The water quality of a tributary of the Thames, the Pang, southern England

The water quality of a tributary of the Thames, the Pang, draining a rural part of the Thames basin is described and related to the influences of inpu ts from farming activity and groundwater from the underlying chalk aquifer as modified by biological within-stream processes. The groundwater inputs e nsures that the waters are calcium and bicarbonate bearing and have relativ ely uniform concentrations. Agricultural inputs result in enhanced levels o f nutrients, nitrate and soluble reactive phosphorus (SRP) in particular. F or nitrate, the concentrations are higher during the winter months due to i ncreased surface runoff. In contrast, SRP shows a more erratic behaviour wi th higher concentrations occurring during the early storm hydrograph rise f ollowing summer baseflow recession. Within the stream, biological activity results in: (a) marked diurnal fluctuations in the dissolved levels of carb on dioxide and oxygen; (b) a strong seasonal pattern in chlorophyll a level s; and (c) dissolved silica concentration reductions during the early sprin g period. Carbon dioxide levels are particularly high in the groundwater (t ypically approximately 60 times the atmospheric value) as observed at a spr ing discharge. However, within the stream, considerable degassing occurs al though values remain an order of magnitude above atmospheric pressure. The findings are discussed in the context of the water quality functioning of a griculturally and sewage impacted southern eastern UK rivers. For example, the work shows that unlike for riverine systems with point sewage discharge s or limited groundwater storage, there is a very poor link between chemica l concentrations and flow even for components such as SRP and boron which a re often connected to sewage discharges. (C) 2000 Elsevier Science B.V. All rights reserved.