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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 from domestic and commercial sources, 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.
The trends in the average concentrations of four key marine pollutants (Dissolved Oxygen, Suspended Solids, Total Inorganic Nitrogen and E. coli) across each of the ten Water Control Zones (WCZs) are shown in Figures 5.1a to 5.1d. Based on the overall mean values calculated for each WCZ using the 1994 - 1998 data, levels of E.coli are highest in the Deep Bay and Victoria Harbour WCZs and that Deep Bay WCZ also has the highest levels of suspended solids and total inorganic nitrogen.
Assimilative capacity of marine waters can be assessed by comparing pollutant levels with Water Quality Objectives (WQOs) for the ten WCZs in Hong Kong. The data in 1998 show that the levels of compliance with the WQOs in Tolo Harbour, Southern, Port Shelter and Mirs Bay WCZs generally remain steady in the past five years. On the other hand, the WQO compliance rates of Junk Bay, Deep Bay, Northwestern, Western Buffer, Eastern Buffer and Victoria Harbour WCZs have shown some decline during the same period. The overall decline in WQO compliance was largely due to the exceptionally low compliance rates (69%) in 1998 which was an EI Nino year with unusual weather and oceanographic conditions. 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%.
Marine waters in Hong Kong have been subject to occurrences of red tides. From 1980 to 1997, 481 red tides have been recorded with most occurring in the eastern WCZs of Tolo Harbour and Channel (48% occurrences), Mirs Bay (16%), Port Shelter (13%) and the Southern WCZ (13%). The annual numbers of red tide peaked in 1988 (88) and declined thereafter to 10 - 25 during the 90s (Figure 5.1e). A significant proportion (between 20% and 50%) of red tides occurred during 1993 and 1997 have affected bathing beaches.
Figure 5.1e: Annual Red Tide Occurrences in Hong Kong, 1980-1997
Monitoring of the quality of marine sediments by EPD indicates that typhoon shelter sediments are particularly contaminated. Water quality (E. coli) is also monitored at Hong Kong's beaches. Although beach water quality declined from 1992 to 1995, in 1998 over 75% of gazetted beaches were ranked as 'good' or 'fair'(Figure 5.1f). Most of the beaches with consistently poor water quality are located in the Tsuen Wan District of the SAR.
Figure 5.1f: Annual Rank of Gazetted Beach Water Quality 1992 - 1998
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 and the deleterious effects of poor water quality in terms of aesthetic, public health and ecological impacts.
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