2.1 As identified in the EIA Report, a key water quality issue of the construction phase will be dredging works for the jetty and catwalk foundation and the approach channel. Marine water quality monitoring shall be carried out during the dredging works to ensure that any unacceptable increase in suspended solids/turbidity and decrease in dissolved oxygen due to dredging activities could be readily detected and timely action be taken to rectify the situation.
2.2 Dissolved oxygen (DO), turbidity and suspended solids (SS) levels shall be monitored at designated marine water quality monitoring stations during the dredging works. DO and turbidity should be measured in situ whereas SS should be determined by laboratory.
2.3 Other relevant data shall also be recorded, including monitoring location / position, time, water depth, pH value, salinity, temperature, tidal stages, weather conditions and any special phenomena or work underway at the construction site.
2.4 The proposed water quality monitoring schedule shall be faxed to EPD and AFCD on or before the first day of the monitoring month. EPD and AFCD shall also be notified immediately for any changes in schedule by fax.
Dissolved Oxygen and Temperature Measuring Equipment
2.5 The instrument should be a portable and weatherproof dissolved oxygen (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.
2.6 It should have a membrane electrode with automatic temperature compensation complete with a cable. Should salinity compensation not be built-in to the DO equipment, in-situ salinity should be measured to calibrate the DO equipment prior to each DO measurement.
Turbidity Measurement Instrument
2.7 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
2.8 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
2.9 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
2.10 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
2.11 Water samples for SS determination 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 2.15.
Monitoring Position
Equipment
2.12 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 during marine water monitoring to ensure the monitoring vessel is at the correct location before taking measurements.
Calibration of In-Situ
Instruments
2.13 The pH meter, 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 3 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.
2.14 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.
Laboratory Measurement/Analysis
2.15 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 0.1 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.
2.16 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 EPD or his representatives when requested.
2.17 The marine water quality monitoring stations during the dredging works are shown in Figure 2.1. These stations are chosen based on the following criteria:
i. To monitor the off-site dispersion of the sediment plume generated during dredging works; and
ii. Two control stations that shall be at locations representative of the project site in its undisturbed condition. Control stations should be located, as far as is practicable, both upstream and downstream of the works area.
2.18 The co-ordinates of the proposed monitoring stations are listed in Table 2.1.
Table 2.1 Proposed Marine Water Quality Monitoring Stations
|
Station |
Easting |
Northing |
|
M1 |
806244 |
827080 |
|
M2 |
806329 |
826408 |
|
M3 |
806235 |
826089 |
|
C1 |
806116 |
827618 |
|
C2 |
806034 |
825308 |
2.19 Control stations are necessary to compare the water quality from potentially impacted sites with the ambient water quality. Control stations shall be located within the same body of water as the impact monitoring stations but should be outside the area of influence of the works and, as far as practicable, not affected by any other works.
2.20 Three monitoring stations (M1 – M3) are proposed to monitor off-site sediment dispersion and are located close to the boundary of the mixing zone, i.e. to just outside the sediment plume generated by the dredging works (as predicted by the sediment spill modelling presented in the EIA Report).
2.21 Measurements shall be taken at 3 water depths, namely, 1 m below water surface, mid-depth and 1 m above sea bed, except where the water depth is less than 6 m, in which case the mid-depth station may be omitted. Should the water depth be less than 3 m, only the mid-depth station will be monitored.
2.22 The status and locations of water sensitive receivers and the marine 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 EPD.
2.24 Other relevant data shall also be recorded, such as: monitoring location / position, time, water depth, tidal stages, weather conditions and any special phenomena underway near the monitoring station. There shall not be any marine construction activities in the vicinity of the stations during the baseline monitoring.
2.25 In exceptional cases when insufficient baseline monitoring data or questionable results are obtained, the ET Leader shall seek approval from EPD on an appropriate set of data to be used as baseline reference. In the event of any significant delay in the construction programme, it is recommended that a second set of baseline monitoring be carried out before the commencement of the dredging works for the approach channel.
2.26 Baseline monitoring schedule shall be faxed to EPD two weeks prior to the commencement of baseline monitoring. The interval between two sets of monitoring shall not be less than thirty-six hours.
2.27 During the dredging works for the jetty and catwalk foundation and approach channel, monitoring shall be undertaken three days per week, at mid-flood and mid-ebb tides, with sampling/measurement at the designated monitoring stations. The interval between two sets of monitoring shall not be less than thirty-six hours except where there are exceedances of Action and/or Limit levels, in which case the monitoring frequency will be increased.
2.28 Two consecutive measures of DO concentration, DO saturation and turbidity will be taken in-situ at 1 m below the surface, mid-depth and 1 m above the seabed at each location. If the water depth is less than 6 m, the mid-depth measurement may be omitted subject to the approval of the ER. If the depth is less than 3 m, only the mid-depth measurements need to be taken subject to the approval of the ER. 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 parameters 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 measurements shall be collected at the same three depths.
2.29
If the impact monitoring results
indicate that dredging works have caused adverse impacts on water quality at
the monitoring stations, appropriate actions including the lowering of the production rate for
dredging, or restriction of dredging works to certain tidal conditions, should
be taken and additional mitigation measures should be implemented as
necessary.
2.30 Upon completion of all marine-based construction activities, a post-project monitoring exercise on water quality shall be carried out for four weeks in the same manner as the impact monitoring.
Event and Action Plan for Water Quality
2.31 Marine water quality criteria, namely Action and Limit levels, are shown in Table 2.2. These criteria should be applied to ensure that any deteriorating water quality could be readily detected. When the monitoring results of the water quality parameters at any designated monitoring stations exceed the water quality criteria, the actions in accordance with the Event and Action Plan in Table 2.3 shall be carried out.
2.32 It is recommended that if the monitoring results indicate that the dredging works have caused an adverse impact on water quality, additional mitigation measures should be recommended to rectify the non-compliance or the construction programme should be carefully reviewed to slow down the rate of dredging. The working schedule and the mitigation measures should be reviewed by the Contractor, the ET Leader and the ER, and if necessary, works should be slowed down or suspended until such impact is reduced to an acceptable level.
2.33 The ET Leader should assess the effectiveness and efficiency of the proposed mitigation measures and/or remedial actions for the on-going construction activities. The performance of the environmental management system (that is, of the overall EM&A programme) should be reviewed by the ET Leader on a quarterly basis. The findings of this review should be included in the quarterly EM&A summary reports, together with any recommendations to improve the performance of the EM&A programme.
Table 2.2 Action and Limit Levels for Marine Water Quality
|
Parameters |
Action |
Limit |
|
DO in mg/L (Surface, Middle & Bottom) |
Surface and
Middle 1 percentile of
baseline data; or midway between 5 percentile of baseline data and limit
level Bottom 1 percentile of
baseline data or midway between 5 percentile of baseline data and limit level |
Surface and
Middle 4 mg/L Bottom 2 mg/L |
|
SS in mg/L (depth-averaged) |
95 percentile of
baseline data or 120% of upstream control station's SS at the same tide of
the same day |
99 percentile of
baseline or 130% of upstream control station's SS at the same tide of the
same day |
|
Turbidity in NTU (depth-averaged) |
95 percentile of
baseline data or 120% of upstream control station's Turbidity at the same
tide of the same day |
99 percentile of
baseline or 130% of upstream control station's Turbidity at the same tide of
the same day |
Notes: 1. "depth-averaged"
is calculated by taking the arithmetic means of reading of all three depths.
2.
For DO, non-compliance of the water quality limits occurs when
monitoring result is lower than the limits.
3.
For turbidity and SS, non-compliance of the water quality limits occurs
when monitoring result is higher than the limits.
4.
All the figures given in the table are used for reference only and the
EPD may amend the figures whenever it is considered as necessary.
Table 2.3 Event and Action Plan for Marine Water Quality
|
Event |
ET Leader |
ER |
Contractor |
|
Action level being exceeded by one sampling day |
Repeat in situ measurement
on next day of exceedance to confirm findings; Identify source(s) of impact; Inform Contractor and ER; Check monitoring data, all plant, equipment and Contractor's working
methods. |
Check monitoring data submitted by ET and Contractor’s working
methods; Confirm receipt of notification of non-compliance in writing; Notify Contractor. |
Inform the ER and confirm notification of the non-compliance in
writing; Rectify unacceptable practice; Amend working methods if appropriate. |
|
Action level being exceeded by two or more consecutive sampling days |
Repeat measurement on next day of exceedance to confirm findings; Identify source(s) of impact; Inform Contractor, ER and EPD; Check monitoring data, all plant, equipment and Contractor's working
methods; Discuss mitigation measures with ER and Contractor; Ensure mitigation measures are implemented; Increase the monitoring frequency to daily until no exceedance of
Action level. |
Check monitoring data submitted by ET and Contractor’s working
methods; Discuss with ET and Contractor on the proposed mitigation measures; Ensure mitigation measures are properly implemented; Assess the effectiveness of the implemented mitigation measures. |
Inform the Engineer and confirm notification of the non-compliance in
writing; Rectify unacceptable practice; Check all plant and equipment and consider changes of working methods; Submit proposal of additional mitigation measures to ER within 3 working
days of notification and discuss with ET and ER; Implement the agreed mitigation measures. |
|
Limit level being exceeded by one sampling day |
Repeat measurement on next day of exceedance to confirm findings; Identify source(s) of impact; Inform Contractor, ER and EPD; Check monitoring data, all plant, equipment and Contractor's working
methods; Discuss mitigation measures with ER and Contractor. |
Check monitoring data submitted by ET and Contractor’s working method; Confirm receipt of notification of failure in writing; Discuss with ET and Contractor on the proposed mitigation measures; Request Contractor to review the working methods. |
Inform the ER and confirm notification of the non-compliance in
writing; Rectify unacceptable practice; Check all plant and equipment and consider changes of working methods; Submit proposal of mitigation measures to ER within 3 working days of
notification and discuss with ET and ER. |
|
Limit level being exceeded by two or more consecutive sampling days |
Repeat measurement on next
day of exceedance to confirm findings; Identify source(s) of impact; Inform Contractor, ER and
EPD; Check monitoring data, all
plant, equipment and Contractor's working methods; Discuss mitigation measures
with ER and Contractor; Ensure mitigation measures are
implemented; Increase the monitoring frequency to daily until no exceedance of Limit level for two consecutive days. |
Check monitoring data
submitted by ET and Contractor’s working method; Discuss with 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; Ensure mitigation measures
are properly implemented; Consider and instruct, if
necessary, the Contractor to slow down or to stop all or part of the
construction activities until no exceedance of Limit level. |
Take immediate action to
avoid further exceedance; Submit proposal of
mitigation measures to ER within 3 working days of notification and discuss
with ET and ER; Implement the agreed
mitigation measures; Resubmit proposals of
mitigation measures if problem still not under control; As directed by the Engineer,
to slow down or to stop all or part of the construction activities until no
exceedance of Limit level. |
Water Quality Mitigation Measures
2.34 The 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
2.35 In order to minimize the impacts on water quality due to dredging, the implementation of the following measures is recommended:
· Use of closed grab dredgers with a maximum daily production rate of 500 m3/day;
· Deployment of silt curtain to enclose the grab while dredging works are in progress. This layer of silt curtain is a frame type silt curtain and is designed to enclose local pollution caused by the grab dredging. It consists of floats, curtain and ballast at the bottom. Mid-ballast may be added as necessary. The Contractor should install the silt curtain prior to dredging and should remove it upon completion of dredging. In case the baseline water quality monitoring data indicates low ambient suspended solids concentrations of less than 12.5mg/l, a second silt curtain should be deployed around the project site (i.e. hanging type silt curtain) and the dredging rate lowered to 250 m3/day. The second layer of silt curtain should be formed from tough, abrasion-resistant permeable membranes, supported on floated booms in such a way as to ensure that egress of turbid waters from the enclosed dredging area shall be restricted.
· Use of small sized hopper barge and dredger for dredging works. The maximum draft of the dredger and hopper barge, when fully loaded, should not exceed 2m. The hopper barge should make more frequent trips to transport the dredged material to the allocated marine disposal ground, instead of remaining at Lung Kwu Chau until it is fully loaded, in order to avoid grounding of the barge during low tide.
2.36 Other good site practices that should be undertaken during the construction works include:
· mechanical grabs, if used, should be designed and maintained to avoid spillage and sealed tightly while being lifted;
· all vessels should be sized such that adequate clearance (i.e. minimum clearance of 0.6m) is maintained between vessels and the seabed in all tide conditions, to ensure that undue turbidity is not generated by turbulence from vessel movement or propeller wash;
· all pipe leakages shall be repaired promptly and plant shall not be operated with leaking pipes;
· the decks of all vessels should be kept tidy and free of oil or other substances that might be accidentally or otherwise washed overboard. Litter and loose objects, e.g. tools, must not be left lying around. Wires and ropes must be coiled and secured, so as to cause least obstruction. Cylindrical shape items (such as pipes), heavy objects/equipment and stagings/scaffoldings on deck should be lashed securely against the rolling movement of vessels. Doors must be properly secured to open or closed position to prevent swinging as the vessel rolls;
· adequate freeboard (i.e. minimum of 200mm) shall be maintained on barges to ensure that decks are not washed by wave action;
· all hopper barges and dredgers should be fitted with tight fitting seals to their bottom openings to prevent leakage of material;
· construction activities should not cause foam, oil, grease, scum, litter or other objectionable matter to be present on the water within the site or dumping grounds; and
· loading of barges and hopper barges should be controlled to prevent splashing of material into the surrounding water. Barges or hopper barges should not be filled to a level that will cause the overflow of materials or sediment laden water during loading or transportation.
2.37 It is recommended that the demolition and removal works of the submerged portion of the existing jetty be carried out around low tide to minimise the duration of works taking place within the marine waters.
Operation Phase
2.38 Adverse water quality impact associated with the operation phase of the jetty is not anticipated. During maintenance dredging, the recommended mitigation measures for the construction phase dredging should also be implemented to minimize the impacts on water quality.