Field measurements of SOD and sediment nutrient fluxes in a land-locked embayment in Hong Kong

The pollution problem in Hong Kong has increased over the past two decades, and the situation in the nearly land-locked Tolo Harbour, located at the north-eastern part of the territories, has been particularly problematic. Although the exogenous organic materials dissolved in the overlying water have been substantially reduced, the soft sediments at the bottom of the harbour continue to act as sources of nutrients. This sediment oxygen demand (SOD) amounts to a significant value, hence increasing the total oxygen demand load of the harbour. In this paper, the current state of the water environment is reviewed. The SOD and the rate of nutrient release in different forms of nitrogen and phosphorus were measured in laboratory experiments. It was found from the laboratory tests that the average SOD value is 38 mgO2/m2-h and that the mean release rates of phosphate phosphorus and ammonia nitrogen are, respectively, 3 and 62 mg/m2-d. These measured field data will be useful in understanding the water quality of the water body and also provide parameters that may be used when developing numerical water quality models for Tolo Harbour.