10.69. The Unit assisted in planning new treatment works and also carried out a number of miscellaneous projects, the most important being an investigation into the quality of salt water flushing supplies. The water testing facilities and advisory services continued to be utilized by other government departments, commercial and industrial firms and the general public, resulting in the examination of 1,127 samples, a similar number to the previous year.
Plover Cove Reservoir
10.70. Summer inflows reduced the salinity of impounded water from 530 parts per million (p.p.m.) expressed as sodium chloride (NaCl) in April to a minimum of 305 parts per million in the upper layers of water by September. For the first time marked thermal and chemical stratification of the stored water occurred from April to October with temperature and salinity differences of up to 12°C and 270 parts per million sodium chloride, respectively, persisting between surface and bottom layers. This was due to the higher water level than in previous years and lack of natural circulation and mixing during these months. However, the overall quality of water stored remained satisfactory throughout the year with no serious proliferation of algae or difficulties in treating the abstracted supply. With the cooling of surface water during the autumn complete mixing of the reservoir contents occurred in November by which time the average salinity was 390 parts per million sodium chloride and total dissolved solids 490 p.p.m. A further 520,500 fresh water fish fry were introduced during the year bringing the total number stocked to just over 1.1 million.
Water Treatment
10.71. Successful trials were conducted on experimental filters at Sha Tin in which part of the sand bed was replaced by a layer of specially graded crushed anthracite permitting increased filter ratings. As a result it was decided to carry out similar modifications to existing filters at Sha Tin and Tsuen Wan in connection with extensions to these works.
Quality of Supplies
10.72. Owing to temporary difficulties in obtaining stocks of chemical, fluoridation of supplies which had been started in 1961 with the aim
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