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Investigation of rising nitrate concentrations in groundwater in the Eden Valley, Cumbria. 2, unsaturated zone studies

By A. Butcher, A. Lawrence, M. Mansour, S. Burke, J. Ingram and P. Merrin

Abstract

This component Work Package of the Eden Valley Project was undertaken with the objective\ud of estimating recharge rates and the timescale for water movement through the unsaturated\ud zone where the Permo-Triassic sandstone aquifer is exposed at the surface (i.e. free of\ud superficial deposits).\ud Given the inherent uncertainties and limitations associated with the various methods for\ud estimating recharge, it was proposed to use three different and independent methods and to\ud compare the results obtained.\ud The three methods proposed were:\ud (i) to date the pore water profile within the unsaturated zone using the historical tracer\ud tritium\ud (ii) to date the pore water profile within the unsaturated zone using nitrate and chloride,\ud released from the soil following the change in land-use from rough grazing to\ud intensive pasture.\ud (iii) To estimate recharge using a soil moisture water balance approach.\ud The pore water profile data (needed for the tracer method for estimating recharge) was\ud obtained from a cored borehole at a site to the east of Penrith where a deep unsaturated zone\ud (c. 120m) was present. The site had a good record of land-use history which showed that a\ud sudden change in landuse, from rough grazing to intensive grass pasture, occurred in 1976\ud and continued until the present. It was anticipated that this land use change would produce a\ud recognisable ‘step change’ in pore water concentrations for both nitrate and chloride.\ud The reason for selecting a site with such a deep unsaturated zone was to maximise the\ud likelihood that infiltration from c. 1962 (when tritium concentrations occurred) would be\ud present in the unsaturated zone, the depth of penetration of the 1962 recharge was variously\ud estimated to be between 80 and 160 m depth, depending on what recharge rate and what\ud moisture content (for the sandstone) is assumed.\ud The borehole was drilled to a total depth of 122m in August 2004. The pore water chemistry\ud results indicated that elevated nitrate and chloride concentrations, associated with 1976\ud recharge, had penetrated to about 100 m depth (this is equivalent to an average recharge rate\ud of about 468 mm/y). The 1962 recharge is believed to have migrated below the water table\ud and therefore tritium could not be used to ‘date’ the pore water profile within the unsaturated\ud zone.\ud The average recharge (for the period 1976-present) was estimated using the soil moisture\ud water balance (deficit) approach, to be 368-424 mm/y depending on the value of the ‘root\ud constant’ selected and rainfall monitoring station data applied. This is equivalent to a depth\ud of penetration of between 79 and 90 m for the 1976 recharge. The higher recharge rate (or\ud deeper penetration depth of the 1978 recharge) is preferred for estimating recharge at this site\ud because this was based on data from the nearest rain gauge (at a site at a similar elevation to\ud that at the drillsite). The rainfall data at this latter gauge was c. 6% higher than the main\ud rainfall monitoring station in Penrith (which had a longer monitoring record).\ud The average recharge rate is therefore likely to be in the range 424-468 mm/y and the rate of\ud water movement through the unsaturated zone is c. 3.5-3.85 m/y. Based on this estimate of\ud water movement in the unsaturated zone, the travel time for recharge to migrate from the soil\ud to the water table (or the delay imposed by the unsaturated zone) over the highest ground\ud (where unsaturated zone thickness can be in excess of 175m) the travel times could exceed 50\ud vi\ud years. However, over large areas of the Eden valley, the recharge currently arriving at the\ud water table is of post 1990 origin. Thus an important conclusion is that, over most of the\ud Eden valley, nitrate concentrations arriving at the water table are unlikely to substantially\ud increase. This does not mean of course that nitrate concentrations will not increase at\ud abstraction boreholes (or in the baseflow to the streams). Indeed, it is anticipated that nitrate\ud concentrations at groundwater outflows will continue to rise until most of the pre 1990 origin\ud recharge has been flushed out; this is likely to take many decades.\ud Similar possibly slightly slower rates of water movement through the unsaturated zone may\ud be anticipated where thin (<2m) but relatively permeable cover overlies the sandstone.\ud Where the superficial cover exceeds 2m thick, and especially where it is clay-rich, recharge\ud rates may be significantly reduced. The influence of the lithology and thickness of\ud superficial cover on recharge rates is an important issue and one that needs to be investigated\ud at a later stage.\ud Of interest, although not directly relevant to the objectives of this Work Package is that the\ud pore water of the unsaturated zone had a different water chemistry to the groundwater in the\ud saturated zone. The former are of mixed ionic composition with low bicarbonate\ud concentrations whilst the latter are a Ca-HCO3 type water. This suggests that any carbonate\ud cement that may have been present in the unsaturated zone has been removed; a process\ud which will have modified the porosity and permeability of the aquifer and which would have\ud been enhanced by acid rain deposition

Publisher: British Geological Survey
Year: 2008
OAI identifier: oai:nora.nerc.ac.uk:14009

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