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Advisory Council on the Environment

Interim Report on River and Marine Water Quality in Hong Kong in 1999 (without Figures)

(ACE Paper 25/2000)
For discussion

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 in our marine environment. The production of these reports involves the lengthy process of field sampling, measurements and laboratory analyses, as well as data analysis, report preparation and printing. The river and marine water quality reports for 1999 are scheduled to be published in November 2000. For the early information of ACE members, this paper provides a brief summary of the water quality data from representative monitoring stations, and a 'snapshot' of the general condition of rivers and marine waters in 1999.

River water quality

2.The rivers in Hong Kong have shown steady improvement since the late 1980s in terms of the Water Quality Index (WQI) (Figure 1). The WQI generally reflects the river's organic pollution level and its ability to support aquatic life. The river water quality in 1999 was largely similar to that in 1998 with slight variations in the different index categories. Most of the river monitoring stations (83.2%) obtained a WQI grading of 'fair' or above, representing a slight decrease as compared with that in 1998 (84.6%). The difference may be due to natural fluctuations as 1999 was drier than the previous year (2129mm of rainfall in 1999 vs 2565mm in 1998) resulting in less baseline flow and dilution in the rivers.

3.Figure 2 shows the location of twelve major rivers in the territory and their most downstream monitoring stations. In 1999, Mui Wo River continued to maintain an 'excellent' WQI grading; whereas Shing Mun Main Channel, Tai Po River, Ho Chung River and Tuen Mun River obtained a 'good' grading (Figures 3 a-b). Within the Deep Bay catchment, the water quality of Yuen Long Creek, River Ganges, River Beas, River Indus and Kam Tin River remained either `bad' or `very bad' mainly due to pollution by livestock waste and from unsewered villages (Figure 3c). Although a slight improvement has been observed in the Upper River Indus and Lower River Ganges, overall, the quality of Deep Bay rivers is still unsatisfactory.

4.The level of E.coli bacteria in the major rivers remained fairly high ranging from a geometric mean of 1,000 no./100 mL in Mui Wo River to 2,700,000 no./100 mL in Yuen Long Creek in 1999. By and large, the bacteriological quality of the major rivers in 1999 was similar to that in 1998.

Marine water quality

5.A summary of the long-term water quality data from ten representative marine monitoring stations, located roughly in the middle of each of the water control zones (Figure 4), is plotted in Figures 5a. to 5j. In 1999, the E.coli and dissolved oxygen levels in the marine waters were generally similar to those in 1998. There had been a significant rise in nitrogen levels throughout the territory in 1997 and 1998, coinciding with the El Nino event. By 1999, the phenomenon had subsided and nitrogen in the marine water had largely returned to the pre-1997 level.

6.In general, Port Shelter and Mirs Bay continued to have the best water quality in the territory with very low concentrations of sewage bacteria and nutrients and high concentrations of dissolved oxygen. On the other hand, the water quality in Victoria Harbour remained poor with a relatively low dissolved oxygen content and a high level of E.coli in 1999.

7.Figure 6 shows the frequency of red tides in different water control zones from 1980 to 1999. Red tide incidents increased markedly in the 1980s, reaching a peak in 1988. Since then, the number of occurrences has declined. The frequency has largely remained stable in the 1990s. A massive toxic red tide bloom occurred in spring 1998, killing most of the cultured fish stock in the territory. In 1999, a total of 31 red tides were reported, but there was no fish kill associated with red tides.

Conclusion

8.In 1999, the general long-term trend of river water quality improvement was maintained and the condition of the territory's rivers was largely similar to that in 1998. However, the bacterial content of the major rivers remained very high.

9.In the marine waters, the overall water quality in 1999 was similar to that in 1998. There was a substantial decline in nitrogenous nutrients in 1999 following a period of high nitrogen in the previous two years.

Water Policy and Planning Group
Environmental Protection Department
July 2000


 

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