The water duality of the River Kennet: initial observations on a lowland chalk stream impacted by sewage inputs and phosphorus remediation

The water quality of seven sites on the upper reaches of the River Kennet r ound the market town of Marlborough is described and related to the introdu ction of phosphorus treatment of effluent from Marlborough sewage treatment works (STW). The River Kennet is mainly groundwater-fed from a Cretaceous chalk aquifer and hence the river water is calcium- and bicarbonate-bearing and has a relatively constant composition of many major water quality dete rminands. In-stream biological activity gives rise to marked diurnal fluctu ations in pH (of approx. 0.8 units). Dissolved carbon dioxide and dissolved oxygen also show marked diurnal fluctuations. Dissolved carbon dioxide var ies from approximately 10 to 70 times atmospheric pressure, indicating net release of carbon dioxide and the dominance of heterotrophic (respiratory) processes over autotrophic processes (photosynthesis). Much of the excess c arbon dioxide is probably associated with carbon dioxide laden groundwater inputs and the relatively short within-stream residence times ensures only limited degassing to the atmosphere. Diurnal fluctuations in dissolved oxyg en vary from approximately 20% to 200% saturation. For both dissolved carbo n dioxide and dissolved oxygen, the amplitude of fluctuations is much lower during the winter period, when biological activity is at its lowest. The c oncentrations of soluble reactive phosphorus (SRP), total phosphorus (TP) a nd boron increase markedly just downstream of the sewage works as a result of this paint source input. These concentrations slowly decline further dow nstream as additional groundwater inputs dilute the effluent further. The i ntroduction of chemical treatment of sewage effluent for phosphorus reducti on at Marlborough STW resulted in a marked decrease in within-river SRP and TP concentrations to levels approximately the same as those upstream of th e STW. A comparison of SRP and boron concentrations reveals a reduction in in-stream SRP concentrations by approximately 75% following effluent treatm ent. In terms of within-river processes controlling in-stream phosphorus co ncentrations, previous studies have indicated that one potentially importan t mechanism within calcium bicarbonate bearing rivers may be related to co- precipitation of phosphorus with calcium carbonate (calcite). The present s tudy shows that the waters are oversaturated with respect to calcium carbon ate, that no equilibrium conditions exist and that phosphorus removal has l ed to undetectable changes in calcium carbonate oversaturation. Hence, it i s concluded that the primary changes in phosphorus levels within the river is directly associated with changing point source contributions from the ST W and physical dilution within the river. However (1) the results relate to only the first year of study and subsequent differences may become apparen t and (2) reactions between the water column and plant and bottom sediment interfaces may be important in regulating phosphorus fluxes within the rive r. The results presented in this paper mark a pilot phase of a longer-term initiative and this paper provides a background setting. The paper discusse s the longer-term objectives and important gaps in knowledge of the system that requires further address. (C) 2000 Elsevier Science B.V. All rights re served.