The phosphorus content of rivers in England and Wales

To determine the extent of phosphorus (P) enrichment of the major rivers in England and Wales, a proportion of the large archive of data held by the National Rivers Authority on river orthophosphate concentrations was collated and statistical summaries produced. Length of sampling record, sampling frequency and method of analysis was not consistant for all rivers, but median orthophosphate concentrations for 98 rivers 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 concentrations were largest in highly urbanised catchments and in some rural catchments with low flow rates, and smallest in rural upland catchments and other high rainfall areas. Estimated loadings of orthophosphate were < 1 kg/ha in 50% of rivers. Where total P data were available, orthophosphate was the dominant form of P present. Mean monthly orthophosphate concentrations derived for 20 of the rivers increased slowly during summer to reach a maximum between July and November and decreased quickly over winter to reach a minimum between January and April. Similar temporal trends were observed in long-term time series for individual rivers. Limited linear regression analysis indicated that orthophosphate concentrations had significantly increased over the last 20 years in some rivers. Reductions in orthophoshate levels in other rivers could be related to recent attempts to control sources of P inputs. 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 representative of base flow conditions and therefore probably underestimate the contribution from agriculture, it can be concluded that the widespread P enrichment of rivers in England and Wales is largely due to point source P inputs.