.23
(0.14 –
0.43)
|
0.14
(0.07 –
0.25)
|
0.15
(0.06 –
0.43)
|
|
Total
Kjeldahl nitrogen, SP
|
mg/L
|
0.15
(0.07 –
0.79)
|
0.71
(0.15 –
2.20)
|
0.32
(0.18 –
0.49)
|
|
Ortho-phosphate
|
mg/L
|
0.11
(0.03 –
0.25)
|
0.11
(0.04 –
0.26)
|
0.04
(0.02 –
0.16)
|
|
Total
phosphorus, SP
|
mg/L
|
0.15
(0.04 –
0.27)
|
0.18
(0.07 –
0.39)
|
0.09
(0.07 –
0.24)
|
|
Sulphide, SP
|
mg/L
|
0.02
(0.02 –
0.02)
|
0.02
(0.02 –
0.02)
|
0.02
(0.02 –
0.02)
|
|
Flow
|
L/s
|
31
(3 – 621)
|
NM
|
216
(40 – 1,014)
|
Notes:
1.
Data presented are in
annual medians of monthly samples, except those for faecal coliforms and E.coli
which are in annual geometric means.
2.
Figures in brackets are
annual ranges.
3.
SP – soluble and
particulate fractions (i.e. total value).
4.
NM indicates no
measurement taken
5.4.1
Identified water sensitive receivers
(WSR) within the assessment area are listed below:
·
Abandoned fish ponds at Luk Tei Tong;
·
Freshwater marsh at Luk Tei Tong;
·
Silver River; and
·
Silver Mine Bay Beach.
5.4.2
The Pak Ngan Heung River, Tai Tei Tong
River, Luk Tei Tong River and Luk Tei Tong (2) River discharge to the
three-river confluence at the head of the Silver River. The Silver River discharges to Silver Mine
Bay. There are no water catchments or
water gathering grounds located within the assessment area. The location of the abandoned fish ponds,
freshwater marsh and Silver River is shown on Figure 7.1.
5.5.1
The assessment area for the water
quality impact assessment is defined in the EIA Study Brief as all areas within
500 m from the project boundary.
5.5.2
The water sensitive receivers that may
be affected by the construction works for the river channel drainage
improvement were identified. The
proposed method of construction and operational activities of the Project were
reviewed, and potential sources of water quality impact that may arise during
the construction and operation phases were described. This task included identifying pollutants from point discharges
and non-point sources to surface run-off.
All the identified sources of potential water quality impact were then
evaluated and their impact significance determined. The need for mitigation measures to reduce any identified adverse
impacts on water quality to acceptable levels was determined.
Construction
Phase
5.6.1
Potential sources of water quality
impact associated with the construction of the proposed drainage improvement
works at Pak Ngan Heung River, Tai Tei Tong River, Luk Tei Tong River and Luk
Tei Tong By-pass Channel have been identified and include:
·
construction site runoff and drainage;
·
general construction activities; and
·
sewage effluent produced by on-site
workforce.
5.6.2
Specific to the proposed drainage
improvement works for the construction of the river channels would be potential
impacts on water quality from the excavation works within the existing rivers.
Construction
Site Runoff and Drainage
5.6.3
Runoff from the construction works
areas may contain increased loads of sediments, other suspended solids and
contaminants. Potential sources of
pollution from site drainage include:
·
runoff and erosion from exposed soil
surfaces, earth working areas and stockpiles;
·
release of grouting and cement
materials with rain wash;
·
wash water from dust suppression
sprays; and
·
fuel and lubricants from maintenance
of construction vehicles and mechanical equipment.
5.6.4
Sediment laden runoff during
construction works for the drainage improvements to Pak Ngan Heung River, Tai
Tei Tong River, Luk Tei Tong River and Luk Tei Tong By-pass Channel if
uncontrolled may carry pollutants (adsorbed onto the particle surfaces) into
the downstream sections of these rivers and the receiving waterbody of Silver
River. Associated effects which may
arise include increased suspended solids concentrations in the water
bodies. Similarly, sediment laden
runoff during construction works for the proposed drainage improvements at Luk
Tei Tong River and Luk Tei Tong By-pass Channel may enter the nearby freshwater
marshes and the abandoned fish ponds if uncontrolled. Mitigation measures should be implemented to control construction
site runoff and drainage from the works areas, and to prevent runoff and
drainage water with high levels of suspended solids from entering the Luk Tei
Tong marshes and fish ponds and the downstream receiving waterbody of Silver
River.
5.6.5
The Silver River discharges into
Silver Mine Bay and therefore the implementation of mitigation measures would
also be required to prevent polluted discharges of site runoff and drainage
water from entering the bay. With the
implementation of adequate construction site drainage and provision of silt
removal facilities as described in Section 5.7.2, it is not anticipated that
adverse water quality impacts would arise at the Silver Mine Bay Beach.
General
Construction Activities
5.6.6
On-site construction activities may
cause water pollution from the following:
·
uncontrolled discharge of debris and
rubbish such as packaging, construction materials and refuse; and
·
spillages of liquids stored on-site,
such as oil, diesel and solvents etc, are likely to result in water quality
impacts if they enter the adjacent watercourses or storm water drains.
5.6.7
Good construction and site management
practices should be observed, as detailed in Sections 5.7.3 & 5.7.4, to ensure
that litter, fuels and solvents do not enter the river channels of Pak Ngan
Heung River, Tai Tei Tong River, Luk Tei Tong River and Luk Tei Tong (2) River,
the downstream receiving waters of Silver River and the local storm water
drainage system.
Sewage
Effluents
5.6.8
Domestic sewage would be generated
from the workforce during the construction phase. However, this temporary
sewage can be adequately treated by interim sewage treatment facilities, such
as portable chemical toilets, which can be installed within the construction
site. It is unlikely that sewage generated from the site would have a
significant water quality impact, provided that sewage is not discharged
directly to the river channels nor local storm water drainage system, and
chemical toilets are used and properly maintained.
River
Channel Excavation Works
5.6.9
In order to minimize the potential
impacts on water quality during the excavation works at the Pak Ngan Heung River,
Tai Tei Tong River, Luk Tei Tong River and Luk Tei Tong By-pass Channel, the
excavation works to widen the river channel would be restricted to within an
enclosed dry section of the channel. A
bund would be constructed in the middle of the channel to surround the area to
be excavated and the water flow diverted to one half of the channel. Excavation would be carried out in the dry
section of the channel. Upon completion,
excavation would be carried out in the other half of the channel by switching
the two ends of the bunds to enclose the area to be excavated. Figure 5.2 illustrates the proposed
working method for the excavation works in the river channel involving the
temporary drainage diversion. The flow area of the existing river would be
maintained. Excavation works would be
small-scale and carried out by land-based plant, and dredging would not be
required.
5.6.10
This works arrangement would provide a
dry zone for excavation works within the river channel and would prevent the
transportation of suspended sediment downstream. With the implementation of the working method controls as
described above, it is anticipated that unacceptable water quality impacts
would not arise at the downstream receiving water body of Silver River during
the channel excavation works.
5.6.11
The disposal of any slurry water would
need to comply with the TM on Effluent Discharge Standards under the WPCO.
Cumulative
Impacts
5.6.12
The identified concurrent projects
which involve construction activities during the construction period of the
river improvement works are:
·
Outlying Sewerage Stage 2 – Mui Wo Village Sewerage
(Package 1A) (late 2005 – mid 2007).
·
Construction of Emergency Vehicular Access (EVA) and
Sewerage Works at Luk Tei Tong, Mui Wo (IS007) (late 2004 – late 2006).
5.6.13
The proposed sewerage works under the
Mui Wo Village Sewerage would take place in the vicinity of the Pak Ngan Heung
River. The village sewerage works would
be small-scale and involve the laying of sewer pipelines. It is anticipated that with the
implementation of mitigation measures to control construction site runoff and
drainage, potential impacts on water quality would be controlled within the
site boundary of the village sewerage works.
Therefore, there is very limited potential for cumulative impacts on
water quality to result from the land-based construction activities of this
concurrent project.
5.6.14
The proposed EVA and sewerage works at
Luk Tei Tong are scheduled for completion in mid 2006, which coincides with the
commencement of site clearance works for the proposed drainage improvement
works. With the implementation of
mitigation measures to control construction site runoff and drainage, there is
limited potential for cumulative water quality impacts to result from the minor
construction activities of this concurrent project.
Operation
Phase
5.6.15
The potential for water quality
impacts during the operation phase of the proposed drainage improvement works
may result from the following:
·
Maintenance desilting of re-profiled
river channel of Pak Ngan Heung River, Tai Tei Tong River, Luk Tei Tong River
and Luk Tei Tong By-pass Channel.
·
Hydrological disturbance to Luk Tei
Tong marsh.
5.6.16
Maintenance desilting of the river
channels would be carried out by land-based plant or labourers and would be
substantially less intensive than the construction phase excavation works. The channel desilting works would be
confined in a small works zone which is isolated from the rest of the channel
by temporary barrier walls. This
arrangement would provide a dry zone for desilting work and would prevent the
transportation of suspended sediment downstream. Furthermore, desilting works would be carried out in the dry
season when the river flow is low to further minimize the potential for impacts
on downstream water quality.
5.6.17
Hydrological disturbance to Luk Tei
Tong marsh would be expected to be minor, as discussed in Section 7.7.29. The
Luk Tei Tong By-pass Channel (which would run through the marsh) would be only
slightly lower (approximately 800mm) than the surrounding area of the marsh,
and no substantial ‘draining down’ of the marsh would be anticipated.
5.6.18
During the operation phase, the river
channel reprofiling works to the Pak Ngan Heung River, Tai Tei Tong River, Luk
Tei Tong River and Luk Tei Tong By-pass Channel would not produce extra
pollution loading to the flow of these three rivers. The new alignments of the
three rivers would provide widened sections to alleviate flooding within the
Mui Wo area during heavy rainfall.
Under the proposed village sewerage improvement works at Luk Tei Tong,
Tai Tei Tong and Pak Ngan Heung, the sewage flows from the unsewered villages
will be collected and conveyed to the sewage treatment plant at Mui Wo. It is anticipated that the water quality of
the three rivers would improve in the future following the implementation of
the village sewerage scheme.
5.7.1
Proposed mitigation measures for containing
and minimizing water quality impacts are summarised below.
Construction
Phase
Construction
Site Run-off and Drainage
5.7.2
The site practices outlined in ProPECC
PN 1/94 “Construction Site Drainage” should be followed as far as practicable during
the drainage improvement works in order to minimise surface runoff and the
chance of erosion, and also to retain and reduce any suspended solids prior to
discharge. These practices include the
following items:
·
Before commencing any site formation work,
all sewer and drainage connections should be sealed to prevent debris, soil,
sand etc. from entering public sewers/drains.
·
Temporary ditches should be provided
to facilitate run-off discharge into appropriate watercourses, via a silt
retention pond. No site run-off should
enter the freshwater marshes at Luk Tei Tong.
·
Sand/silt removal facilities such as
sand traps, silt traps and sediment basins should be provided to remove
sand/silt particles from runoff to meet the requirements of the Technical Memorandum
standard under the Water Pollution Control Ordinance. The design of silt removal facilities should be based on the
guidelines provided in ProPECC PN 1/94.
All drainage facilities and erosion and sediment control structures
should be inspected monthly and maintained to ensure proper and efficient
operation at all times and particularly during rainstorms.
·
Water pumped out from foundation
excavations should be discharged into silt removal facilities.
·
During rainstorms, exposed slope
surfaces should be covered by a tarpaulin or other means. Other measures that need to be implemented
before, during, and after rainstorms are summarized in ProPECC PN 1/94.
·
Exposed soil areas should be minimized
to reduce potential for increased siltation and contamination of runoff.
·
Exposed soil surfaces should be
protected by paving or fill material as soon as possible to reduce the
potential of soil erosion.
·
Open stockpiles of construction
materials or construction wastes on-site of more than 50 m3 should
be covered with tarpaulin or similar fabric during rainstorms
General
Construction Activities
5.7.3
Debris and refuse generated on-site
should be collected, handled and disposed of properly to avoid entering the river
channels, the Luk Tei Tong freshwater marshes and local storm water
drains. Stockpiles of cement and other
construction materials should be kept covered when not being used.
5.7.4
Oils and fuels should only be used and
stored in designated areas which have pollution prevention facilities. To prevent spillage of fuels and solvents to
nearby water bodies, all fuel tanks and storage areas should be provided with
locks and be sited on sealed areas, within bunds of a capacity equal to 110% of
the storage capacity of the largest tank.
The bund should be drained of rainwater after a rain event.
Sewage
from Construction Workforce
5.7.5
Temporary sanitary facilities, such as
portable chemical toilets, should be employed on-site. A licensed contractor would be responsible
for appropriate disposal and maintenance of these facilities.
River
Channel Excavation Works
5.7.6
The excavation and widening works for
the drainage improvements to the Pak Ngan Heung River, Tai Tei Tong River, Luk Tei
Tong River and Luk Tei Tong By-pass Channel should be carried out in sections
(approximately 300 – 400 m in length) and in dry condition. Containment measures such as bunds and
barriers should be used within the river channel and the excavation works
restricted to within an enclosed dry section of the channel. The excavation and widening works should be
carried out in the dry season as far as practicable.
Operation
Phase
5.7.7
For maintenance desilting of the
re-profiled river channels of the Pak Ngan Heung River, Tai Tei Tong River, Luk
Tei Tong River and Luk Tei Tong By-pass Channel, temporary barrier walls should
be used to provide a dry zone for desilting work. Maintenance desilting should be carried out during periods of low
flow in the dry season.
5.8.1
With the full implementation of the
recommended mitigation measures for the construction and operation phases of
the proposed Project, no unacceptable residual impacts on water quality are
anticipated. It is recommended that
regular audits of the implementation of the recommended mitigation measures at
all work areas for the drainage improvements to the Pak Ngan Heung River, Tai
Tei Tong River, Luk Tei Tong River and Luk Tei Tong By-pass Channel be carried
out during the construction phase.
5.9.1
The water quality assessment has
identified that the key issue in terms of water quality would be related to
excavation works for the reprofiling of the river channels. To minimize potential
impacts on water quality, the excavation works would be carried out in dry
condition through the use of the recommended working method controls. Given that Pak Ngan Heung River, Tai Tei
Tong River and Luk Tei Tong River discharge to the three-river confluence at
the head of the Silver River which ultimately discharges to Silver Mine Bay,
monitoring of downstream water quality within the rivers is recommended during
the construction phase. In addition,
it is recommended that regular site audits be undertaken to inspect the
construction activities at all works areas to ensure the recommended mitigation
measures are properly implemented.