6.1.1
Water Pollution Control
Ordinance
6.1.1.1 The Water Pollution Control Ordinance 1980 (WPCO) (Cap. 358) is the principal legislation governing marine water quality in Hong Kong. Under the provisions of this Ordinance Hong Kong’s waters have been divided into a series of Water Control Zones. Water Quality Objectives (WQOs) have been declared to protect the specific beneficial uses and conservation goals of each of the zones.
6.1.1.2 The proposed PAFF at Tuen Mun Area 38, the proposed submarine pipeline routes and the HKIA all lie within the North Western Waters Water Control Zone (NWW WCZ). Marine waters within the NWW WCZ are identified as having the following beneficial uses :
¨ a source of food for human consumption;
¨ a commercial fisheries resource;
¨ a habitat for marine organisms generally;
¨ recreational bathing;
¨ secondary contact recreation including diving, sailing and windsurfing;
¨ domestic and industrial supply;
¨ navigation and shipping; and
¨ aesthetic enjoyment.
6.1.1.3 Subsequent to the Gazettal of the NWW WCZ the waters around Lung Kwu Chau and Sha Chau have been declared a Marine Park primarily to protect marine mammals which are prevalent in this part of Hong Kong. Also the NWW WCZ is known to support a small coral population. Protection of these species represents key conservation goals for the Hong Kong Government.
6.1.1.4 Relevant Marine WQOs applicable to the NWW WCZ are summarised in Table 6.1 below.
Table 6.1. Water Quality Objectives for the North Western Waters Water Control Zone
|
Water
Quality Objectives |
|
Aesthetic
Appearance Ø
There
should be no objectionable odours or discolouration of the water. Ø
Tarry
residues, floating wood, articles made of glass, plastic, rubber or any other
substances should be absent. Ø
Mineral oil
should not be visible on the surface. Ø
There
should be no recognisable sewage derived debris. Ø
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. Bacteria Ø The levels of Escherichia coli should not exceed 180 counts per 100 ml at
bathing beaches, calculated as the geometric mean of the 5 most recent
samples collected by EPD. Ø The levels of Escherichia coli should not exceed 610 counts per 100 ml at
secondary contact recreation sub-zones, calculated as the geometric annual
mean. Dissolved
Oxygen Ø
The depth
averaged concentration of dissolved oxygen should not fall below 4 mg/l for
90% of the sampling occasions during the whole year Ø
The
concentration of dissolved oxygen should not be less than 2 mg/l within 2m of
the seabed for 90% of the sampling occasions during the whole year. pH Ø
The pH of
the water should be within the range 6.5 – 8.5 units. Ø
Human
activity should not cause the natural pH range to be extended by more than
0.2 units. Temperature Ø
Waste
discharges shall not cause the natural daily temperature range to change by
more than 2.0oC. Salinity Ø Waste Discharges shall not cause the
natural ambient salinity to change by more than 10%. Suspended
Solids Ø
Human
activity should neither cause the natural ambient level to be raised by more
than 30% nor give rise to accumulation of suspended solids which may
adversely affect aquatic communities. Ammonia Ø The un-ionised ammoniacal nitrogen level
should not be more than 0.21 mg/l calculated as the annual average
(arithmetic mean). Nutrients Ø
Nutrients
should not be present in quantities sufficient to cause excessive or nuisance
growth of algae or other aquatic plants Ø
Without
limiting the generality of the above point, the level of inorganic nitrogen
should not exceed 0.5 mg/l, or 0.3 mg/l within Castle Peak sub-zone,
expressed as the annual water column average. Toxins Ø Waste discharges shall not cause the
toxins in water to attain such a level as to produce significant toxic,
carcinogenic, mutagenic or teratogenic effects in humans, fish or other
aquatic organisms, with due regard to biologically cumulative effects in food
chains and to interactions of toxic substances with each other. Ø Waste discharges shall not cause a risk
to any beneficial use of the aquatic environment. |
6.1.2
WPCO Technical Memorandum
on Effluent Discharges
6.1.2.1 Any polluting wastewater effluent from the PAFF and associated infrastructure either during construction or subsequent operations would require a discharge permit issued in accordance with the provisions of the WPCO. In setting conditions on required effluent quality the Authority would be guided by the Technical Memorandum issued under Section 21 of the Ordinance. The Technical Memorandum indicates maximum permissible concentrations for a comprehensive suite of pollutants depending on the point of discharge.
6.1.3
EIAO Technical Memorandum
on EIA Process
6.1.3.1 Annex 6 and Annex 14 of the EIAO Technical Memorandum stipulate Criteria for Evaluating Water Pollution and Guidelines for the Assessment of Water Pollution, respectively. These must be adhered to in the preparation of an EIA report submitted under the EIAO in pursuance of an Environmental Permit for the project.
6.1.4
Works Bureau Technical
Circular on Management of Dredged Sediment
6.1.4.1 No ambient sediment quality acceptability criteria equivalent to the Water Quality Objectives have been defined for Hong Kong. However an approach has been promulgated under Work Bureau Technical Circular 3/2000 for the purposes of assessing the disposal requirements for dredged sediment. The assessment requirements and waste management implications of this document are elaborated in more detail in Section 14 of this report. Within the Technical Circular two sets of chemical criteria are identified for potentially toxic sediment contaminants
¨ Lower Chemical Exceedance Level (LCEL); and
¨ Upper Chemical Exceedance Level (UCEL).
6.1.4.2 The LCEL is intended to represent levels below which adverse biological effects (or ecotoxicity) are considered to be unlikely. In contrast the UCEL is intended to represent a level beyond which adverse biological effects are considered likely to occur. These criteria therefore represent a convenient means to broadly characterise sediment quality and identify potentially polluted sediments that might be of concern if they are disturbed in the course of the Project. These two sets of criteria are presented in Table 6.2.
Table 6.2. WBTC 3/2000 Sediment Quality Criteria for the Classification of Sediment
|
Contaminants |
Lower Chemical Exceedance Level (LCEL) |
Upper Chemical Exceedance Level (UCEL) |
|
Metals (mg/kg dry
wt.) |
|
|
|
|
|
|
|
Cadmium (Cd) Chromium (Cr) Copper (Cu) Mercury (Hg) Nickel (Ni)* Lead (Pb) Silver (Ag) Zinc (Zn) |
1.5 80 65 0.5 40 75 1 200 |
4 160 110 1 40 110 2 270 |
|
|
|
|
|
Metalloid (mg/kg dry
wt.) |
|
|
|
|
|
|
|
Arsenic (As) |
12 |
42 |
|
|
|
|
|
Organic-PAHs (mg/kg dry wt.) |
|
|
|
|
|
|
|
Low
Molecular Weight PAHs |
550 |
3160 |
|
High
Molecular Weight PAHs |
1700 |
9600 |
|
|
|
|
|
Organic-non-PAHs (mg/kg dry wt.) |
|
|
|
|
|
|
|
Total
PCBs |
23 |
180 |
|
|
|
|
|
Organometallics (mg TBT/L in Interstitial
water) |
|
|
|
|
|
|
|
Tributyltin* |
0.15 |
0.15 |
|
|
|
|
6.1.5.1 Marine water pollution control is also effected by means of the Dumping at Sea Ordinance which covers the dumping of waste material, including dredged spoil, at sea. The implications of this are discussed separately in Section 14 of this report.
6.2.1.1 The North Western Waters are situated at the mouth of the Pearl River Estuary and as such are heavily influenced by the massive freshwater flows from the hinterland. There is a distinct seasonality. The estuarine influence is especially pronounced in the wet summer months when the freshwater flows are greatest and a strong salinity and temperature stratification is evident. During winter months water conditions are more typically marine and salinity and other parameters vary less with depth.
6.2.1.2 Water temperature ranges between about 150C and 300C over an annual cycle with a mean of about 220C to 230C. Salinity typically varies within the range 10 to 32 ppt.
6.2.1.3 The Pearl River carries very heavy loads of suspended sediment and nitrates and as a consequence concentrations of these parameters within North Western Waters are variable but generally far higher than in the more oceanic influenced waters to the south and east of Hong Kong.
6.2.2
Existing Pollution Sources
and Activities
6.2.2.1 The NWW WCZ contains several significant sewage outfalls (Pillar Point, Urmston Road, Siu Ho Wan) and cooling water discharges from a number of users including Castle Peak Power Station, Hong Kong International Airport (HKIA) and Shui Wing Steelworks.
6.2.2.2 In the past, dredging of marine mud and sand extraction has been extensive in the North Western Waters coastal area for reclamation projects including the proposed PAFF site at Tuen Mun Area 38, the River Trade Terminal, Tin Shui Wai New Town and the HKIA platform at Chek Lap Kok. Mud dredging to construct and maintain the navigation channel in the Urmston Road and the berthing area at Castle Peak Power Station is periodic and on-going and the present temporary Aviation Fuel Receiving Facility (AFRF) at Sha Chau required the construction of a navigation channel and berthing area, which is also subject to maintenance dredging. To date, no adverse environmental impacts have been reported for any of these previous dredging exercises which have involved the removal of much larger quantities of marine mud than will be required for excavation of the pipeline route.
6.2.2.3 Disposal of contaminated dredged material, which began in 1992, is also on-going at the Contaminated Mud Pits at East of Sha Chau. The next mud pits are already being formed north east of HKIA. Disposal of uncontaminated dredged material continues intermittently at the North Lantau Borrow pit.
6.2.2.4 Commercial trawling is undertaken over much of the North Western Waters and the Urmston Road is a very busy shipping channel for river trade vessels, high speed ferries, large coal vessels servicing Castle Peak Power Station and the existing temporary Aviation Fuel Receiving Facility delivery vessels.
6.2.2.5 The locations of the principal areas of seabed disturbance and the major sewage outfalls in the study area are indicated in Figure 6.1.
6.2.3.1 There are a number of potentially important sensitive receivers within the study area. These are illustrated in Figure 6.2. They include areas of ecological sensitivity and conservation importance areas, a commercial fishing resource, areas of direct human contact such as bathing beaches and various points where sea water is abstracted for domestic, commercial or industrial purposes. Notwithstanding the importance of protecting water quality generally throughout the NWW WCZ in accordance with the statutory Water Quality Objectives it is especially important to maintain the integrity of the water quality at these specific sites.
6.2.3.2 The Indo-Pacific humpback dolphin (Sousa chinesis) is frequently observed within the study area both in and around the Sha Chau and Lung Kwu Chau Marine Park. The North Western Waters of Hong Kong actually represent the eastern range of the Pearl River Estuary dolphin population which extends far into mainland Chinese waters.
6.2.3.3 Tuen Mun is home to a large offshore fishing fleet and the North Western Waters support an important spawning ground and commercial fishery industry for a variety of fish species and also Penaeid shrimps.
6.2.3.4 Other features of conservation concern in the wider study area include mangrove stands and seagrasses (Zostera japonica, Halophilia ovata and Halopila beccarii) at Tai Ho and along the Tung Chung Channel south of HKIA at Sha Lo Wan and San Tau. The area also provides preferred habitat for species of horseshoe crab (Tachypleus gigas, Tachypleus tridentatus) which have been seen near the beaches of Lung Kwu Tan, Lung Kwu Chau and The Brothers. Previous surveys in the Sha Chau area have identified the presence of the stone coral Faviidea as well as gorgonians and sea pens which are also of ecological interest.
6.2.3.5 The study area contains two ungazetted bathing beaches at Lung Kwu Tan as well as a number of Gazetted bathing beaches. Butterfly Beach is the nearest of these, located about 3.8 km to the east of the proposed PAFF site. Further east still can be found Castle Peak Beach, Kadoorie Beach, Cafeteria Old and New Beaches and Golden Beach. These have historically suffered from high sewage derived bacterial loads. However as a result of recent pollution enforcement activities and sewerage infrastructure improvements water quality at all beaches in the NWW WCZ is now deemed ‘fair’ according to EPD criteria and suitable for bathing.
6.2.3.6 As part of the mitigation for the temporary aviation fuel line at Sha Chau, artificial reefs have been deployed in the Sha Chau and Lung Kwu Chau Marine Park. These reefs are designed to enhance fisheries resources and promote feeding opportunities for the Chinese White Dolphins which frequent the area. In addition, the Hong Kong Jockey Club, with support from the Airport Authority, financed a project to deploy artificial reefs in the Chek Lap Kok Marine Exclusion Zone off the north-eastern corner of the HKIA.
6.2.3.7 There are a number of major seawater intakes in the study area serving Tung Chung new town, the HKIA and industrial users, particularly the Castle Peak Power Station and the Shui Wing Steelworks immediately to the west of Tuen Mun Area 38. For the Castle Peak Power Station intake there is a specific requirement that suspended sediment concentrations must be maintained below a level of 150 mg/l within a 5 km radius of the intake. This radius encompasses much of the northern sections of the proposed pipeline route, see Figure 6.2.
6.2.4
Baseline Water Quality
Conditions
6.2.4.1 Existing water quality and sediment quality in the North-western waters have been monitored for many years as part of the EPD Routine Monitoring Programme. Water quality is monitored bi-monthly at six stations within the NWW WCZ as shown in Figure 6.3. Of these, station NM6 lies close to the southern section of the existing pipeline. The stations NM3, NM5 and NM8 lie in convenient close proximity to the represent conditions within the likely area of influence of the Project. A summary of the data published by EPD for the NWW WCZ for 1999 and 2000 is presented in Appendix D.
6.2.4.2 Some temporal and spatial variability is evident in this dataset but compliance is usually observed with the key WQOs for dissolved oxygen, total inorganic nitrogen and unionised ammonia notwithstanding generally eutrophic conditions resulting from the heavy nutrient load carried by the Pearl River. During 1997 and 1998 there had been a detectable deterioration in water quality apparently reflecting a general regional trend. However the data for 1999 and 2000 are more representative of the longer-term data set.
6.2.4.3 Of particular relevance to this assessment, the EPD for these two years indicate that suspended solid concentrations typically lie in the range of 3 to 30 mg/l with the highest recorded value being 86.9 mg/l at the Urmston Road station NM5 in 1999. Compliance with the WQO for dissolved oxygen was achieved throughout despite occasional dips in depth average measurements below the compliance value of 4 mg/l at stations including NM3 and NM5 to the east and west of the project site, respectively. Oxygen supersaturation is observed, particularly at the southwestern most stations NM6 and NM8, again indicative of eutrophication predominantly influenced by the Pearl River discharge.
6.2.4.4 A summary of the data for suspended solids and dissolved oxygen during 1999 and 2000 is provided in Table 6.3 below.
Table 6.3
Summary
of EPD data for Suspended Sediments and Dissolved Oxygen in 1999 and 2000
|
Station |
SS mg/l Depth Av Mean |
SS mg/l Depth Av Range |
DO mg/l Depth Av Mean |
DO mg/l Depth Av Range |
DO mg/l Bottom Mean |
DO mg/l Bottom Range |
|
NM3 |
8.9 |
1.9 – 20.3 |
5.9 |
3.3 – 8.1 |
5.7 |
2.2 – 8.6 |
|
NM5 |
12.6 |
2.0 – 86.9 |
6.0 |
3.7 – 8.6 |
5.7 |
2.3 – 8.8 |
|
NM6 |
9.1 |
2.8 – 21.0 |
6.6 |
4.6 – 11.8 |
6.6 |
4.7 – 11.8 |
|
NM8 |
13.2 |
3.0 – 32.3 |
6.6 |
4.3 – 11.6 |
6.5 |
3.9 – 11.7 |
6.2.4.5 In addition to EPD’s long term programme, comprehensive water quality data sets have been obtained from various construction related environmental monitoring programmes, the most significant ongoing programme being that for the management of the Contaminated Mud Pits at East of Sha Chau. These data are essentially comparable with the longer-term EPD dataset and show general compliance with the WQOs in the region of the mud dumping operations. Data obtained to date from this programme support the hypothesis that the disposal activities have not had any significant adverse effect on water quality beyond the immediate confines of the pit areas during dumping (Mouchel, 2000, ERM 2000, ERM, 2001). Suspended sediment concentrations are again observed to range from less than 10 mg/l to over 80 mg/l although mean values tend to be a bit higher than reported by EPD. Over the period 1994 to 1997, for example, annual mean depth averaged suspended sediment concentrations ranged from about 18 to 40 mg/l illustrating the considerable variability for this parameter. Depth averaged dissolved oxygen varies at East Sha Chau within the range from 4 to 10 mg/l with a mean value of about 6.7 mg/l (Chan & Dawes 1999).
6.2.4.6 The statutory Water Quality Objective for suspended sediments is not defined in absolute numerical terms but instead is worded to require that human activities should not result in an elevation of more than 30 % above ambient levels. This in part reflects the difficulty in trying to apply a single numerical value for environmental management purposes in the context of natural high variability characteristic of Hong Kong’s marine waters. Previous workers assessing the environmental impacts associated with the temporary AFRF at Sha Chau adopted values up to 34 mg/l to represent ambient suspended solid concentrations in essentially the same study area as is being considered in this assessment (ERM, 1996). For the purposes of this assessment a slightly more conservative value of 30 mg/l has been adopted. Based on the review above it can be seen that this value lies well within the natural range experienced in this region and the existing biological system is clearly able to tolerate concentrations at these levels.
6.2.5
Baseline Sediment Quality
6.2.5.1 There is an abundance of data on sediment quality in the study area. Sediment quality is monitored by EPD every six months at four stations in the North Western Waters, namely NS2, NS3, NS4 and NS6. The locations of these stations are indicated in Figure 6.3. Maximum concentrations of key potential toxicants reported by EPD over the 5-year period 1995 – 1999 for these sites are presented in Table 6.4. The corresponding LCEL criteria are presented in the same table for convenient reference.
Table 6.4 EPD Routine Sediment Quality Data for North Western Waters (1995-99)
|
Contaminant |
Concentration (dry weight) |
LCEL (dry weight) |
|
Metals Cd Cr Cu Pb Hg Ni Zn Metalloid As Organics PCBs PAHs |
mg/kg < 0.9 <49 <54 <64 <0.7 <34 <140 mg/kg <14 ug/kg 6-10 51-100 |
mg/kg 1.5 80 65 75 0.5 40 200 mg/kg 12 ug/kg 23 low MW : 550, high MW : 1700 |
6.2.5.2 There is some marginal discrepancy between the lower reporting limits adopted by EPD and the LCEL for Hg and As. It is clear that all other parameters comply comfortably with the LCEL. EPD amended their reporting limits in 2000 to align with the numerical values adopted for the LCEL and UCEL criteria. EPD data for stations NS2, NS3, NS4 and NS6 for the updated 5-year period 1996 – 2000 met the LCEL criteria for all the parameters listed in Table 6.4 with the exception of arsenic. Upper range arsenic concentrations were observed to exceed the LCEL. EPD note in their annual water quality report for 2000 that these arsenic concentrations might be related to the high natural arsenic levels in the soil of some areas of the Northern New Territories which could then be transported to the marine environment through river discharges and storm run off.
6.2.5.3 On no occasion over the past 5 years have EPD detected arsenic concentrations in North Western Waters marine sediment above the UCEL criterion adopted to define a level above which adverse biological toxicological effects would be expected. Given that the arsenic concentrations in this region are likely to represent the result of gradual natural erosive processes over geologic timescales it seems reasonable to assume that the existing ecosystem is tolerant to the widespread presence of this element.
6.2.5.4 In summary it can be concluded that no significant toxicity would be expected at the sampling stations routinely sampled by EPD to assess sediment quality in these waters. NS3, NS4 and NS6 are conveniently situated around the study area for this assessment and thus it is reasonable to extrapolate this finding as being true also for the general conditions in the area of seabed likely to be impacted during construction of the proposed pipeline trench.
6.2.5.5 Dredged sediments identified to be contaminated have been disposed of to series of sea bed pits in the East Sha Chau area since December 1992. These pits are subsequently capped with clean mud to isolate any potential toxicants from the wider environment. The facility has been closely scrutinised by means of a comprehensive monitoring programme administered by the Civil Engineering Department assisted by EPD.
6.2.5.6 The most recent comprehensive publication on this work available at the time of this assessment (ERM, 2000) reports on the 2 year period November 1997 to October 1999. This work principally involved sediment sampling within and very close to the active disposal pits to detect any possible losses of contaminated sediment during the disposal operations. In addition, sampling was undertaken at a series of regional stations to monitor conditions in the wider region. The locations of these stations are indicated in Figure 6.4. All determinands for which LCEL criteria have been defined were tested for on a total of eight occasions. All parameters were observed to pass the LCEL criteria with the notable exception of arsenic which was commonly measured to exceed the LCEL throughout the entire area monitored. Background arsenic concentrations are know to typically exceed the LCEL throughout this region generally and this is attributed to natural geologic concentrations rather than any specific anthropogenic activity. The monitoring programme also detected DDT throughout the sampled area at levels comparable to those recorded elsewhere in Hong Kong’s waters. There is no LCEL criterion for this substance.
6.2.5.7 Since publication of the data discussed above, the East Sha Chau programme has been amended (Mouchel 2001b) although the regional sediment sampling stations remain unchanged (see Figure 6.5). The most up to date information reported at the time of this assessment is essentially comparable with that reported in previous years and, taking the LCEL criteria as a reference, there is no reason to believe that sediments in this general region are of any ecotoxicological concern.
6.2.5.8 A detailed sediment survey was undertaken in 1995 to assess sediment quality along the route for the existing AFRF pipeline from Sha Cha to HKIA and also in the vicinity of the existing AFRF jetty and dredged approach channel (see Figure 6.6). Some 50 locations were sampled and tested for seven heavy metals in accordance with the EPD Technical Circular 1-1-92, which preceded the current Works Bureau technical Circular 3-2000. The parameters tested for and the then ‘Class A’ uncontaminated mud criteria are presented in Table 6.5. The table includes the currently used LCEL criteria for ease of comparison.
Table 6.5 Comparison of historical Class A Criteria with current LCEL Criteria
|
Metal |
Class A (mg/kg) |
LCEL (mg/kg) |
|
Cd |
<0.9 |
<1.5 |
|
Cr |
<49 |
<80 |
|
Cu |
<54 |
<65 |
|
Hg |
<0.7 |
<0.5 |
|
Ni |
<34 |
<40 |
|
Pb |
<64 |
<75 |
|
Zn |
<140 |
<200 |
6.2.5.9 All samples were determined to comply with the Class A criteria in force at that time. The area was subsequently dredged and the resultant spoil categorised as “uncontaminated”. It can be seen that the then Class A criteria are more stringent than the existing LCEL with the exception of that for mercury which has been tightened to < 0.5 mg/kg. A reanalysis of the original source data (ERM, 1996) reveals that at no station was mercury measured at a concentration of more than 0.23 mg/l which is comfortably within the existing LCEL. There is no reason to suspect that sediments in this area should be more contaminated at the present time than they were at the time that these samples were taken. On this basis no ecotoxicity from the 7 metals tested would be expected in this part of the study area. This area specifically includes the section of the proposed pipeline trench between Urmston Road and the existing AFRF at Sha Chau.
6.2.5.10 AAHK commissioned two rounds of sediment sampling in March 1999 and February 2000 at 10 stations around the perimeter of the airport platform (Figure 6.6). The suite of determinands included all LCEL parameters with the exception of TBT. The programme was principally intended to identify the possibility of contamination wash out from storm discharges from the airport site. TBT would not be expected to enter the marine environment from this source. The resultant data (Mouchel 2000) showed that none of the heavy metal concentrations observed were above the LCEL criteria. No PAHs were observed above a detection limit of 0.5 mg/kg again indicating compliance with the relevant LCEL criteria. No PCBs were observed above a detection limit of 0.1 mg/kg. This detection limit actually lies between the corresponding LCEL and UCEL criteria. However the fact that two rounds of sampling over 10 stations on each occasion failed to identify any PCB at all gives confidence that there is no contamination of concern from these substances. As seen elsewhere, arsenic was detected at concentrations equalling or exceeding the LCEL for about 50% of samples. No exceedance of the UCEL was detected.
6.2.5.11 This programme of work also indicated that there was no significant temporal difference between the annual sampling rounds which gives support to the extrapolation of this data set to the present time.
6.2.5.12 A sediment quality assessment was commissioned in summer 1999 by Shiu Wing Steel Co. Ltd in respect of a requirement to undertake maintenance dredging at their jetty at Tap Shek Kok. The site adjoins the proposed location for the PAFF jetty and there are no pollution sources in between. Three boreholes were sampled and 5 depths and analysis was conducted for Cd, Cr, Cu, Ni, Pb Zn and Hg. All results were below the existing LCEL criteria and the sediments at the site were confirmed to be Class A or uncontaminated by EPD in August 1999 as defined by the relevant Technical Circular in force at that time, WBTC 22/92.
6.2.5.13 In conclusion there is an abundance of existing sediment quality data pertaining to the study area for this assessment which has been thoroughly characterised for various reasons in the past. There is a clear weight of evidence to indicate that the sediments are not contaminated such that they might reasonably be expected to exert any significant ecotoxicological impact if disturbed during the course of the Project.
6.3.1.1 The principal water quality concern associated with this project relates to disturbance to the seabed during the construction period. There will be a need to dredge a long channel to enable the pipelines to be placed below the existing seabed and then backfilling works to provide a protective rock armour cover. These operations will inevitably result in the loss of sediments and backfilling materials into the water column where they will add to the suspended sediment load.
6.3.1.2 During dredging works fine material will be displaced and may be carried downstream of the works area. The extent of the suspended sediment plume will depend on the rate of release and thus the working methods adopted, the particle size of the dredged material and its characteristic settling velocity, the prevailing currents and hydrodynamic conditions. Similar disturbance may be experienced during backfilling. However, the backfill material will be very much coarser grained and heavier and, thus, the principal concern relates to the period of dredging rather than backfilling.
6.3.1.3 Sediment laden plumes may directly affect marine organisms through abrasion and clogging of fish gills and other organs or possibly as a result of reduced light penetration.
6.3.1.4 From the review of sediment quality data Sections 6.2.5.1 to 6.2.5.13, above it can be concluded that the dredging operations would be most unlikely to release contaminants of potential ecotoxicological concern into the wider environment. In some situations dredging operations can give rise to concerns about possible release of nutrients or organically rich material which could result in water column oxygen depletion. These would typically be issues for long term dredging operations in relatively still or poorly flushed waters. However, there is very extensive experience of dredging operations for construction works similar to those intended for this project within the marine waters of Hong Kong. Nutrient enrichment or oxygen depletion has never been reported as a major concern for marine dredging works in Hong Kong previously and there is no reason to believe that these processes would be of concern in the well flushed North Western Waters. No such impacts were observed during construction of the AFRF next to Sha Cha island, for example.
6.3.1.5 The project would entail significant land based works to construct the tank farm and associated facilities. These will be built on existing reclaimed land. The main water quality related issues will be to prevent erosion on site and minimise suspended sediment loads washed out in stormwater and well as the need to control waste water streams such as temporary sewage facilities, cementitious waters and general construction refuse. Toilets will be connected to the local sewerage system if possible during construction but if not feasible chemical toilets will be used. There is vast experience of managing this type of project in Hong Kong and it is considered that these issues would not be of any substantive concern provided that good site practice is adhered to.
6.3.1.6 The other principal construction activity associated with the project is the construction of the reception jetty. The jetty would be supported on piles. There is unlikely to be any requirement for dredging works on the seabed. As with the land based works, provided good site practice is maintained there should be no particular concerns for water quality during the construction period.
6.3.2.1 During the operational phase the likelihood of major spill events occurring is very low.
6.3.2.2 Aside from the risk of major spills there will be a need to prevent minor slops during routing operations e.g. coupling and uncoupling of vessel discharge hoses. This will be managed through design and operational practice. Similar concerns apply to the tank farm and mitigatory measures will also be required there to guard against leakage or other unintentional discharges. Mitigation measures to prevent and contain leakages from land based facilities are also identified in Section 13.5.
6.3.2.3 There will be no routine discharge of wastewater or contaminated surface drainage to sea or surface watercourse in the operational phase. Sewage from site offices will be minimal, amounting to only about 2.9m3/day and as such it is predicted that spare capacity will be available in the local system to accommodate this insignificant amount.
6.4
Construction Phase Impact Assessment
6.4.1.1 As discussed above, the principal water quality concerns in the construction period are those associated with dredging and backfilling for the submarine fuel pipeline. In this assessment, representative sediment plumes have been simulated using the Delft 3D tidal flow and water quality models. Since the sediment losses and migration during backfilling with coarse material will be very much less than those during dredging when fine silts may be resuspended, the latter case is simulated here for conservative assessment purposes.
6.4.1.2 It has been assumed that the pipeline trench could either be dredged mechanically using a barge mounted grab and hydraulically by means of a trailer suction hopper dredger. Both have their pros and cons from an operational and also environmental point of view. From the environmental perspective the instantaneous sediment loss rates associated with grab dredging are likely to be less than that for trailer dredging. Thus, the intensity and extent of a sediment plume emanating downstream of a working dredge is likely to be less for a grab than a trailer and thus trailer suction dredgers are generally regarded as being more environmentally damaging than grab dredgers. However the plume from a trailer dredger will predominantly be formed at depth close to the drag head whereas grab-dredging plumes, though less concentrated, are likely to originate throughout the water column as the grab is pulled to the surface. Also, grab dredging is considerably slower and thus the period of disturbance is much greater. There are important operational constraints also. The pipeline route must cross the busy Urmston Road marine fairway and it is unlikely that the Marine Department would allow the use of a relatively immobile slow working grab dredger in this congested area. Deployment of a self powered trailer would be considerably safer. However, trailers can not operate in shallow or very nearshore areas where their manoeuvrability is impeded. Thus, some combination of methods can realistically be expected. For robust assessment purposes dredging for this project is simulated to represent deployment of both grab and trailer suction dredgers. Cross-sections of the pipelines in the trench are provided in Figure 3.3.
6.4.1.3 It is anticipated that a trailer suction hopper dredger (TSHD) will be used to excavate a trench in the Urmston Road, a major shipping channel with strong tidal flows and where anchoring a barge would be difficult and almost certainly unacceptable to the Marine Department. Over the shallower areas crossed by the pipeline and possibly inaccessible to a large TSHD at times during the tidal cycle, it is assumed that a grab dredger would be used.
6.4.2.1 Delft Hydraulics have established well calibrated three-dimensional hydrodynamic and water quality models of the Pearl Estuary and the whole of Hong Kong Territorial waters. These models have been calibrated and validated using a number of historic data sets. The latest model is referred to as the Update model and it could be applied directly in the present assessment of the fuel pipeline. However, the model grid resolution in the area of interest is considered to be too coarse and therefore another existing model of a smaller area covering the Western Harbour and North West New Territories, referred to as the Western Harbour Model, has been applied. The Western Harbour model of tidal flows extracts boundary conditions from the Update model and has also been fully validated by Delft Hydraulics for EPD. The grid size of this model in the area of interest is 200m x 200m. The areas covered by both the Update Model and the Western Harbour Model are shown in Figure 6.8. Details of the Western Harbour Model’s mesh is presented in Figure 6.9.
6.4.2.2 The Delft3D model of tidal flows has been used to simulate tidal flows in the both the wet and dry seasons. While the Delft3D model of tidal flows has been used to simulate the water movements, the Delft3D model of water quality has been used to simulate the fate of sediment lost to suspension during dredging operations. The Delft3D water quality model has been used rather than the Delft3D random walk sediment plume model because the sediment plume model has a much simpler representation of erosion and deposition than the water quality model and this simpler representation can result in inaccurate simulations.
Settling
Velocities
6.4.3.1 The settling velocity of suspended cohesive sediment is concentration dependent. The simulations were carried out using a constant settling velocity of 0.5mm/s which is typical of low suspended solids concentrations. This represents a conservative assumption in that a higher sedimentation rate would actually be expected in the dense plume close to the drag head.
Erosion and
Deposition
6.4.3.2 Erosion and deposition in the water quality model are defined in terms of a critical stress for deposition above which no deposition can take place and a critical stress for erosion above which erosion can take place. The critical stress for deposition was set at 0.2N/m2 with a restriction being imposed on deposition in shallow water where wave action would inhibit deposition. A water depth of 2m was selected as the minimum depth in which deposition can take place. The critical stress for erosion was set at 0.3N/m2 which is applicable to relatively soft new deposits with a density of around 200kg/m3 (HWR, 1993).
Assumed Sediment
Losses
6.4.3.3 For the grab dredgers, it is assumed that 17kg/m3 dredged will be lost to suspension and, for the TSHD, it is assumed that 10kg/m3 dredged will be lost to suspension when dredging mud (Hyder, 1998). For the grab dredgers, it is assumed that the losses are evenly distributed over the water column while, for the TSHD, the losses are assumed to take place in the lowest layer in the model.
TSHD Dredging
6.4.4.1 The TSHD is assumed to have a capacity of 8,000m3 and an assumed production rate of around 6,000m3/hour. As discussed above, it is likely that this method of dredging will be used in the Urmston Road and possibly for other stretches of the pipeline length. In terms of sediment release rates, the TSHD represents a worst case situation as compared to the grab dredging and as such, modelling runs have been carried out to simulate losses from the TSHD based upon this method being used for most of the pipeline alignment. The volume of material assumed to be dredged by the TSHD is estimated to be 274,000m3. Based upon this and, assuming continuous working with no downtime, all TSHD dredging will be completed within 6 days. This is a conservative estimate (in environmental impact terms) as production rates are not likely to be higher than this. It is possible the dredgers will not be able to achieve these production rates in which case the dredging could be for a longer period of time. However it should be noted that the dredging period is only 6 days, so a change in the production rate of (say) 25% would only increase the dredging time by 1.5 days. A longer dredging time would result in a proportionately lower sediment release rate for the longer period. The net result would be a lower impact. Thus the case assessed is a worst case.
6.4.4.2 Losses from the TSHD have been simulated over a 6 day period spanning both large and small amplitude tides to ensure all representative tidal conditions are considered. Sediment losses were simulated along the pipeline route between the Tuen Mun Area 38 shore and the boundary of the Marine Park. The dredging point was moved continuously along the pipeline route to simulate a dredger progressing at a rate of 0.5 ms-1 for a period of one hour and twenty minutes followed by a two hour gap to represent the end of the dredging cycle and a period in which the dredger leaves the site to travel to and from the disposal site.
Grab Dredgers
6.4.4.3 For grab dredgers, it is assumed that dredging is continuous with a production rate of 7,000m3/day. Sediment losses have been simulated over a spring-neap tidal cycle with the grab dredger assumed to move progressively away from the existing AFRF jetty at Sha Chau toward the Urmston Road at a rate of 100m per day.
6.4.5
Environmental Assessment
Criteria
6.4.5.1 For the purposes of this assessment, the results from the model simulations in terms of suspended solids concentrations and deposition rates are assessed with respect to the Water Quality Objectives (WQO) for the North Western Waters Water Control Zone. Sensitive receivers likely to be impacted have been identified on Figure 6.2 and include the Lung Kwu Chau and Sha Chau Marine Park, artificial reefs and other important habitat, cooling water intakes and bathing beaches. As discussed in section 6.2.4.6, for the purposes of this assessment a background suspended sediment concentration of 30mg/l is assumed. The WQO allows for a 30% increase as a result on anthropogenic activity and thus a maximum contribution from dredging activity of 9mg/l.
6.4.5.2 It is considered most unlikely that sediment losses will have a significant impact on dissolved oxygen concentrations. Nevertheless, for completeness, the possibility of dissolved oxygen depletion is calculated from the predicted suspended solids concentrations using the method of ERM, 1997. Any oxygen sag can be compared against background levels (Section 6.2.4.5 refers) and the relevant WQOs.
6.4.5.3 It has already been concluded in Section 6.2.5.13 that there is no reason to believe that the dredged sediments would be contaminated such that they might reasonably be expected to exert any significant ecotoxicological hazard. It has, therefore, not been necessary to quantitatively assess the leaching of potentially toxic substances such as heavy metals or trace pollutants.
6.4.6.1 The water quality modelling results are presented in full in Appendix E. For each scenario, modelled plots are provided of:
¨ depth averaged suspended sediment contours at high and low water for each day of the model simulation;
¨ time history plots in the upper, middle and bed layers at sensitive receivers in the study area throughout the period of the model simulation; and
¨ sediment deposition plots at high and low water for each day of the model simulation.
6.4.6.2 The location of the sensitive receivers considered in the modelling work is indicated on Figure 6.10.
6.4.6.3 Vector diagrams showing the current flow in the surface, middle and lower layers of the water column during wet and dry seasons at ebb and flood tides are also presented in Appendix E.
Trailer Suction Hopper Dredging
6.4.6.4 The model output suggests consistently acceptable water quality conditions. Depth averaged suspended sediment concentrations are predicted to be below the assessment criterion of 9 mg/l above ambient at all times and in fact rarely increase by more than about 5 mg/l throughout the study area. The water quality objective would be complied with at all times.
6.4.6.5 The time history output from the model indicates that there would be complete compliance with the water quality objective for suspended sediments at all times for all sensitive receivers during the dry season.
6.4.6.6 In the wet season the waters within the dredged area are markedly influenced by freshwater flows from the Pearl River. The waters are strongly stratified particularly on the ebb tide. This would tend to decrease mixing of sediment disturbed at depth by the trailer arm and higher suspended sediment concentrations might be expected at depth compared to in the dry season when there is vertical mixing across the whole water column. These effects are reflected in the model output.
6.4.6.7 Periodic suspended sediment elevations in the bed layer are predicted within the marine park when the dredger approaches the marine park boundary during the wet season simulation. A maximum value of about 25 mg/l is predicted in the bottom layer to the east of Sha Chau. These peaks are short lived and quickly fall to zero on the turn of the tide. Suspended sediment levels at the surface and in the middle of the water column are predicted to be low, below 5 mg/l at this site at all times. These bottom layer conditions are well within the range of natural variability and do not present any ecological concern. Suspended sediment concentrations are not expected to impact on the artificial reef site in the marine park where compliance with the water quality objectives is predicted.
6.4.6.8 The model output shows a one-off and short lived suspended sediment concentration spike of about 40mg/l in vicinity of the Shui Wing steelworks intake. This incident represents an occasion in the wet season when the dredger is working close to the Tuen Mun Area 38 shore. The elevation is only observed in the very bottom layer of the water column. Increases in sediment concentrations are not expected in middle and upper layers of the water column and on no other occasion are levels predicted to exceed 5 mg/l. This incident would not detrimentally affect the quality of water abstracted by the steelworks.
6.4.6.9 The model does not predict that there would be any substantial accumulation of re-deposited sediments likely to adversely affect benthic ecology or particularly susceptible species such as corals. No locations beyond the immediate vicinity of the worked area would experience settlement rates greater than a value of 200 g.m-2day-1 that has been used by previous workers as an indicator level above which sustained deposition could harm sediment sensitive hermatypic corals. (Binnie, 1996). Soft corals typical of the study area would be even more tolerant and thus there is no reason to believe that they would be impacted. Some recent studies have used an impact criterion of 100 g.m-2day-1 for the protection of corals. No locations beyond the immediate vicinity of the worked area would experience settlement rates greater than this value.
6.4.6.10 The degree of oxygen depletion exerted by a sediment plume is a function of the sediment oxygen demand of the sediment, its concentration in the water column and the rate of oxygen replenishment.
6.4.6.11 For the purposes of this assessment, the impact of the sediment oxygen demand on dissolved oxygen concentrations has been calculated based on the following equation (ERM, 1997):
DODep = C * SOD * K * 0.001
where DODep = Dissolved Oxygen depletion (mg/l)
C = Suspended Solids concentration (kg/m3)
SOD = Sediment Oxygen Demand
K = Daily oxygen uptake factor (set at 0.23 in ERM 1997)
6.4.6.12 An SOD of 15,000 mg/kg has been taken with reference to EPD Marine Monitoring data as a suitably representative value for sediments in the North Western Waters region.
6.4.6.13 The analysis using the above equation does not allow for re-aeration which would tend to reduce any impact of the suspended sediment on the water column DO concentrations. The analysis, therefore, errs on the conservative side so as not to underestimate the extent of DO depletion. Further, it should be noted that, for sediment in suspension to exert any oxygen demand on the water column will take time and, in that time, the sediment will be transported and mixed/dispersed with oxygenated water. As a result, the oxygen demand and the impact on dissolved oxygen concentrations will diminish as the suspended sediment concentrations decrease.
6.4.6.14 The highest levels of suspended sediment predicted for any sensitive receiver in the modelling run described above is to be found in the layer of water at a point to the east of Sha Chau during the wet season simulation. For convenient reference the time history plot for this site as presented in Appendix E is reproduced in Figure 6.11. It can be seen that the occasionally higher near bed suspended sediment concentrations only persist for a relatively short time intervals. No corresponding suspended sediment elevation is observed in the upper layers at this location.
6.4.6.15 Sediment depletion is not instantaneous and thus previous workers have assumed that the impact of suspended sediment on dissolved oxygen will depend on tidally averaged suspended sediment concentrations (ERM, 1997). However for the purposes of this demonstration the actual time history plot for suspended sediment has been used as the basis for the calculation in order to identify the hypothetical worst case. In addition, following comments from EPD, the daily uptake factor, K, in the equation above was set to be equal to 1 which indicates instantaneous oxidation of the sediment oxygen demand. The resulting calculated dissolved oxygen deficit, therefore, is expected to be much larger than would be experienced in reality.
6.4.6.16 A plot of the predicted dissolved oxygen depletion based on these various conservative assumptions for the bed layer during the time of plume incursion is presented in Figure 6.12. The ‘worst case’ calculated dissolved oxygen deficit is just over 0.36 mg/l. This calculated dissolved oxygen deficit is expected to be much larger than would occur during the dredging operations but, as indicated in Figure 6.13, there would still be no perceptible oxygen sag at this location. Thus it is concluded that the sediment plumes predicted from the modelling run described above would have negligible effect on dissolved oxygen conditions in the receiving waters. The Water Quality Objectives for dissolved oxygen would easily be complied with at all times.
6.4.6.17 In summary, the impact on water quality from the use of a trailer dredger to form the pipeline trench is considered to be within acceptable bounds.
Grab Dredger
6.4.6.18 The principal difference between grab dredging scenarios compared with trailer dredging scenarios is that the instantaneous sediment release rate for the grab dredging scenarios is approximately 1.4 kg/s as compared to 11.1 kg/s for the trailer dredger. For the trailer dredger it is assumed that the main disturbance is at the sea bed close to the drag head and thus the sediment release is to the bed layer. For the grab dredging scenario account must be taken of sediment losses as the grab is pulled to surface and thus the model simulates sediment release over all depths. These differences are reflected in the model output provided in Appendix E.
6.4.6.19 For the dry season full compliance with the WQOs is predicted at all sensitive receivers at all times. Away from the immediate dredging point suspended sediment is quickly dissipated and the sediment plume is weak and short-lived. The depth average contour plots show that levels in the study area are well below the assessment criterion of 9 mg/l and in fact very rarely exceed a level of 5 mg/l.
6.4.6.20 For the wet season the model again predicts compliance with the assessment criterion of 9 mg/l throughout the study area and as with the other scenarios simulated, sediment levels are rarely increased by more than 5 mg/l above ambient. Complete compliance with the Water Quality Objectives is predicted at all sensitive receivers at all times. On one occasion a spike in suspended sediment concentration approaching 8 mg/l was predicted at a location east of Sha Chau close to a working dredger. This concentration rapidly falls back to near zero in just a few hours. Elsewhere suspended sediment increases are not expected to amount to more than 2-3 mg/l on any occasion.
6.4.6.21 In both seasons sediment re-deposition rates are not expected to exceed the conservative assessment criterion of 200 g/m2/day.
6.4.6.22 In summary, the impact on water quality from the use of a grab dredger to form the pipeline trench is considered to be well within acceptable bounds at all times.
6.5
Operational Phase Impact Assessment
6.5.1.1 In the operational phase the principal concern for water quality relates to the risk of aviation fuel spillage. The likelihood of a major spill is quantifiably very low but nevertheless in the worst case, the potential impacts on water quality and for sensitive receivers including biota are severe. but are transient only. These risks will be minimised through design, operational practice and contingency planning as discussed in Section 11. The consequences of aviation fuel spills from a number of conjectured incident scenarios are simulated by means of the Delft 3D model suite and discussed in Section 11. These scenarios cover leaks and spills from the tank farm, the jetty, approaching tankers and the submarine pipeline.
6.6
Consideration of Alternatives
6.6.1 Subject to implementation of the various mitigatory measures identified in Section 6.8 below, the only significant water quality impact associated with the project will be the generation of sediment plumes in the water column during dredging and, to a lesser extent, backfilling of the protective rock armour. The assessment above has been made on a worst case scenario basis and it is concluded that these impacts are within acceptable bounds. The plumes will not adversely affect any sensitive receiver and they are of no biological concern. Nevertheless it is prudent to fully explore possible alternatives to ensure that any adverse impacts can be kept to a practical minimum.
6.6.2 The assessment above has assumed flexible use of grab or trailer dredgers to suit operational need. To minimise the impacts of dredging in sensitive areas such as within the marine park it would be preferable to constrain the use of trailers and just use a closed grab if this possibility is not precluded in any case on account of the shallow depth. This would reduce the instantaneous sediment loss rates and the intensity of sediment plumes in the bed layer and thus keep any disturbance to a practical minimum.
6.6.3 Current technology does not offer any environmentally preferable means to lay the submarine pipeline into the seabed. As discussed in Section 3, post trenching techniques such as ploughing are not obviously advantageous and in fact would offer less security in the operational phase because the protective rock armour would likely have to protrude above the seabed. Bedding fluidisation methods such as jetting would potentially release even more sediment into the water column than dredging itself.
6.7.1.1 Works within the Marine Park need to be particularly carefully controlled. There should be no access to the shore or working from land within the Marine Park. No marine anchors should be used within the Marine Park.
6.7.1.2 There is a need to dredge a short length of pipeline trench (approximately 400m) within the Marine Park. Whilst the impact of a trailer dredger working in these waters is expected to be within acceptable bounds, it is recommended that the use of hydraulic suction dredgers within the Marine Park should be avoided to minimise instantaneous sediment losses and the intensity of plumes at the point of dredging. Any dredging within the Marine Park should instead be carried out by means of a closed mechanical grab.
6.7.1.3 The pipeline would run in an easterly direction from Sha Chau underneath the existing maintained dredged channel which provides tanker access to the AFRF. The existing AFRF will be retained as a strategic emergency backup facility once the PAFF is commissioned. It must be maintained in working order with tanker access at all times. Maintenance dredging of this channel will continue even on commissioning of the PAFF. The opportunity therefore exists to combine the one-off dredging for the trench with a regular maintenance dredging operation to avoid any duplication of dredging activity and to minimise overall dredging requirements.
6.7.1.4 Dredging operations should be undertaken in such a manner as to minimise resuspension of sediments. Standard good dredging practice measures should therefore be implemented including the following requirements which should be written into the dredging contract. These measures are also summarised in the Environmental Mitigation Implementation Schedule in Appendix B.
¨ trailer suction hopper dredgers shall not allow mud to overflow;
¨ use of Lean Material Overboard (LMOB) systems shall be prohibited;
¨ mechanical grabs shall be designed and maintained to avoid spillage and should seal tightly while being lifted;
¨ barges and hopper dredgers shall have tight fitting seals to their bottom openings to prevent leakage of material;
¨ any pipe leakages shall be repaired quickly. Plant should not be operated with leaking pipes;
¨ loading of barges and hoppers shall be controlled to prevent splashing of dredged material to the surrounding water. Barges or hoppers shall not be filled to a level which will cause overflow of materials or pollution of water during loading or transportation;
¨ excess material shall be cleaned from the decks and exposed fittings of barges and hopper dredgers before the vessel is moved;
¨ adequate freeboard shall be maintained on barges to reduce the likelihood of decks being washed by wave action;
¨ all vessels shall be sized such that adequate clearance is maintained between vessels and the sea bed at all states of the tide to ensure that undue turbidity is not generated by turbulence from vessel movement or propeller wash; and
¨ the works shall not cause foam, oil, grease, litter or other objectionable matter to be present in the water within and adjacent to the works site.
6.7.1.5 Placement of pipeline trench backfill should be undertaken in a controlled manner to minimise impacts. Backfilling with rock should be undertaken either down pipe or by a reverse grab operation or other controlled technique to ensure that this material does not mound on the seabed.
6.7.1.6 General construction activities on land should also be governed by standard good working practice. Specific measures to be written into the works contracts should include :
¨ wastewater from temporary site facilities should be controlled to prevent direct discharge to surface or marine waters;
¨ sewage effluent and discharges from on-site kitchen facilities shall be directed to Government sewer in accordance with the requirements of the WPCO or collected for disposal offsite. The use of soakaways shall be avoided;
¨ storm drainage shall be directed to storm drains via adequately designed sand/silt removal facilities such as sand traps, silt traps and sediment basins. Channels, earth bunds or sand bag barriers should be provided on site to properly direct stormwater to such silt removal facilities. Catchpits and perimeter channels should be constructed in advance of site formation works and earthworks;
¨ silt removal facilities, channels and manholes shall be maintained and any deposited silt and grit shall be removed regularly, including specifically at the onset of and after each rainstorm;
¨ temporary access roads should be surfaced with crushed stone or gravel;
¨ rainwater pumped out from trenches or foundation excavations should be discharged into storm drains via silt removal facilities;
¨ measures should be taken to prevent the washout of construction materials, soil, silt or debris into any drainage system;
¨ open stockpiles of construction materials (e.g. aggregates and sand) on site should be covered with tarpaulin or similar fabric during rainstorms;
¨ manholes (including any newly constructed ones) should always be adequately covered and temporarily sealed so as to prevent silt, construction materials or debris from getting into the drainage system, and to prevent storm run-off from getting into foul sewers;
¨ discharges of surface run-off into foul sewers must always be prevented in order not to unduly overload the foul sewerage system;
¨ all vehicles and plant should be cleaned before they leave the construction site to ensure that no earth, mud or debris is deposited by them on roads. A wheel washing bay should be provided at every site exit;
¨ wheel wash overflow shall be directed to silt removal facilities before being discharged to the storm drain;
¨ the section of construction road between the wheel washing bay and the public road should be surfaced with crushed stone or coarse gravel;
¨ wastewater generated from concreting, plastering, internal decoration, cleaning work and other similar activities, shall be screened to remove large objects;
¨ vehicle and plant servicing areas, vehicle wash bays and lubrication facilities shall be located under roofed areas. The drainage in these covered areas shall be connected to foul sewers via a petrol interceptor in accordance with the requirements of the WPCO or collected for off site disposal;
¨ the contractors shall prepare an oil / chemical cleanup plan and ensure that leakages or spillages are contained and cleaned up immediately;
¨ waste oil should be collected and stored for recycling or disposal, in accordance with the Waste Disposal Ordinance;
¨ all fuel tanks and chemical storage areas should be provided with locks and be sited on sealed areas. The storage areas should be surrounded by bunds with a capacity equal to 110% of the storage capacity of the largest tank; and
¨ surface run-off from bunded areas should pass through oil/grease traps prior to discharge to the stormwater system.
6.7.1.7 Following construction of the pipeline it will be necessary to clean and de-water the aviation fuel pipeline prior to commissioning. Any water lying within the pipeline will be driven out by means of a pressure driven pig. It is recommended that any wastewater thus collected be temporarily stored in a wastewater / slop tank at the tank farm for chemical analysis, pre-treatment as necessary and safe disposal in accordance with the WPCO. The water may be enriched with iron oxides although the presence of toxic substances is not anticipated. Water from this one-off operation that cannot be adequately treated on site should be tankered to the Chemical Waste Treatment Centre for treatment and disposal.
6.7.2.1 The single most important mitigation measure in the operational
phase is the placement of a rock armour protective layer positioned to cover
the pipeline but not protrude above the sea bed. This will prevent possible mechanical damage, for example
from trailing anchors or trawling nets. To provide additional security it is
recommended that the pipeline shall be fitted with a leak detection system. The
system shall be monitored on a 24 hour basis by the control centre at the Tuen
Mun Area 38 site. In the unlikely
event of any failure this warning system should trigger the emergency shutdown.
6.7.2.2 Other mitigation measures recommended for the operational phase
include the following. These measures are also summarised in the Environmental
Mitigation Implementation Schedule in Appendix B:
¨
detailed emergency response procedures
shall be drawn up. These will
include requirements to maintain floating oil booms, absorbent materials and
skimmers on site at all times;
¨ routine losses and spills will also be prevented through design. Coupling points on the jetty will be protected with slop collection utilities;
¨ auxiliary tanks shall be permanently maintained at the tank farm for recovered fuel and slops;
¨ oily drainage systems and slop collection systems will connect to an oil/water separator;
¨
all tanks shall be bunded to a
capacity of at least 110% of the largest individual tank in each compound. Tank pits shall be protected by an
impermeable bed (e.g. geotextile sheeting) to prevent seepage of aviation fuel
to ground. A leak detection system
shall be installed beneath the containment membrane;
¨
there shall be no direct outlet from
the bund. A collection sump shall
be included in the base. Removal of accumulated rainwater shall be activated
manually and discharged to storm drain via an oil / water separator;
¨ contingency procedures shall be drawn up to ensure containment and safe disposal of any fuel lost from tanks or pipework. Suitable absorbent materials (e.g. sand or earth) shall be kept on site to deal with spillages; and
¨ valves shall be installed within the storm drainage system to facilitate the retention of spillages.
6.8.1 With the implementation of the identified mitigation measures the only disturbance to water quality during construction will arise during dredging and construction of the submarine pipeline and placement of its protective wall and rock armour. However the above assessment has demonstrated that any such disturbance will be well within acceptable bounds and thus no adverse residual impacts are anticipated.
6.8.2 With the implementation of the mitigation measured recommended for the operation phase no adverse residual impacts would be expected.
6.9.1 Backfilling operations have been on-going at the contaminated mud disposal facility at East Sha Chau since 1992 (Figure 6.2). The impact on water quality has been assessed both predictively using mathematical models similar to those employed in this assessment and in the field through a comprehensive field monitoring programme.
6.9.2 Mathematical modelling studies carried out in support of the EIA for the disposal site (ERM, 1997) indicated that mud dumped at the site would predominantly settle within the confines of the pit. On strong flood tides the water current is such that sediment suspended in the water column would be carried at low concentrations in a northwesterly direction towards Sha Chau and Lung Kwu Chau before dissipating. Concentrations of suspended sediments at these locations were predicted to be generally very low, typically under 1 mg/l with peak at levels of the order of 3-4 mg/l for short periods within the tidal cycle. These levels are not of any environmental concern and such a ‘plume’ would probably be indistinguishable from ambient conditions in the absence of mud dumping.
6.9.3 Project specific monitoring work has been undertaken at East Sha Chau for nearly a decade and has enabled a definitive conclusion to be drawn that the disposal activities have not had any significant effect on water quality beyond the immediate confines of the pit during dumping (ERM, 2000).
6.9.4 In summary it can be seen that previous studies have clearly established that the mud disposal operations have a negligible impact on water quality away from the immediate vicinity of the disposal pits. There is no reason to suspect that overlapping plumes from the disposal operations at East Sha Chau would contribute to any adverse cumulative impacts if carried our concurrently with construction works for the PAFF pipeline.
6.9.5 It is noted that, in the coming years, the North Brothers Marine Borrow Area will be backfilled and a new contaminated mud facility will be constructed at East of Sha Chau or the Airport East. The PAFF project will be operational by end 2005 and thus, the PAFF dredging will not take place while the new disposal facility is being constructed (planned for 2006) and the backfilling of the North Brothers can be managed to avoid inconvenience to CED while avoiding cumulative impacts.
6.9.6 There are no other confirmed marine works projects likely to give rise to overlapping plumes concurrent with those caused by dredging and pipelaying for the PAFF project. It is therefore concluded that there will not be any adverse cumulative impacts resulting from construction of the PAFF facility at the same time as other known works projects are in progress.
6.10
Environmental Monitoring and Audit
6.10.1 The implementation of good construction works practice and good dredging practice as well as the various specific mitigation measures identified above is important to prevent pollution of marine water in the construction phase. It is therefore recommended that construction activities both on land and offshore should be subject to a routine audit programme throughout the construction period. Further details on the scope of this audit are provided in the project EM&A manual.
6.10.2 With the implementation of the recommended mitigatory measures no residual adverse impacts on water quality would be expected. Nevertheless in view of the close proximity of the key sensitive receiver, the Lung Kwu Chau and Sha Chau Marine Park, it is considered appropriate to implement a water quality monitoring programme when construction works are taking place within 1000m of the Marine Park boundary to verify that the intensity of sediment plumes caused by activities associated with dredging and pipelaying are within the predicted acceptable bounds. The monitoring programme shall form an integral part of a management and control programme with a clearly defined Action Plan to trigger implementation of any necessary revision to works practice or provision of supplementary mitigation measures in the unlikely event that adverse impacts are identified. Further details of the monitoring programme and accompanying Action Plan are provided in the EM&A manual.
6.10.3 The principal mitigation features in the operational phase will be established through the design and construction of the facility and the implementation of an aviation fuel spill emergency response contingency plan. No adverse impacts on water quality are anticipated in the operational phase and thus there is no particular need for any specific long term auditing programme in the context of the EIAO.
Binnie Consultants Ltd. (1996) Fill Management Study Phase IV. Investigation and Development of Marine Borrow Areas : Coral Growth at High Island Dam. Prepared for Civil Engineering Department.
Chan, A.L.C. and Dawes, A.C. (1999) Disposal of Contaminated Mud in Hong Kong: A Review of the Environmental Monitoring Programme. In : Environmental Hydraulics (Lee, J.H.W., Jayawardena, A.W., and Wang, Z.Y. Eds.) Rotterdam, Netherlands. Balkema.
ERM Hong Kong (1995). Proposed Fuel Aviation Receiving Facility at Sha Chau: Environmental Impact Assessment. Prepared for Provisional Airport Authority.
ERM Hong Kong (1996). Aviation Fuel Receiving Facility at Sha Chau: Detailed Design Basis Supplementary EIA. Prepared for Leighton Contractors (Asia) Ltd.
ERM (1997) Environmental Impact Assessment Study for Disposal of Contaminated Mud in the East of Sha Chau Marine Borrow Pit. Prepared for Civil Engineering Department of the Government of the HKSAR.
ERM (2000). Environmental Monitoring and Audit for Contaminated Mud Pit IV at East Sha Chau. Second Annual Programme Review Report. Prepared for Civil Engineering Department of the Government of the HKSAR.
ERM (2001). Environmental Monitoring and Audit for Contaminated Mud Pit IV at East Sha Chau. 12th Quarterly Report. August – December 2000. Prepared for Civil Engineering Department of the Government of the HKSAR.
Hyder (1998) Supplement EIA for the Proposed Sand Extraction from the Brothers’ Marine Borrow Area. Prepared for Civil Engineering Department of the Government of the HKSAR.
HWR (1993) Disposal of Spoil at West Po Toi. An Assessment of the Stability and Losses of Dumped Spoil. Hydraulics and Water Research (Asia) Ltd Report HWR 067, July 1993 for Greiner Maunsell.
Mouchel (2000). Marine Environmental Monitoring. Prepared for Airport Authority Hong Kong.
Mouchel (2001a). Environmental Services for Permanent Aviation Fuel Facility. Final Comparative Assessment Report. Prepared for Airport Authority Hong Kong.
Mouchel (2001b). Environmental Monitoring and Audit for Contaminated Mud Pit IV at East Sha Chau (2000-2005). 1st Quarterly Report. Prepared for Civil Engineering Department of the Government of the HKSAR.