(ACE Paper 20/2002)
1. The Environmental Protection Department publishes two water quality reports at the end of every year to inform the public about the health status of Hong Kong's river and marine environment of the previous year. 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 river and marine waters in 2001.
River water quality
2. Since the late 1980s, the water quality of the rivers in Hong Kong has shown a steady improvement as shown in Figure 1 . In 2001, half of the watercourses (50%) has attained an "Excellent" Water Quality Index (WQI) grading, showing another 2.6% increase from that in 2000. The improvement was due to the efforts made in implementing environmental legislation and provision of sewerage facilities. For the second consecutive year, none of the river monitoring stations in the territory was in the "Very Bad" category.
3. Figure 2 shows the WQI grading of the most downstream monitoring stations in 12 major rivers in the territory. For the first time in 10 years, Sam Dip Tam Stream was upgraded to "Excellent" in 2001 (Figure 3b ). Tai Po River, Mui Wo River and Ho Chung River also maintained their "Excellent" water quality; while the Shing Mun Main Channel, Lam Tsuen River and Tuen Mun River remained "Good" during the year (Figures 3 a-b). However, Yuen Long Creek, River Ganges, River Beas, River Indus and Kam Tin River in the Deep Bay catchment remained "Fair" or "Bad" (Figures 3 b-c ) mainly due to the presence of livestock waste and unsewered villages.
4. The overall E.coli levels in the major rivers in 2001 remained more or less similar to those in 2000, ranging from an annual geometric mean of 3,700 cfu/100 ml in Mui Wo River to 1,300,000 cfu/100 ml in Yuen Long Creek.
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 shown in Figures 5 a-j. The E.coli, total nitrogen and dissolved oxygen (DO) levels in marine waters in 2001 in the territory were largely similar to those in 2000. Some increases in total nitrogen and decreases in dissolved oxygen were found in Deep Bay, which may be due to an increase in organic pollution in the catchment.
6. In general, the water quality in Port Shelter and Mirs Bay continued to be the best in the territory with very low sewage bacteria, nutrients and high in dissolved oxygen content. On the other hand, the water quality in Victoria Harbour remained poor, with relatively low dissolved oxygen and high E.coli levels in 2001.
7. Figure 6 shows the number of red tides in Hong Kong waters from 1980 to 2001. Most red tides occurred in the eastern and southern waters of Hong Kong. Red tide incidents increased significantly in the 1980s, reaching a peak in 1988. The number has decreased and remained relatively stable since the 1990s. In 2001, a total of 40 red tide cases were reported. Among them, one red tide in Mirs Bay and another in Port Shelter was associated with fish kills in the mariculture zones.
8. In general, the improvement of river water quality continued in 2001. The percentage of monitoring stations attaining an 'Excellent' WQI grading (50%) was the highest on record. However, the bacterial contents of the major rivers remained high, in particular those in the Deep Bay catchment.
9. The overall marine water quality in 2001 was largely similar to that in 2000 except for some deterioration detected in Deep Bay.
Water Policy and Planning Group
Environmental Protection Department