Water quality of treated sewage effluent in a rural area of the upper Thames Basin, southern England, and the impacts of such effluents on riverine phosphorus concentrations

Data for water quality surveys of effluent from sewage treatment works (STWs) in the rural Kennet/Dun sub-catchments of the upper Thames Basin are presented to characterize treated sewage effluent. Water quality determinand relationships with boron (B) are presented to provide information that can be used, with stream water quality information, to assess the relative inputs of treated sewage effluent pollutants to streams in rural areas. The approach is based on three points: (1) information on sewage effluent and agricultural pollution is of concern in relation to the management of UK lowland river systems in rural environments; (2) the lack of detailed information on sewage runoff chemistry and flow means that direct assessment of sewage effluent pollution to surface waters cannot be gauged; (3) B provides a clear chemically conservative marker of sewage sources in surface and ground waters. Three types of relationship to B were observed. Firstly, determinands such as Na, Cl, soluble reactive phosphorus (SRP) and NO3 showed a positive linear relationship with B and there is a near zero intercept; these components are essentially derived from sewage sources. Secondly, Mg and SO4 show linear relationships with B, but there is a non-zero intercept; these components have both a sewage component and a background component linked to water supplies from surface and groundwater sources. Thirdly, there are determinands that show no relationship with boron. In this study, an erratic pattern was observed for ammonium. This probably reflects the variable removal of this pollutant from sewage sources. Near constant concentrations of components such as Ca and alkalinity, which come from the background aquifer sources, were also found. SRP and B relationships for rivers in the upper Thames Basin showed the potential importance of (a) removal processes in the stream/groundwater for SRP derived from STW effluent inputs and (b) tertiary P stripping at the STWs on river water SRP levels. The importance of agricultural sources of SRP is questioned.