Nitrate concentrations in river waters of the upper Thames and its tributaries
The spatial and temporal patterns of in-stream nitrate concentrations for the upper Thames and selected tributaries are described in relation to point and diffuse sources for these rural catchments. The rivers associated with catchments dominated by permeable (Cretaceous Chalk) bedrock show a smaller range in nitrate concentrations than those associated with clay and mixed sedimentary bedrock of lower permeability. The differences reflect the contrasting nature of water storage within the catchments and the influence of point and diffuse sources of nitrate. Nitrate concentrations often increase in a gradual way as a function of flow for the rivers draining the permeable catchments, although there is usually a minor dip in nitrate concentrations at low to intermediate flow due to (1) within-river uptake of nitrate during the spring and the summer when biological activity is particularly high and (2) a seasonal fall in the water table and a change in preferential flow-pathway in the Chalk. There is also a decrease in the average nitrate concentration downstream for the Kennet where average concentrations decrease from around 35 to 25 mg NO3 l- 1. For the lower permeability catchments, when point source inputs are not of major significance, nitrate concentrations in the rivers increase strongly with increasing flow and level off and in some cases then decline at higher flows. When point source inputs are important, the initial increase in nitrate concentrations do not always occur and there can even be an initial dilution, since the dilution of point sources of nitrate will be lowest under low-flow conditions. For the only two tributaries of the Thames which we have monitored for over 5 years (the Pang and the Kennet), nitrate concentrations have increased over time. For the main stem of the Thames, which was also monitored for over 5 years, there is no clear increase over time. As the Pang and the Kennet river water is mainly supplied from the Chalk, the increasing nitrate concentrations over time clearly reflect increasing nitrate concentrations within the groundwater. It primarily reflects long-term trends for agricultural fertilizer inputs and significant aquifer storage and long water residence times
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ChREAM homepage
Colin Neal
Margaret Neal
Helen Jarvie
Linda Hill
Heather Wickham

Nitrate; RELU; river; chalk; aquifer; Cherwell; Dun; Kennet; Lambourn; LOCAR; LOIS; pang; ray; Thame; Thames; water framework directive

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Issue number
Wednesday, January 3, 2018
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Date of publication
Sunday, January 1, 2006
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Place of publication
Amsterdam, The Netherlands
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Title of journal
Science of the total environment
Neal, Colin et al (2006) Nitrate concentrations in river waters of the upper Thames and its tributaries. Science of the total environment. 365 (1-3), pp. 15-32 Amsterdam, The Netherlands: Elsevier.
Neal Colin et al. Nitrate concentrations in river waters of the upper Thames and its tributaries. Science of the total environment 2006; 365 (1-3): 15-32.