The water quality of the River Thames at a rural site downstream of Oxford

Water quality information is presented for the River Thames 34 km downstrea m of the market town of Oxford in Oxfordshire to provide an overview of the hydrochemical functioning of a major agriculturally impacted river enterin g the North Sea. The data, which cover the period from the spring of 1997 t o the spring of 1999, relate to three types of data. These types are: (1) w eekly spot sampling for determination of major, minor and trace elements, p H, alkalinity and herbicides; (2) tri-weekly spot sampling for pH, alkalini ty and dissolved silicon; and (3) continuous measurements of pH and dissolv ed oxygen. Calcium and bicarbonate provide, respectively, the dominant cati on and anion in solution and their compositions remain relatively constant through time, irrespective of flow levels. In contrast, many determinands s how seasonally related fluctuations. Concentrations for most of the major a nions, sodium, potassium as well as soluble reactive phosphorus and several soluble trace elements such as boron, antimony, arsenic and molybdenum dec rease as flow increases. A reverse pattern is observed for nitrate, some he rbicides and trace elements associated with particulate phases: concentrati ons increase with increasing flow. These patterns reflect the influence of: (1) a calcium carbonate rich groundwater system which provides the main st ream flow component; (2) dilution of point source pollutant inputs associat ed with sewage and possibly light industry at high flows for several major, nutrient and trace elements; (3) enhanced nitrate and herbicide runoff fro m agricultural land at high flows; (4) enhanced microparticulate trace meta l levels associated with increased suspended sediment loads at high hows; a nd (5) biological processes which affect pH, dissolved silicon, dissolved c arbon dioxide and dissolved oxygen levels. An examination of soluble reacti ve phosphorus (SRP) and boron relationships reveals a reduction in concentr ations for SRP associated possibly with phosphorus removal from a major sew age treatment works on an upstream tributary of the Thames, the Thame. (C) 2000 Elsevier Science B.V. All rights reserved.