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Nitrate concentrations in the Morestead borehole, Twyford

By M.E. Stuart, P.J. Chilton, A.J. Newell and A.S. Butcher

Abstract

This report describes work carried out at Morestead, Twyford as part of a BGS research\ud project “Nitrate Mass Balance in the Saturated Zone”. The project aimed to evaluate the role\ud of the diffusive exchange of nitrate between fracture water and porewater in the saturated\ud zone of the aquifer. The approach adopted attempted to obtain a mass balance for the\ud catchment to a public supply borehole by comparing nitrogen released from the soil with\ud nitrogen held in the aquifer and nitrogen pumped to an abstraction borehole.\ud A new, cored borehole was drilled at Morestead, Twyford in an area of set-aside land on the\ud margins of a field normally used for cereals. The borehole was completed to 75 m depth in the\ud former Upper Chalk penetrating about 25 m of the Seaford Chalk Formation and about 50 of\ud the Lewes Nodular Chalk Formation. The water table lay in the lower part of the Seaford\ud Chalk.\ud The core obtained was fractured but most of these fractures appeared to be to be parallel to the\ud bedding (e.g. along marl seams) and were probably drilling-induced. Some high-angle\ud fractures with mineralised fracture faces were found in the uppermost 10 m and mineralised\ud fractures with slickensides (possibly associated with a minor fault) were observed at 31 m\ud depth, a few metres below the water table at the time of drilling.\ud A detailed profile of porewater quality was obtained by centrifugation of core samples. The\ud results indicate that there do not appear to be any zones of unfractured chalk where porewater\ud has retained pre 1960s concentrations of nitrate. Zones close to major fractures did not show\ud steep nitrate concentration gradients, suggesting that there were not large differences in\ud quality between the fracture water and porewater. Porewater concentrations followed a typical\ud nitrate profile for chalk overlain by arable land with elevated concentrations (up to 18 mg N/l)\ud in the unsaturated zone and declining concentrations in the saturated zone (up to 9 mg N/l),\ud except in a 15 m thick zone of the Lewes Nodular Chalk about 25 m below the water table.\ud Here a number of marl bands appear to result in a zone of slow-moving water with low nitrate\ud concentration, but elevated concentrations of a range of trace elements often associated with\ud clay minerals and residence time, particularly Br, Co, Cr, F, Li, Mo, Sb and U.\ud The results of packer testing of the borehole confirmed that the highest permeabilities were in\ud the zone close to the water table, with low values at depth, consistent with results from\ud boreholes in the nearby Candover catchment. Marl seams appear to be much more important\ud than fractures in controlling groundwater movement to this borehole. Groundwater samples\ud obtained during packer testing were all of similar composition and were interpreted as being\ud drilling water which had not been fully flushed from the borehole before the test. The\ud modelling step of the work was therefore not attempted as this required nitrate concentrations\ud in the mobile water in the aquifer.\ud The unsaturated zone porewater profile indicates nitrate concentrations mainly at and above\ud the current drinking-water standard of 50 mg/l nitrate (11.3 mg N/l). Annual mean nitrate\ud concentrations at Twyford are rising towards 50 mg/l with seasonal peaks exceeding these\ud concentrations. If this is representative of local conditions, and given the significant\ud proportion of similar arable land in the immediate catchment of the Twyford boreholes then\ud groundwater concentrations are likely to continue rising under the present landuse and\ud agricultural regime. Moreover much of the cultivated land is located in the upper part of the\ud catchment and the nitrate is likely to still be present in the unsaturated and saturated\ud groundwater flow path. Even close to the Twyford borehole, the ‘improved grassland’ may\ud have significant applications of inorganic fertiliser and organic manure. Present conditions do\ud not therefore suggest any immediate reduction in the upward groundwater nitrate trend

Publisher: British Geological Survey
Year: 2008
OAI identifier: oai:nora.nerc.ac.uk:13354

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