6.                       Air Quality Assessment


6.1                    Introduction


6.1.1              This section presents a qualitative assessment of air quality impact associated with the construction and operation of the secondary drainage channel KT13. Potential construction phase air quality impacts consists of increased fugitive dust at the nearby air quality sensitive receivers and some odour issue when contaminate sediments are removed from the stream.


6.1.2              Operation of storm water drainage channel is not expected to have any significant air quality impacts. Maintenance cutting of vegetation and sediments removal may occasionally increase odor in the Study Area but in the long term such incidents are expected to be reduced, as the quality of the sediments improve due to continued implementation of Livestock Waste Control Scheme.


6.2                    Air Quality Assessment Criteria


6.2.1              The principal legislation regulating air emissions in Hong Kong is the Air Pollution Control Ordinance (APCO) [Cap 311] of 1983 and its subsidiary regulations. The whole of the HKSAR has been covered by Air Control Zones. The Hong Kong Air Quality Objectives (AQOs) stipulate maximum acceptable concentration of air pollutants. The AQOs for 1 hour, 24 hour and annual concentrations of four major pollutants are shown in Table 6.1.


Table 6.1

Hong Kong Air Quality Objectives (AQOs)



Concentration in

Micrograms per Cubic Meters (µg/m3)

Averaging Time

1 hourA

24 hourB

1 yearC

Nitrogen Dioxide (NO2)




Sulphur Dioxide (SO2)




Total Suspended Particulates (TSP)




Respirable Suspended Particulates (RSP)D






Concentrations measured at 298K (25°C) and 101.325 kPa (one atmosphere)

A      1 hour criteria not to be exceeded more than 3 times per year.

B      24 hour criteria not to be exceeded more than once per year.

C      Arithmetic means.

D      Respirable suspended particulates means suspended particulates in air with a nominal aerodynamic diameter of 10 micrometer (µm) or smaller

E      This control limit has no statutory basis but is used as a target level for limiting fugitive dust emissions generated by construction activities.


6.3                    Air Sensitive Receivers


6.3.1              The representative air sensitive receivers (ASRs) that may potentially be impacted by KT13 are listed in Table 6.2 and shown in Figure 6.1.


Table 6.2

Representative Air Sensitive Receivers (ASRs)





Approximate Distance from KT13 Site Boundary


Ma On Kong Village

Village House

35 m


Ho Pui Village

Village House

36 m


Ma On Kong Village

Village House

4 m


Ho Pui Village

Village House

6 m


North of Ma On Kong Village

Village House

12 m


Note: For location of ASRs, please refer to Figure 6.1.


6.4                    Baseline Air Quality


6.4.1              The Study Area is located in the northwest of Hong Kong and to the north is Shenzhen. This area falls within the Deep Bay Airshed and is topographically confined. The area generally exhibits a limited dispersive capacity because of the occurrence of inversion layers in the morning and evening[1]. In other words, the stable layer associated with the inversion layer acts as a lid, trapping any pollutants[2].


6.4.2              The wind rose from the Shek Kong Station (Figure 6.2) can be used as an indicator for the Study Area. The wind rose indicates that the dominant wind direction is north-easterly and easterly.

Existing Air Conditions


6.4.3              As there is no monitoring station at the Study Area, EPD's Air Quality Monitoring Station at Yuen Long was used to establish ambient air quality over the Study Area in the western end. According to data collated from the Yuen Long Station, the averages of the NO2, SO2, TSP and RSP for period between 2003[3] are shown in the Table 6.3 below.

Table 6.3

Air Pollutants Concentrations at Yuen Long EPD Station (2003)



Concentration in Micrograms per Cubic Meter (μg/m3)


1 hour

24 hour







































N/A means Not Applicable.

*              This control limit has no statutory basis but is used as a target level for limiting fugitive dust emissions generated by construction activities.


6.4.4              The table indicates that the concentrations of short and long-term SO2 and NO2 are below the AQOs. However, the short-term and long-term concentrations of TSP and RSP exceed the stipulated AQOs.


6.4.5              The high TSP and RSP levels recorded at EPD's Yuen Long Station were probably attributable to construction activities in the vicinity of the Station including works for Village Flood Protection for Yuen Long, Kam Tin & Ngau Tam Mei (30CD), Drainage Channels for Yuen Long & Kam Tin (43CD and 60CD), infrastructure provision for Yuen Long South development and the heavy industrial activities near Shenzhen as well as vehicular emission from nearby roads. In the long term, TSP and RSP levels in the Yuen Long area are likely to remain high as construction and industrial activities will continue.


6.5                    Impact Assessment and Mitigation


Construction Phase


6.5.1              The principal potential source of air quality impact arising from the construction of KT13 will be fugitive dust. In order to minimize impact on air quality, no on-site concrete batching plant will be erected. Any concrete to be used will be transported to the work site. When construction tenders/contracts are prepared, these documents will clearly specify that the Contractor shall not erect and operate on-site concrete batching plant. The temporary dust nuisance from construction activities, measurable as TSP (Total Suspended Particulates) and RSP (Respirable Suspended Particulates) will be generated mainly by site formation works where large quantity of silty material movement may be involved. The nuisance from emission of construction vehicles and plant will be relatively small for the area involved, as the number of construction vehicles and plant is expected to be limited and small. Moreover, the dust impact is expected to be relatively insignificant as the excavated material of the area will be mainly from river channel which have a high moisture content.


6.5.2              The construction duration for KT13 is expected to last for about 36 months, from mid 2007 to mid 2010.


6.5.3              Thus the most affected air sensitive receivers will be those village houses near and around the works areas. Only minor impacts are expected due to off-site transportation of spoil materials as the number of haulage trips outside the site is limited to approximately 4 per hour (round trip).  Most of the spoil will also be excavated in a moist state and directly loaded to trucks for disposal.


6.5.4              Due to the limited scale of construction works required and the fact that the use of gabions for lining the channel is proposed for KT13, no concrete batching plants and blasting will be required. Construction tenders/contracts will stipulate that blasting and on-site concrete batching shall not be practised.


6.5.5              The proposed construction works will involve site clearance in the works areas, excavation to formation levels of secondary channels, formation of banks of channels and restoration landscaping works after the construction activities.


6.5.6              The existing stream is a semi-natural stream, west of Tai Kek and near Ho Pui. The proposed works involved construction of two separate gabion lined secondary channels to the north and to the south of Ho Pui Area. The northern section (Section A) is about 400m long and the resultant channel will be about 5m wide and 3m high. While the southern section (Section B: trapezoidal channel) is about 300m long and the formed channel is about 5m wide and 2m high. The two will be connected with a twin box concrete culvert for bypass of storm water flow designed to cope with 1 in 50 years storm.


6.5.7              The proposed construction works will mainly involve site clearance in the works areas and formation of access road, excavation to formation levels of secondary channels, formation and concreting of bypass culvert, formation of channel banks and landscaping restoration works after the construction activities.


6.5.8              For the construction of secondary drainage channel KT13, the amount of spoil generated and filling material required is estimated to be about 81,283 m³. Based on a typical truck load of 6 m3 spoil, for a period of about 30 months and 10 hours per working day, the amount of vehicle trips is estimated to be 4 per hour (roundtrip). Therefore, dust impact arising from construction traffic is considered negligible.


6.5.9              As the construction works will likely be constructed section by section, construction dust nuisance is not expected to cause any significant dust impacts to the nearby sensitive receivers. The quantity of the spoil materials from site clearance and excavation is unlikely to be large enough to cause a significant dust nuisance. It is anticipated that excavated material will only be stockpiled on each local works area and the duration of the stockpiling will be as short as possible, since some of the excavated materials will be used back as backfilling materials. As some materials will be used back in the construction processes, the quantity of material need to be disposed of or import will be limited. Hence construction dust nuisance at nearby representative sensitive receivers will expect to be insignificant. In addition, the distance between the construction works and nearest air sensitive receiver is about 4m. This buffer distance is adequate to alleviate the ASR from the construction dust nuisance provided that proper construction dust mitigation measures are in place as required by the Air Pollution Control (Construction Dust) Regulation. As part of the ecological mitigation measure to minimise impact on the existing stream course, a restriction to minimised the working area to 75m length of stream has been proposed. Such restriction would also reduced the likely dust impact.


6.5.10          Dedicated haul road is not expected as access to the site is accessible via existing public road, e.g. Kam Ho Road.




6.5.11          In order to minimize the impact of potential odour during construction, excavation works should be conducted in the shortest time span possible. Programming of the works should be carried out well in advance to ensure that the time for such works can be minimized. If necessary, the contractors should also liaise with local resident to inform them of the duration of potential odour nuisance, and that efforts are made to minimize and mitigate such nuisance. For earth and other materials excavated from the ground, these should be transported away from the site within 1 day. In the event that on-site storage cannot be avoided, the stockpiling should be temporary. Also, all such stockpiled materials must be stored in covered skips. Any leachate from storage skips should be stored in covered buckets and tanks. It should be collected along with toilet waste by licensed contractor to be disposed at government sewer.


Dust and Odour Mitigation Measures


6.5.12          In order to comply with the Air Pollution Control Ordinance (APCO), the Contractor should undertake at all times to prevent dust nuisance as a results of his activities. The Contractors are required to follow all the requirements for dust control stipulated in the Air Pollution Control (Construction Dust) Regulation. Dust suppression measures should be installed as part of proper construction practice, and they should be incorporated in the Contract Specification and implemented to minimize dust nuisance to within acceptable levels arising from the works. The followings are examples of the dust and odour suppression measures.


Dust Mitigation Measures


(i)         The Contractor shall frequently clean and water the site to minimize fugitive dust emissions.


(ii)        Effective water sprays shall be used during the delivery and handling of aggregate, and other similar materials, when dust is likely to be created and to dampen all stored materials during dry and windy weather.


(iii)       Watering of exposed surfaces shall be exercised as often as possible depending on the circumstances.


(iv)       Areas within the site where there is a regular movement of vehicles shall be regularly watered as often as necessary for effective suppression of dust or as often as directed by the Engineer.


(v)        Where dusty material are being discharged to vehicle from a conveying system at a fixed transfer point, a three-sided roofed enclosure with a flexible curtain across the entry shall be provided. Exhaust fans shall be provided for this enclosure and vented to a suitable fabric filter system.


(vi)       The Contractor shall restrict all motorised vehicles within the site, excluding those on public roads, to a maximum speed of 15 km per hour and confine haulage and delivery vehicles to designated roadways inside the site.


(vii)      Wheel washing facilities shall be installed and used by all vehicles leaving the site. No earth, mud, debris, dust and the like shall be deposited on public roads. Water in the wheel cleaning facility shall be changed at frequent intervals and sediments shall be removed regularly. The Contractor shall submit details of proposals for the wheel cleaning facility. Such wheel washing facilities shall be usable prior to any earthworks excavating activity on the site. The Contractor shall also provide a hard-surfaced road between any washing facility and the public road.


(viii)      All vehicle exhausts shall be directly vertically upwards or directed away from the ground.


(ix)       Any materials dropped on paved roads shall be cleaned up immediately to prevent dust nuisance.


Odour Mitigation Measures


               (x)        Any odourous excavated material shall be placed away from sensitive receivers. The material shall be removed within 1 day.


               (xi)       Any odourous material stockpiled shall be of the shortest duration. Also, all stockpiled materials shall be stored in covered skips. Any leachate from these storage skips shall be collected in covered tanks or buckets and removed from site with toilet waste by licensed collectors for discharging to government sewer.


Operation Phase


6.5.13          During the operation phase, any excavated sediments arising from maintenance dredging should be stored in covered impermeable skips and removed from site within 1 day.


Concurrent Construction Activities


6.5.14          Concurrent major projects in the North West New Territories (NWNT) include the proposed Deep Bay Link (6.5 km away), Yuen Long Highway Widening Works (4 km away) and Yuen Long Floodway Bypass (4 km away). Since all these projects are located far away from this Project, potential cumulative dust nuisance is not expected.


6.5.15          Other secondary drainage channels will be constructed within the same timeframe as KT13. All are spread out across the Kam Tin area, the nearest of which (KT12, 200m in length) is at least 350m to the east of KT13. The other nearby projects include the secondary drainage channels KT14 and KT15 which is approximately 650m and 1300m away from KT13. Despite the three years construction period of KT13, the cumulative dust impacts as a result of concurrent channel works within the Kam Tin areas are not expected to be significant given the large distance separation and provided that the recommended dust suppression measures are implemented.


6.6                    Residual Impacts


6.6.1             With the implementation of the proposed mitigation measures, no adverse residual impacts are expected during the construction and operation of KT13.


6.7                    Monitoring and Audit Requirements


6.7.1              Dust impacts during the construction phase can be readily mitigated through implementation of standard mitigation measures and proper housekeeping practices.


6.7.2              An Environmental Monitoring & Audit (EM&A) program in respect of dust issues will be carried out at ASR14 and ASR15 at Ma On Kong and Ho Pui Villages respectively to ensure that the proposed mitigation measures are implemented and that they are effective. These representative sensitive receivers are close to the construction works areas and hence, are most likely to be impacted if dust suppression measures are not properly implemented.


6.7.3              No monitoring is required during operational phase.


6.7.4              The details of EM&A requirements are presented in Chapter 11 of this Report.


6.8                    Summary


6.8.1              The construction dust impact assessment has identified Air Sensitive Receiver (ASR) within the Study Area. Potential works that may cause potential construction dust nuisance is also identified. Mitigation measures, as required by Air Pollution Control (Construction Dust) Regulation, are also highlighted. Provided that the Contractor implements all the necessary mitigation, construction dust nuisance at nearby representative sensitive receivers is expected to be complied with AQOs. Mitigation measures have been proposed in order to minimise the impact of potential odour during construction.


6.8.2              Operational stage odour nuisance is expected to be complied with the EPD’s criteria as regular cleaning and drainage improvement will improve the sediment deposition situation in all the proposed drainage.

    [1]            Royal Observatory Hong Kong, 1984: Meteorological Assessment of Atmospheric Transport Condition in the Deep Bay Airshed.

    [2]            Roland, B.S. (1988): An Introduction to Boundary Layer Meteorology, Kluwer Academic Publishers, p.666.

    [3]            EPD : Air Quality in Hong Kong 2003, Hong Kong Government