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Whilst many of the SAR's rivers are still not compliant with each individual WQO, significant improvements in water quality have been achieved in recent years, and the overall compliance level has steadily increased. Much of this improvement can be attributed to extension of the sewerage network to rural areas previously unserved by sewers or treatment plant, improvements in agricultural and chemical waste management, as well as a decline in agricultural and industrial intensity generally. Whilst acknowledging the recent progress and continuing responses by EPD and other Governments concerned to improve the river quality of Hong Kong, implementation of further work programmed in the SMPs should continue to bring about benefits through reduced organic pollutant loadings in rivers.
Nevertheless, a number of issues threaten the long term quality and assimilative capacity of freshwater systems in Hong Kong. Pollution of watercourses through illegal dumping of waste materials and contravention of discharge consents poses a threat to enhancement of assimilative capacity. Measures to address these problems lie either in encouraging better practice and environmental protection through education programmes or through penalising offenders through stiffer fines and greater enforcement of regulations. Abstraction of freshwater from rivers also affects water quality, particularly during dry weather conditions when river flows are already low, and results in reduced dilution capacity such that in some cases, the dry season flows of rivers are largely made up of effluent discharges.
The alteration and conversion of natural streams to drainage channels enhances the natural capacity of freshwaters to assimilate floodwaters and drainage, but can result in water quality impacts. In addition, channelisation results in a loss of ecological capital stock since drainage channels cannot be colonised by normal stream assemblages of flora and fauna. Unless more advanced channel designs which allow for some ecological mitigation are used, this process is simply substituting ecological capital stock for assimilative capital stock. There are examples of EIA studies which have resulted in drainage channel designs which provide for ecological mitigation, such as the Main Drainage Channels for Yuen Long and Kam Tin which feature specially constructed mangrove platforms.
In addition, the development of drainage works to relieve flooding or provide for urbanisation in areas of the north west New Territories also has implications for ecological capital in Hong Kong. Since these wetland areas are of particular ecological importance, the effects of new works on habitats such as wetlands, fish ponds and gei wais should be considered at a strategic level (eg through Stormwater Drainage Master Plans) as well as through individual project level assessment such as EIA.
The future sustainability of the freshwater resource depends upon a range of measures to prevent polluting discharges, enforcement of regulation to protect watercourses from waste disposal and illegal pollution, and to consider rivers and streams as part of the natural and ecological capital stock. Despite reductions in pollutant loadings to Hong Kong's rivers, there are still many point source discharges of both treated and untreated wastewaters from domestic, commercial, industrial and agricultural activities. Given the low flows of most rivers in Hong Kong, it seems unlikely that there will be sufficient assimilative capacity to deal with discharges other than those from non-point sources (ie run-off). Future water quality and assimilative capacity will therefore rest with measures to close off point source discharges to rivers, and direct wastewater discharges to sewer for eventual treatment.
Further research into determining assimilative capacity of freshwaters may help to better understand assimilative capacities of river systems and provide a scientific foundation to establish the discharge loadings which could collectively be accommodated within the assimilative capacity of the waterbody, taking account of the level of pollution already being discharged. It is recognised, however, that this approach would require a different analysis of each individual watercourse and may be a more complex task than its equivalent for marine waters.
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