Water and Sediment Quality in Typhoon Shelters

Besides its work monitoring open seas across Hong Kong, the EPD also monitors typhoon shelters, boat anchorages, marinas and the Government Dockyard, using a total of 18 water monitoring stations and 15 sediment monitoring stations. These areas are unique in character due to their nature and use.

[Photo of Causeway Bay Typhoon Shelter]

Typhoon shelters are embayments with low tidal flushing. Because of the fact that they are largely surrounded by a sea wall, they are particularly vulnerable to pollution from vessels using them and from the shore. Historically, water quality at certain typhoon shelters in urban areas has been bad, with very low levels of dissolved oxygen and high levels of E. coli and other pollutants. In addition, sediment from these typhoon shelters typically has low electrochemical potential and contains high levels of organic matter and heavy metals.

 

This pollution occurs largely because of the inability of the typhoon shelters to easily rid themselves of any contaminants that make their way into them. This means that the best way to improve water quality in the typhoon shelters in the long run is to track down and remove sources of the pollution flowing into them.

 

The EPD has been active in doing this around typhoon shelters across Hong Kong, especially those located in the heart of built-up urban areas. Typical pollution sources include street-washing, expedient connections (where sewers are connected to storm drains), and illegal discharges, where wastewater such as dishwater or water from wet markets is discharged directly into storm drains. The EPD has followed up very many such incidents over the past two decades, helping reduce the amount of effluent and wastewater flowing into typhoon shelters from the vicinity.

 

Other larger-scale developments have also helped. Some old residential areas, including groups of squatter huts, have been cleared and redeveloped. Occasionally a typhoon shelter has been relocated because of urban redevelopment. The Yau Ma Tei Typhoon Shelter is an example. In the case of the Kwun Tong Typhoon Shelter, improved levels of dissolved oxygen have arisen both because of the EPD's ongoing efforts to reduce the discharge of wastewater into it, and because of the Government's Tolo Harbour Effluent Export Scheme, in which relatively clean treated effluent has been piped from the Sha Tin and Tai Po Sewage Treatment Works in the Tolo Harbour area and released near the Kwun Tong Typhoon Shelter, raising oxygen levels in the water and improving water flow. As a result both of specific EPD pollution control measures and other projects, water quality in a number of typhoon shelters in urban areas (including those in Kwun Tong, Sam Ka Tsuen, Yau Ma Tei and Aberdeen) has improved in recent years. Levels of dissolved oxygen have increased while pollutants such as E. coli bacteria have been significantly reduced.

[Photo of Comparison of E. coli levels in typhoon shelters in 1996 and 2005]

In general, typhoon shelters around Victoria Harbour suffer from more chronic problems than those located in the territory and islands, due to their long history in highly urbanised and industrialised parts of the city. In 2005, E. coli levels at the typhoon shelters in Sai Kung and Hei Ling Chau stood at very low levels (<10 cfu/100mL), but levels at those within Victoria Harbour were much higher (1,000-10,000 cfu/100mL). The distribution of other pollutants such as ammonia nitrogen and total inorganic nitrogen was similar. In addition, higher levels of cadmium, chromium, copper and other metals are also typically found in the sediment of typhoon shelters around Victoria Harbour. This contamination appears to be related to discharges between the 1960s and the 1980s from industries involved in electroplating and in the manufacture of printed circuit boards and electronics goods.

 

Bio-remediation in the Sam Ka Tsuen Typhoon Shelter

[Photo of Bio-remediation of anoxic sediment being carried out in the Sam Ka Tsuen Typhoon Shelter in 2004 ]

The Government has been addressing many of the water quality problems faced by typhoon shelters as outlined above. One of the most noticeable problems associated with water pollution is the bad smell of polluted water, an odour problem occurring when hydrogen sulphide gas is released from sediment that is rich in organic matter and low in oxygen.

 

This was a particularly severe problem in the lower Shing Mun River in Sha Tin, but dredging and a bio-remediation project undertaken in 1997 made a huge difference to the smell of the river. Bio-remediation can be achieved in different ways, but in the case of the Shing Mun River, it involved injecting calcium nitrate into the sediment, thus converting anoxic pollutants into odourless gases such as nitrogen or carbon dioxide, and in the process removing offensive odours. Based on the success of that project prompted the Government carried out a similar project in the Sam Ka Tsuen Typhoon Shelter in 2004.

 

The EPD's long-term monitoring data indicates that the electrochemical potential of the sediment at Sam Ka Tsuen Typhoon Shelter (which reflects its oxygen content) was substantially raised (became less negative) as a result of the bio-remediation programme, and the total sulphide level (hydrogen sulphide gas, the source of the bad odour) was significantly reduced. Nearby residents could breathe easier as a result of the initiative!

[Photo of Electrochemical potential and total sulphide levels at Sam Ka Tsuen Typhoon Shelter (2000-2005) ]