ORTHOPHOSPHATE CONCENTRATIONS IN THE RIVER THAMES

Observed concentrations of orthophosphate in the River Thames, from it s source to its tidal limit, exceed 0.1 mg P/l for most of its length, reasons for annual and seasonal variations are explained. By using da ta routinely collected by the Environment Agency throughout the Thames catchment, the orthophosphate load in the river, derived from agricul tural sources and sewage-treatment works, is estimated. A water qualit y model, TOMCAT, has been adapted to simulate observed data and used t o estimate river concentrations if orthophosphate loads from sen age-t reatment works are reduced.