THE PHOSPHORUS-CONTENT OF RIVERS IN ENGLAND AND WALES

To determine the extent of phosphorus (P) enrichment of the major rive rs in England and Wales, a proportion of the large archive of data hel d by the National Rivers Authority on river orthophosphate concentrati ons was collated and statistical summaries produced. Length of samplin g record, sampling frequency and method of analysis was not consistant for all rivers, but median orthophosphate concentrations for 98 river s ranged from < 0.01 to 7.85 mg/l. Median concentrations exceeded 0.1 mg/l in 78 of the rivers and 1.0 mg/l in 16 rivers. Orthophosphate con centrations were largest in highly urbanised catchments and in some ru ral catchments with low flow rates, and smallest in rural upland catch ments and other high rainfall areas. Estimated loadings of orthophosph ate were < 1 kg/ha in 50% of rivers. Where total P data were available , orthophosphate was the dominant form of P present. Mean monthly orth ophosphate concentrations derived for 20 of the rivers increased slowl y during summer to reach a maximum between July and November and decre ased quickly over winter to reach a minimum between January and April. Similar temporal trends were observed in long-term time series for in dividual rivers. Limited linear regression analysis indicated that ort hophosphate concentrations had significantly increased over the last 2 0 years in some rivers. Reductions in orthophoshate levels in other ri vers could be related to recent attempts to control sources of P input s. Comparison of two different rural catchments demonstrated that the distribution of sewage treatment works and river flow characteristics had a major influence on the spatial variation of river orthophosphate concentrations in catchments. Although the data Were more representat ive of base flow conditions and therefore probably underestimate the c ontribution from agriculture, it can be concluded that the widespread P enrichment of rivers in England and Wales is largely due to point so urce P inputs. Copyright (C) 1996 Elsevier Science Ltd.