Physical factors such as Hong Kong's limited land area, topography, sheltered marine waters and confined airsheds mean that assimilative capacity is naturally constrained. The influence of anthropogenic impacts further stresses the environmental quality of these media. The high density of development in the Territory, the large (and growing) population and its sophistication with regard to demands for housing, energy, products, services, mobility and so on, means that significant amounts of pollution are generated and in many cases, are discharged to the receiving environments untreated. Assimilative capacity is affected not only by current levels of activity such as transport which significantly contributes to deteriorating air quality, but also by a legacy of post-war development and pollution of sediment, soils and groundwater contamination which will take many years to rectify.

Assimilative Capacity of Marine Waters - Issues and Trends

• The assimilative capacity of marine waters in Hong Kong is affected by a combination of factors including marine currents, discharges of freshwater from the Pearl River and high energy events such as typhoons. Inputs of polluting material from human activities acts to reduce the natural assimilative capacity available. Key pollutant inputs are untreated sewage discharges, inputs from rivers containing agricultural wastes and fertilisers, plumes from dredging activities, nutrient rich wastes from mariculture operations and illegal dumping. Natural assimilative capacity may be further reduced by physical works such as reclamation, particularly in waterbodies with naturally poor flushing capacities.

• Assimilative capacity of marine waters can be assessed by comparing pollutant levels with Water Quality Objectives (WQOs) for the 10 Water Control Zones (WCZs) in Hong Kong. The data show an improvement in average compliance with WQOs for Tolo Harbour, Port Shelter, Mirs Bay and Victoria Harbour WCZs over recent years. Compliance levels have generally remained steady for Southern, North Western, Western and Eastern WCZs while there has been a decline in compliance levels for the Deep Bay and Junk Bay zones, both of which experience particularly low Dissolved Oxygen (DO) levels.

• Compliance across the WCZs for individual pollutants between 1993 and 1998 show that compliance with the WQOs for Total Inorganic Nitrogen (TIN) have been consistently low in the Southern, Deep Bay and Victoria Harbour WCZs, whilst compliance with WQOs for unionized ammonia (NH3-N) and E. coli have generally been close to 100%.

• Surveys of toxic organic compounds have indicated that levels of Total DDT are significant enough in waters off North Point and south of Tsing Yi to be classed as moderately contaminated. Typhoon shelter sediments are particularly contaminated.

• Although beachwater quality declined from 1992 to 1997, in 1998 over 75% of gazetted beaches were ranked as 'good' or 'fair'. Most of the beaches with consistently poor water quality are located in the Tsuen Wan and Tuen Mun districts of the SAR.

• Overall, Hong Kong waters are under increasing pressure from local wastewater discharges such that the natural capacity of the marine environment to assimilate waste inputs appears to be exceeded in some areas such as Deep Bay. However, where pollution control measures have been implemented (eg in Tolo Harbour), improvements in water quality have been registered. A combination of policy/regulatory measures (such as the Sewerage Master Plans) and physical assimilative capacity enhancers (such as the Strategic Sewage Disposal Scheme) are likely to be needed to cope with anticipated increases in population without further compromising marine assimilative capacity.