4                   Water Quality

4.1                 Introduction

4.1.1           The water quality assessment in the EIA Report identified that the key issue in terms of water quality would be related to excavation works for the construction phase of the proposed drainage improvement works to the river channels.  To minimize potential impacts on water quality during the drainage improvement works, construction methods with minimal disturbance to the watercourses have been recommended and the excavation works would be restricted to within an enclosed dry section of the channel.

4.1.2            The EIA Report concluded that the other identified water quality impacts associated with the river channel drainage improvement could be minimized by implementing the recommended mitigation measures, such as control measures on runoff and drainage from the works areas to minimise construction run-off, and proper site management and good housekeeping practices.  It was recommended that regular site inspections be undertaken to inspect the construction activities at all works areas in order to ensure the recommended mitigation measures are properly implemented.  In addition, 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 was recommended during the construction phase.   

 

4.2                 Water Quality Parameters

4.2.1           Dissolved oxygen (DO), turbidity and suspended solids (SS) level shall be monitored at designated water quality monitoring stations during the construction works.  The parameters of DO and turbidity should be measured in situ whereas SS should be determined by laboratory analysis.

4.2.2           Other relevant data shall also be recorded, including monitoring location / position, time, water depth, temperature, salinity, weather conditions and any special phenomena or work underway at the construction site.

4.2.3           The proposed water quality monitoring schedule shall be faxed to EPD on or before the first day of the monitoring month.  EPD shall also be notified immediately for any changes in schedule by fax.  A monitoring record sheet is presented in Appendix B for reference.

 

4.3                 Monitoring Equipment

Dissolved Oxygen and Temperature Measuring Equipment

4.3.1           The instrument should be a portable and weatherproof DO measuring instrument complete with cable and sensor, and use a DC power source.  The equipment should be capable of measuring:

·           A DO level in the range of 0 ‑ 20 mg L-1 and 0 ‑ 200% saturation; and

·           A temperature of 0 ‑ 45 degree Celsius.

4.3.2           It should have a membrane electrode with automatic temperature compensation complete with a cable.

Turbidity Measurement Instrument

4.3.3           The instrument should be a portable and weatherproof turbidity-measuring instrument using a DC power source.  It should have a photoelectric sensor capable of measuring turbidity between 0 - 1000 NTU (for example, Hach model 2100P or an approved similar instrument).

Sampler

4.3.4           A water sampler is required.  It should comprise a transparent PVC cylinder, with a capacity of not less than 2 litres, which can be effectively sealed with latex cups at both ends.  The sampler should have a positive latching system to keep it open and prevent premature closure until released by a messenger when the sampler is at the selected water depth (for example, Kahlsico Water Sampler or an approved similar instrument).

Water Depth Detector

4.3.5           A portable, battery-operated echo sounder should be used for the determination of water depth at each designated monitoring station.  This unit can either be hand held or affixed to the bottom of the work boat, if the same vessel is to be used throughout the monitoring programme.

Salinity

4.3.6           A portable salinometer capable of measuring salinity in the range of 0 – 40 parts per thousand (ppt) should be provided for measuring salinity of the water at each monitoring location.

Sample Containers and Storage

4.3.7           Water samples for SS measurement should be stored in high density polythene bottles with no preservative added, packed in ice (cooled to 4°C without being frozen) and delivered to the laboratory and analysed as soon as possible after collection.  Sufficient volume of samples should be collected to achieve the detection limit stated in Section 4.15. 

Monitoring Position Equipment

4.3.8           A hand-held or boat-fixed type digital Differential Global Positioning System (DGPS) with way point bearing indication and Radio Technical Commission for maritime (RTCM) Type 16 error message ‘screen pop-up’ facilities (for real-time auto-display of error messages and DGPS corrections from the Hong Kong Hydrographic Office), or other equipment instrument of similar accuracy, should be provided and used to ensure that the water sampling locations are correct during the water quality monitoring work.

Calibration of In-Situ Instruments

4.3.9           The DO meter and turbidimeter shall be checked and calibrated before use.  DO meter and turbidimeter shall be certified by a laboratory accredited under HOKLAS or any other international accreditation scheme, and subsequently re-calibrated at three monthly intervals throughout all stages of the water quality monitoring.  Responses of sensors and electrodes should be checked with certified standard solutions before each use.  Wet bulb calibration for a DO meter shall be carried out before measurement at each monitoring location.

4.3.10       Sufficient stocks of spare parts shall be maintained for replacements when necessary.  Backup monitoring equipment shall also be made available so that monitoring can proceed uninterrupted even when some equipment is under maintenance, calibration, etc.

 

4.4                 Laboratory Measurement / Analysis

4.4.1           Duplicate samples from each independent sampling event are required by EPD for all parameters.  Analysis of suspended solids shall be carried out in a HOKLAS or other international accredited laboratory.  Sufficient water samples shall be collected at the monitoring stations for carrying out the laboratory SS determinations.  The detection limit shall be 1 mg/L or better.  The SS determination work shall start within 24 hours after collection of the water samples.  The analyses shall follow the American Public Health Association (APHA) Standard Methods for the Examination of Water and Wastewater, 19th edition, 2540D or an equivalent method subject to the approval of EPD.

4.4.2           Additional duplicate samples may be required by EPD for inter laboratory calibration.  Remaining samples after analysis shall be kept by the laboratory for 3 months in case repeat analysis is required.  If in-house or non-standard methods are proposed, details of the method verification may also be required to submit to EPD.  In any circumstance, the sample testing shall have comprehensive quality assurance and quality control programmes.  The laboratory shall prepare to demonstrate the programme to DEP or his representatives when requested.

 

4.5                 Monitoring Locations

4.5.1           The proposed water quality monitoring stations during the construction works are shown in Figure 4.1.  Since the excavation works would be restricted to within an enclosed dry section of the channel, one control station shall be located in each stream / river course upstream of the works areas which shall act as a control station, and one monitoring station shall be located at the end of each stream / river course downstream of the works areas.  A monitoring station shall also be located downstream of the works areas to monitor the overall construction impact or the impact from the incoming tide. 

4.5.2           Measurements shall be taken at mid-water depth.  The status and locations of water sensitive receivers and the works activities may change after issuing this Manual.  If such cases exist, the ET Leader shall propose with justification for changes to monitoring locations or other requirements of the EM&A programme, and seek approval from the IC(E) and EPD.

 

4.6                 Baseline Monitoring

4.6.1           Baseline conditions in the streams and rivers shall be established and agreed with EPD prior to the commencement of construction works.  The purpose of the baseline monitoring is to establish ambient conditions prior to the commencement of the works and to demonstrate the suitability of the proposed monitoring stations. The baseline conditions shall normally be established by measuring the water quality parameters specified in Section 4.3.  The measurements shall be taken at the designated monitoring stations three days per week, at ebb tides (to reduce the impact of the incoming tide on the monitoring results), for at least four weeks prior to the commencement of works. 

4.6.2           Other relevant data shall also be recorded, such as:  monitoring location / position, time, water depth, weather conditions and any special phenomena underway near the monitoring station.  There shall not be any construction activities in the vicinity of the stations during the baseline monitoring.

4.6.3           In exceptional cases when insufficient baseline monitoring data or questionable results are obtained, the ET Leader shall seek approval from the IC(E) and EPD on an appropriate set of data to be used as baseline reference.

4.6.4           Baseline monitoring schedule shall be faxed to EPD one week prior to the commencement of baseline monitoring. 

4.7                 Impact Monitoring

4.7.1           During the course of the construction works, impact monitoring shall be undertaken three days per week, at ebb tides, with sampling / measurement at the designated monitoring stations.  Upon completion of the construction works, the monitoring exercise at the designated monitoring locations shall be continued for four weeks in the same manner as the impact monitoring.

4.7.2           Two consecutive measurements of DO concentration, DO saturation and turbidity will be taken at mid-depth at each location.  The monitoring probes shall be retrieved out of water after the first measurement and then redeployed for the second measurement.  Where the difference in value between the first and second readings of DO or turbidity is more than 25% of the value of the first reading, the reading shall be discarded and further readings shall be taken.  Water samples for SS measurement shall be collected at the same mid-depth. 

4.7.3           The water quality monitoring schedule shall be faxed to EPD on or before the first day of the monitoring month.  EPD shall be notified immediately of any changes in schedule by fax.

 

4.8                 Event and Action Plan for Water Quality

4.8.1           The water quality criteria, namely Action and Limit levels, are shown in Table 4.1.  These criteria should be applied to ensure that any deterioration of water quality is readily detected and timely action is taken to rectify the situation.  Should the monitoring results of the water quality parameters at any designated monitoring station exceed the water quality criteria, the actions in accordance with the Event and Action Plan in Table 4.2 shall be carried out.

 

Table 4.1   Action and Limit Levels for Water Quality

Parameters

Action

Limit

DO in mg/L

(mid-depth)

5 %-ile of baseline data

 

4 mg/L

 

SS in mg/L (mid-depth)

95 %-ile of baseline data or 120% of control station’s SS on the same day of measurement

99 %-ile of baseline or 130% of control station's SS on the same day of measurement

 

Turbidity in NTU (mid-depth)

 

95 %-ile of baseline data or 120% of control station’s turbidity on the same day of measurement

99 %-ile of baseline or 130% of control station's turbidity on the same day of measurement

Notes:  1.   For DO, non-compliance of the water quality limits occurs when monitoring result is lower than the limits.

2.        For SS and turbidity, non-compliance of the water quality limits occurs when monitoring result is higher than the limits.

 


Table 4.2         Event and Action Plan for Water Quality

Event

ET Leader

IC(E)

ER

Contractor

Action level being exceeded by one sampling day

Repeat in situ measurement to confirm findings;

Identify reasons for non-compliance and source(s) of impact;

Inform IC(E) and Contractor;

Check monitoring data, all plant, equipment and Contractor's working methods;

Discuss mitigation measures with IC(E) and Contractor;

Repeat measurement on next day of exceedance.

Discuss with ET and Contractor on the mitigation measures;

Review proposals on mitigation measures submitted by Contractor and advise the ER accordingly;

Assess the effectiveness of the implemented mitigation measures.

Discuss with IC(E) on the proposed mitigation measures;

Make agreement on the mitigation measures to be implemented.

Assess the effectiveness of the implemented mitigation measures.

Inform the ER and confirm notification of the non-compliance in writing;

Rectify unacceptable practice;

Check all plant and equipment;

Consider changes of working methods;

Discuss with ET and IC(E) and propose mitigation measures to IC(E) and ER;

Implement the agreed mitigation measures.

 

Action level being exceeded by more than two consecutive sampling days

Repeat in situ measurement to confirm findings;

Identify reasons for non-compliance and source(s) of impact;

Inform IC(E) and Contractor;

Check monitoring data, all plant, equipment and Contractor's working methods;

Discuss mitigation measures with IC(E) and Contractor;

Ensure mitigation measures are implemented;

Prepare to increase the monitoring frequency to daily;

Repeat measurement on next day of exceedance.

Discuss with ET and Contractor on the mitigation measures;

Review proposals on mitigation measures submitted by Contractor and advise the ER accordingly;

Assess the effectiveness of the implemented mitigation measures.

Discuss with IC(E) on the proposed mitigation measures;

Make agreement on the mitigation measures to be implemented;

Assess the effectiveness of the implemented mitigation measures.

Inform the ER and confirm notification of the non-compliance in writing;

Rectify unacceptable practice;

Check all plant and equipment; Consider changes of working methods;

Discuss with ET and IC(E) and propose mitigation measures to IC(E) and ER within three working days;

Implement the agreed mitigation measures.

 

Limit level being exceeded by one sampling day

Repeat in situ measurement to confirm findings;

Identify reasons for non-compliance and source(s) of impact;

Inform IC(E) Contractor and EPD;

Check monitoring data, all plant, equipment and Contractor's working methods;

Discuss mitigation measures with IC(E), ER and Contractor;

Ensure mitigation measures are implemented;

Increase the monitoring frequency to daily until no exceedance of Limit level.

Discuss with ET and Contractor on the mitigation measures;

Review proposals on mitigation measures submitted by Contractor and advise the ER accordingly;

Assess the effectiveness of the implemented mitigation measures.

Discuss with IC(E), ET and Contractor on the proposed mitigation measures;

Request Contractor to critically review the working methods;

Make agreement on the mitigation measures to be implemented;

Assess the effectiveness of the implemented mitigation measures.

Inform the ER and confirm notification of the non-compliance in writing;

Rectify unacceptable practice;

Check all plant and equipment;

Consider changes of working methods;

Discuss with ET, IC(E) and ER and propose mitigation measures to IC(E) and ER within three working days;

Implement the agreed mitigation measures.

 

4.9                 Water Quality Mitigation Measures

4.9.1           Mitigation measures recommended for the construction and operation phases of the Project are summarized below.  The implementation schedule of the recommended water quality mitigation measures is presented in Appendix A.

 

Construction Phase

Construction Run-off and Drainage

4.9.2           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

4.9.3           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. 

4.9.4           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

4.9.5           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

4.9.6           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

4.9.7           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.