7.0       WATER QUALITY

Introduction

7.1               The potential water quality impacts associated with the construction and operation of the proposed concrete batching facility are assessed in this section.  Where necessary, mitigation measures have been proposed to ensure that all residual impacts are in compliance with the relevant environmental legislation, standards and guidelines.

Assessment Criteria

7.2               The Water Pollution Control Ordinance (Cap.358) provides the major statutory framework for the protection and control of water quality in Hong Kong.  According to the Ordinance and its subsidiary legislation, all Hong Kong waters are divided into Water Control Zones (WCZ).  The proposed site is located at the western end of Hong Kong Island and is adjacent to the East Lamma Channel. The East Lamma Channel falls within the Western Buffer WCZ, which was declared in 1993.  The relevant Water Quality Objectives (WQOs) for this WCZ are given in Table 7.1 below.

 

Table 7.1       Water Quality Objectives – Western Buffer Water Control Zone

Parameters

Criterion

Beneficial Use

Suspended Solids

Waste discharges shall 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

Marine waters

Dissolved Oxygen

Waste discharges shall not cause the level of dissolved oxygen to fall below 4 mg/L (depth averaged) for 90% of the sampling occasions during the whole year

 

The dissolved oxygen concentration should not be less than 2mg/L within bottom 2m of seabed for 90% of the sampling occasions during the whole year

Marine subzone except FCZ

 

 

Marine waters

PH

The pH of water should be within the range of 6.5-8.5 units. Changes due to waste discharges should not exceed 0.2 pH units

Marine waters except bathing beaches subzone

BOD

BOD should not exceed 3 mg/L

Water Gathering Ground

Nutrients

Annual depth averaged total inorganic nitrogen not to exceed 0.4mg/L

Marine waters

Ammonical Nitrogen

Annual mean not to exceed 0.021mg/L

Whole zone

Water Sensitive Receivers

7.3               The water quality sensitive receivers identified in Section 7.3 of the Scheme 1 EIA Report which are applicable to the Project are listed below (Figure 7.1).

·                 Queen Mary Hospital seawater intake at Sha Wan Drive (WSR1);

·                 The as-built salt water intake (as part of the CED reclamation work) for the proposed WSD pumping station at the southern end of the reclamation (WSR2);

·                 WSD seawater intake at Wah Fu Estate (WSR3).

 

Description of Environmental Baseline Conditions

7.4               The water quality baseline conditions described in Section 7.4 of the Scheme 1 of the approved EIA Report are applicable to the Project and are summarized below.  Existing ambient marine water quality in the area was established based on the EPD routine monitoring data obtained for the years 1996 to 1998 for stations WM1 and WM2. Table 7.2 summarises the key monitoring data. A review of the data indicated that:

·                 Dissolved oxygen (DO).  The DO level at the surface and the bottom was between 4.7 mg/L to 8.7 mg/L and 2.9 mg/L to 8.1 mg/L, respectively. The WQO for depth-averaged DO was only satisfied in 1996 and 1997.  More than 10% of the samples collected from both stations in 1998 had DO content less than 4 mg/L. Non-compliance with the WQO for depth-averaged DO was recorded in the October and November 1998 monitoring data;

·                 pH. This was within the WQO range of 6.5 to 8.5 throughout the monitoring period;

·                 Suspended solids (SS). The recorded SS levels fluctuated throughout the monitoring period. The annual mean depth-averaged levels were below 10mg/L. A decreasing trend was observed over the three year period;

·                 Total Inorganic Nitrogen (TIN). Full compliance with the WQO for TIN was recorded at both monitoring stations;

·                 E-Coli. There is no secondary contact subzone in the vicinity of the site and therefore the WQO is not applicable;

·                 BOD5. The annual mean BOD5 was consistently between 0.5 mg/L to 0.8 mg/L with the depth-averaged maximum levels being well below the WQO (3 mg/L);

·                 NH4-N. Annual NH4-N levels at both stations ranged from 0.09 mg/L to 0.13 mg/L. The results indicated that the WQO for marine waters could not be met.

 

7.5               In summary, the monitoring results indicated that marine water quality in the East Lamma Channel in general complied with the WQOs except NH4-N and depth-averaged DO.

 

Table 7.2                     Summary of EPD Marine Water Quality Monitoring Data

Parameter

WM1

WM2

1996

1997

1998*

1996

1997

1998*

DO

 

[mg/L]

Surface

6.4

(4.7-8.1)

6.6

(4.7-11.1)

6.5

(3.5-8.2)

6.1

(4.4-8.5)

6.4

(5.0-9.2)

6.0

(3.5-7.6)

Bottom

5.5

(2.9-7.2)

5.8

(2.7-8.0)

5.4

(2.2-8.7)

5.7

(3.3-8.5)

5.5

(3.6-7.0)

5.3

(2.5-8.0)

Depth- averaged

10% - 3.7 mg/L

90% - 7.24 mg/L

10% - 3.4 mg/L

90% - 7 mg/L

 

pH

 

8.1

(7.8-8.3)

7.8

(6.8-8.2)

7.7

(7.0-8.2)

8.1

(7.8-8.3)

7.8

(7.2-8.2)

7.9

(7.5-8.2)

SS

[mg/L]

4.8

(3.0-8.1)

4.3

(2.2-7.2)

3.9

(2.7-4.8)

7.5

(2.6-14.6)

5.9

(2.5-12.3)

5.4

(1.9-11.2)

 

TIN

[mg/L]

0.19

(0.04-0.38)

0.2

(0.11-0.43)

0.24

(0.12-0.44)

0.25

(0.14-0.40)

0.30

(0.18-0.42)

0.25

(0.14-0.39)

BOD5

[mg/L]

0.6

(0.36-1.07)

0.66

(0.21-1.6)

0.71

(0.34-1.1)

0.56

(0.27-1.39)

0.6

(0.08-1.9)

0.79

(0.23-1.36)

E-Coli

[cfu/100ml]

188

(56-1333)

201

(35-634)

65

(14-170)

111

(13-1167)

251

(21-1327)

345

(40-1869)

NH4-N

[mg/L]

0.09

(0.01-0.26)

0.08

(0.03-0.13)

0.09

(0.04-0.17)

0.09

(0.02-0.16)

0.13

(0.03-0.28)

0.12

(0.04-0.18)

Source:     EPD Routine Monitoring Data

Note: *      Data for 1998 were calculated based on the EPD monitoring results obtained up to November 1998.

 

Assessment Methodology

7.6               The water sensitive receivers that may be affected by the construction works and operational activities for the on-site concrete batching plant were described.  Potential sources of water quality impact that may arise during the construction and operation phases of the Project were identified.  This task included identifying pollutants from point discharges and non-point sources to surface run-off, and wastewater arisings during the operational phase.  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.  Residual impacts were also evaluated and requirements for environmental monitoring and audit identified.

Construction Phase Impact Assessment

Potential Sources of Impact

7.7               Potential sources of water quality impact associated with the construction of the proposed concrete batching facility have been identified and include:

Ÿ         construction run-off and drainage;

Ÿ         general construction activities; and

Ÿ         sewage effluent produced by the on-site workforce.

The construction of additional piers or berthing facilities will not be required.  There will not be any marine-based construction activities associated with the proposed concrete batching plant.

Construction Runoff and Drainage

7.8               During site formation works, soil surfaces would be exposed and an elevated level of suspended particles would be present in the surface run-off.  As the proposed site for the concrete batching plant is located near the seawall, the coastal waters could potentially be impacted by sediment laden and polluted runoff if construction runoff from the site is uncontrolled.  Sources of water pollution include release of cement materials with rain wash, wash water from dust suppression sprays, and fuel, oil and other lubricants from maintenance of construction vehicles and mechanical equipment.

7.9               Mitigation measures should be implemented to control construction site runoff, and to minimise the chances of introducing sediment and pollutants into the nearby coastal waters. With the implementation of adequate construction site drainage and the provision of sediment removal facilities, it is expected that unacceptable water quality impacts on the coastal waters would not arise.

7.10            The nearest saltwater intake for the proposed WSD saltwater pumping station at the southern end of the reclamation is located at a distance of approximately 200 m from the boundary of the proposed works site for the concrete batching plant.  Any stormwater discharge locations from the works site should be located at a distance of over 100 m from the seawater intake point.  With the implementation of the recommended mitigation measures and good site practices, it is anticipated that adverse water quality impacts would not arise at the intake point of the saltwater pumping station during the works period.

General Construction Activities

7.11            On-site construction activities may cause water pollution from the following:

Ÿ         Uncontrolled discharge of debris and rubbish, such as packaging and used construction materials, could result in floating refuse with associated impacts on the aesthetic quality of the coastal waters; and

Ÿ         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 water column.

7.12          Good construction practices and site management measures should be observed to ensure that rubbish, fuels and solvents do not enter the nearby coastal waters.

Sewage Effluent

7.13            Domestic sewage would be generated from the workforce during the construction phase.  It is unlikely that sewage generated from the site would have a significant water quality impact, provided that sewage is not discharged directly into storm water drains adjacent to the construction site.  Temporary sanitary facilities such as portable chemical toilets should be used on-site and properly maintained.

Operation Phase Impact Assessment

7.14            All water used within the concrete batching plant will be collected, stored and recycled to reduce resource consumption.  This includes water used in the concrete batching process, truck cleaning, yard washing and dust suppression spraying.  All spent dust suppression effluent will be collected and recycled.  Stormwater run-off at the plant will drain under gravity towards a sedimentation basin located at the lowest site formation level.  The overlying water from the sedimentation basin will be recycled for reuse within the plant.  The deposited sediment will be dewatered and the dry matter will require disposal off-site.  As no water will be discharged outside the boundary of the plant, there will not be any potential water quality impacts associated with the operation of the facility.

7.15            The use of conveyor barge is proposed instead of derrick barge for aggregate storage and transfer.  The transfer of material from barge to hopper is via conveyor system (no lifting of material) and the conveyors are enclosed.  The loss of material and floating refuse is therefore not an issue of concern. 

Mitigation Measures

7.16            Proposed mitigation measures for containing and minimising construction phase water quality impacts are summarised below.

Construction Runoff and Drainage

7.17            The site practices outlined in ProPECC PN 1/94 Construction Site Drainage should be followed as far as practicable 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, inter alia, the following items :

Ÿ         Provision of perimeter channels to intercept storm-runoff from outside the site.

Ÿ         Programming of the works to minimise earthworks during the rainy season whenever possible.

Ÿ         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 on Effluent Standards under the Water Pollution Control Ordinance.  The silt traps should be designed as per the guidelines given in Appendix A1 of ProPECC PN 1/94. Sedimentation basins should be configured so as to provide sufficient time for the suspended solids to settle out.  These facilities should be regularly desilted to maintain their effectiveness.

Ÿ         Channels or earth bunds should be constructed to direct the surface runoff to sand/silt removal facilities.

Ÿ         Exposed soil surfaces should be protected by paving as soon as possible to reduce the potential for soil erosion.

Ÿ         Open stockpiles of construction materials on site should be covered with tarpaulin or similar fabric to prevent surface erosion during rainstorms.

 

General Construction Activities

7.18            Debris and rubbish generated on-site should be collected, handled and disposed of properly to avoid entering the nearby coastal waters.  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. 

Sewage Effluent

7.19            Temporary sanitary facilities, such as portable chemical toilets, should be employed on-site.  A licensed contractor must be responsible for appropriate disposal and maintenance of these facilities.

Operational Phase

7.20            Environmental protection measures proposed for the operation phase of the concrete batching plant are described below :

·         All water used within the concrete batching plant will be collected, stored and recycled.  This includes water used in the concrete batching process, truck cleaning, yard washing and dust suppression spraying.  No water will be discharged outside the plant boundary.

·         Barge effluent should be properly contained and diverted to adequately designed treatment facilities, and treated to Technical Memorandum standard prior to discharge.

7.21            Regular environmental inspections should be conducted to check the environmental performance of daily operation.  These inspections will ensure proper installation and maintenance of pollution control measures, such as checking of sedimentation basin, wastewater recycling facility and enclosure of stockpiles, and the implementation of other mitigation measures.

Conclusions

7.22            The potential water quality impacts arising from the construction and operation of the proposed concrete batching plant have been assessed.  Key concerns of water quality issues are related to the construction phase, as no wastewater will be discharged from the plant during the operation of the plant.  It is considered that construction run-off and drainage generated during the construction works for the Project will have minimal impacts on the receiving waters provided that mitigation measures are implemented.  With the adoption and incorporation of the recommended mitigation measures for the construction phase, unacceptable residual impacts on water quality are not expected.