Sediment oxygen demand and nutrient balance in Tolo Harbour - A land-locked embayment in Hong Kong

As a land-locked embayment in Hong Kong, Tolo Harbour has been receiving massive discharges of untreated municipal sewage, agricultural waste and cottage industrial effluent via river and watercourses over the last three decades. After implementation of the pollution control action plan in the early 1980s, the exogenous organic materials have been substantially reduced. However, the bottom sediments in the harbour have become a source of nutrients and exert a measurable oxygen demand upon overlying waters. In this paper, in vitro measurements of the sediment oxygen demand (SOD) were carried out. SOD values were found between 0.42-1.30 gO2/m2d. Nutrient data for the period 1982 to 1996 were also analyzed for temporal trends. Over the period of observation, the total inorganic nitrogen and total phosphorus both show an increasing trend, despite a decade of efforts in reducing nutrient loading. The release rates of potentially mobile nitrogen (N) and phosphorus (P) from the sediments collected from Tolo Harbour were determined by N and P release experiments under oxic conditions. The experimental results showed that the sediment released significant amounts of nutrients, especially orthophosphated and ammonia nitrogen. The maximum release rates were 15.0 and 206.0 mg/m2d, respectively. Although the external nutrient loading has been reduced, nutrients can gradually be released back into the water column from the contaminated sediments and delay improvement of the water quality.