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Advisory Council on the Environment

Interim Report on River and Marine Water Quality
in Hong Kong in 1998

(ACE Paper 32/99)
for information by circulation

Introduction

The Environmental Protection Department publishes two reports every year to inform the public about the quality of water in Hong Kong's rivers and marine environment. The preparation of these reports involves the lengthy process of field sampling, measurements and laboratory analyses, as well as gathering pollution load information, analysis of data, report preparation and printing. The river and marine water quality reports for 1998 are scheduled to be published in November 1999. For the early information of ACE members, this paper provides a summary of the water quality data from representative monitoring stations, and a 'snapshot' of the general condition of rivers and marine waters in 1998.

River water quality

2.The general river water quality in 1998 was similar to that in 1997 although the amount of rainfall in 1998 was substantially lower than that in the preceding year (2565 mm vs. 3343mm, thus less dilution of pollutants). Most of the river monitoring stations (84.6%) had a Water Quality Index (WQI) grading of 'fair' or above. There was a slight increase in the 'excellent' category (38.5%) as compared with that in 1997 (37.2%). Figure 1 shows the steady improvement of river water quality, reflecting reduction in organic pollution since the late 1980's.

3.Figure 2 shows the twelve major rivers in the territory and their most downstream monitoring stations. In 1998, the rivers which acquired an 'excellent' grading (Figures 3 a-c) included Mui Wo River, Ho Chung River and Tai Po River; whereas Shing Mun Main Channel, Tuen Mun River and Sam Dip Tam Stream obtained a 'good' grading in the year. Within the Deep Bay catchment, the water quality of Yuen Long Creek, River Ganges, River Beas and Kam Tin River remained either `bad' or `very bad' mainly due to pollution by livestock waste and from unsewered villages; the River Indus showed a small improvement to the 'fair' category.

4.The water quality index is calculated from the measurements of dissolved oxygen, 5-day biochemical oxygen demand and ammonia nitrogen. It does not account for bacterial levels. In this regard, while the improvement in general river water quality continued, the level of E.coli bacteria remained fairly high ranging from a geometric mean of 3,420 E. coli /100 mL in Mui Wo River to 3,290,000 E. coli /100 mL in Yuen Long Creek in 1998.

Marine water quality

5.A summary of the long-term water quality from ten representative marine monitoring stations located roughly in the middle of each water control zone (Figure 4) is plotted in Figures 5a. to 5j. Overall, the marine water quality in 1998 was similar to that in 1997 except for a slight decline in dissolved oxygen and E.coli in many areas. The high levels of nitrogen observed in the previous year were sustained in 1998. In addition, a rise in salinity and seawater temperature was observed, which may have been related to the unusual weather conditions in 1998.

6.In general, Port Shelter and Mirs Bay continued to have the best water quality in the territory with very low levels of sewage bacteria and nutrients and high levels of dissolved oxygen. On the other hand, the water quality in Victoria Harbour and Deep Bay remained poor with relatively low levels of dissolved oxygen and high levels of E.coli in 1998.

7.In March - May 1998, a massive toxic red tide broke out in the coastal waters of Hong Kong. A total of 8 fish kills were recorded, wiping out most of the fish stocks in the mariculture industry. Some twenty beaches were closed to the public during the bathing season due to red tides. Figure 6 shows the frequency of red tides in Hong Kong waters from 1980 to 1998. Most red tides occurred in the eastern and southern waters of Hong Kong. In 1998, a total of 38 red tide incidents were reported, the highest since 1989.

Conclusion

8.In 1998, while the momentum of steady long-term improvement was maintained, the general river water quality in the territory was largely similar to that in 1997. However, the bacterial content of the major rivers was still very high.

9.The overall marine water quality in 1998 was similar to that in 1997. A slight decline in dissolved oxygen and a rise in salinity and seawater temperature may have been related to the unusual weather conditions during the year.



Water Policy and Planning Group
Environmental Protection Department
August 1999


 

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