LCQ7 : Water quality of Plover Cove Reservoir remains satisfactory

Following is a question by the Hon Kwong Chi-kin  and a written reply by the Secretary for the Environment, Transport and Works, Dr Sarah Liao, at the Legislative Council meeting today (November 16) :


It has been reported that Plover Cove Reservoir, the major recipient reservoir of water from Dongjiang, recently experienced an unusual algal bloom which caused low oxygen levels in the water, resulting in extensive deaths of fish.  The scene of dead fish all over the place arouses much concern about the water quality there.  In this regard, will the Government inform this Council:

(a)  whether the reasons for the deterioration in water quality of the Reservoir have any connection with the Dongjiang water; the types of pollutants in the water of the Reservoir and whether the existing filtering facilities of the Water Supplies Department can filter out all the pollutants;

(b)  whether the existing facilities of the filter station and the "aerated system" to be installed can eliminate all the heavy metals and organic matter contained in the water of the Reservoir; if not, whether the authorities have other measures to improve the water quality of the Reservoir; and

(c)  of the estimated expenditure on improving the water quality of the Reservoir and whether it will exceed the budget; if so, whether such over-spending will become recurrent every year?


Madam President,

According to the monitoring records of the Water Supplies Department (WSD), the water quality of the Plover Cove Reservoir for the past year generally remained stable and satisfactory.  There was no pollution or deterioration in water quality, nor was there any unusual algal bloom.  However, an upsurge in sand particles in the water resulting from continuous heavy downpour in June this year has caused the death of a small quantity of fish.

(a)  Since the commissioning of the Dongjiang-Shenzhen closed aqueduct in June 2003, there has been significant improvement in the quality of Dongjiang water on all fronts.  The amount of heavy metals or pollutants in Dongjiang water detected by the WSD, such as lead, mercury, cadmium and agricultural pesticide, etc., continuously remains at a very low or even undetectable level.  The WSD has been publishing on its website information on the examination of Dongjiang water quality at Muk Wu Pumping Stations as well as the monitoring results of local potable water for public information on a regular basis.  Hong Kong's potable water has always complied with the Guidelines for Drinking Water Quality drawn up in 1993 by the World Health Organization (WHO) in chemical and bacteriological aspects and is safe for drinking.

A natural phenomenon called "thermal stratification" often occurs in the Plover Cove Reservoir.  The phenomenon is not rare among all deep water lakes or reservoirs.  In summer, the surface temperature of the reservoir increases with solar radiation while the bottom of the reservoir remains cool.  The difference in density leads to stratification, which isolates the bottom layer water from the air at the surface and results in an anaerobic condition.  When this phenomenon occurs, further re-solution of chemicals takes place at the bottom sediment, and, with abundant sunshine, will promote the growth of algae on the water surface.  As a result, the pH values of the water will rise while the amount of dissolved oxygen will drop.  Therefore, controlling the growth of algae plays an important role in the management of reservoirs.  However, when the water quality of a reservoir is affected by the growth of algae, the WSD can lower the water's pH values and filter out the algae by adjusting the water treatment process to ensure that the water quality meets WHO standards.

(b)  In order to eliminate the "thermal stratification" phenomenon in the Plover Cove Reservoir, the WSD plans to install the "aerated system", which can increase the dissolved oxygen in the water as well as control and manage the water quality of the reservoir effectively so as to alleviate any water quality problem in case of a huge algae bloom.  Moreover, the WSD also takes other measures such as maintaining the ecological balance of the reservoir and preventing the water catchment areas from contamination in order to protect and improve the water quality of the Plover Cove Reservoir more effectively.

(c)  The estimated construction cost of the proposed "aerated system" is about HK $4.5M while the estimated annual recurrent operating cost is about HK $0.3M.  The required costs do not exceed the budget. At present, the WSD is applying to the Environmental Protection Department for an environmental permit. If approved, the construction works can commence immediately and the project is expected to be commissioned in March 2006.

Apart from the "aerated system", the WSD draws up an annual estimate of about HK$ 0.1 M for the purchase of fish fry to be put in the Plover Cove Reservoir as an algae control measure to maintain the ecological balance and improve the water quality of the reservoir.

End/Wednesday, November 16, 2005