Southern Waters

The Southern Waters consist of one large WCZ, the Southern Water Control Zone, which covers an area of around 400 km² located to the south of Hong Kong Island and to the east of Lantau Island, directly open to the South China Sea. It is a large expanse of open sea and as such enjoys good levels of dissolved oxygen, but parts of it are affected by the discharge from the Pearl River further to the west, in particular during the wet summer months when the river's fresh water flow increases. The northern part of the WCZ is influenced by the flow from Victoria Harbour. In recent years, the rapid economic development of the Pearl River Delta region has led to increased urbanisation and industrialisation, and the river has carried higher levels of nutrients and pollutants as a result. The western part of the Southern Waters shows some symptoms that appear to be directly related to the Pearl River's outflow.

A number of parameters are affected by the Pearl River, including salinity, suspended solids (SS) and total inorganic nitrogen (TIN). Typically, higher levels of SS and TIN are found at the western stations than at those further east, while the situation is the reverse for salinity. This phenomenon becomes more obvious in the summer months.

Averaged levels of salinity, total inorganic nitrogen and suspended solids at four stations across the Southern WCZ during wet season (April - September) of 2005

In terms of WQO compliance, as the TIN levels at most stations in the WCZ are relatively high, especially in the summer, they exceed the WQO for this parameter. In the 1990s, the WQO compliance rate for TIN was consistently below 20%, and even in 2005 the only station to comply with the TIN objective was situated in the far eastern side of the WCZ. Because of TIN, the Southern WCZ achieved a relatively low overall WQO compliance level of 71% in 2005. Similar levels have been recorded since 1991.

Deep Water Bay Beach -one of the beautiful beaches on the southern side of Hong Kong Island that enjoy consistently good water quality

E. coli levels are generally low and stable across the WCZ, mostly below 10 cfu/100mL. In 2005, the WCZ achieved full compliance with the E. coli WQO. This included all the secondary contact recreation subzones (i.e. the eastern coast of Lantau Island and the south coast of Hong Kong Island), indicating their suitability for windsurfing, sailing and other water sports. In addition, over 20 popular beaches located within the WCZ (on the southern part of Hong Kong Island and the outlying islands of Lantau, Lamma and Cheung Chau) enjoyed consistently good water quality. All these beaches achieved either 'Good' or 'Fair' rankings in 2005.

However, in some places the bacteriological water quality was affected by coastal sewage discharges, with long-term increases in E. coli levels recorded at stations near Sok Kwu Wan and Kau Yi Chau. The first of these is apparently being affected by pollution from Sok Kwu Wan on Lamma Island, while the latter seems to be related to the discharge from the Stonecutters Island Sewage Treatment Works, in connection with the implementation of HATS Stage 1.

Most of the Southern WCZ monitoring stations are situated in open seas well away from urban areas, so sediment samples at these stations tend to have lower levels of heavy metals and toxic organics than many of those closer to the city.