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Investigation of rising nitrate concentrations in groundwater in the Eden Valley, Cumbria. 4, estimating recharge rates through glacial till using an applied tracer technique

By A. Butcher, K. Griffiths, D. Lapworth, A. Humpage and S. Burke

Abstract

This component Work Package of the Eden Valley Groundwater Nitrate Project was\ud undertaken with the objective of determining the velocity of recharge through Glacial Till.\ud Glacial Till overlies the sandstone aquifer in much of the study area and depending on its\ud thickness and composition, it is thought could exert a significant control on the amount of\ud recharge entering the sandstone.\ud Tracer tests were undertaken at research plots established at the edges of the Till. The results\ud will be used to assess the impact on recharge caused by the presence of lower permeability\ud Till compared to areas where sandstone outcrops.\ud Accepting that there are inherent uncertainties and limitations associated with the various\ud methods for estimating recharge, it was proposed to use a direct method that could be\ud undertaken within the proposed project timescale of two years. The method used was to\ud measure the rate of movement of a peak concentration of applied tracer within the pore water\ud profile of the Till (and depending on drilling capability, the top part of the sandstone).\ud The tracers applied were deuterium and bromide which were irrigated onto small research\ud plots. The plots were left uncovered for duration of c. 400 days. Five plots were established,\ud three at the edges of intensive pasture fields, one adjacent to a previous deep borehole site\ud and one on fallow land.\ud A percussion drilling rig was then used to extract undisturbed cores through the Till. The\ud shallow boreholes were drilled in April 2007. The thickness of Till encountered on drilling\ud ranged from c. 0.9m to 4.2m.\ud The cores were sealed and transported to BGS Aquifer Properties Laboratories where\ud following logging and photography samples were taken every 20cm along the cores and\ud porewaters were extracted using a centrifuge. Extracted porewaters were then analysed in the\ud BGS Environmental Indicators Laboratories for deuterium and in the Hydrochemical\ud Analysis Laboratories for bromide.\ud The pore water chemistry results indicated that elevated deuterium and bromide had\ud penetrated to between 1 and 2 m depth, equivalent to a downward velocity of 0.85 to1.7 m/y\ud probably depending on the amount and composition of Till present and the nature of\ud cultivation. The correspondence between deuterium and bromide profiles was very good in\ud all profiles. The coefficient of determination, R2, in three profiles was between 0.66 and 0.84.\ud Where Till was thicker it was rather more difficult to determine the ‘peak’ of tracer present\ud particularly with the bromide tracer, although this appears to be at a depth of approximately\ud 0.7m, equivalent to a downward velocity of 0.6 m/y. Till appears to disrupt the vertical flow\ud of tracer through to the sandstone aquifer below. Hence there should be enhanced run-off\ud associated with the thicker Till.\ud This contrasts with some conclusions from preceding Work Packages in this study where the\ud rate of water movement through the unsaturated zone in the sandstone underlying any\ud superficial deposits is between 3.5 - 3.85 m/y at an adjoining plot where the average recharge\ud rate is in the range 424-468 mm/y.\ud There is an interesting distinction between profiles of nitrate and chloride. Where Till is thin\ud these demonstrate a general correspondence. However, where Till is thicker or in the fallow\ud plot, chloride profiles follow trends in other profiles (a concentration effect) whilst nitrate\ud profiles remain very low

Publisher: British Geological Survey
Year: 2009
OAI identifier: oai:nora.nerc.ac.uk:14165

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Citations

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