Nitrogen isotope hydrochemistry and denitrification within the Chalk aquifer system of north Norfolk, UK

Although there has been much debate over the potential health risks of nitr ate in drinking water, there is a real issue of the costs associated with r emoving nitrate from drinking water supplies. In the Chalk aquifer system o f north-east Norfolk, modern contaminants of a mainly agricultural origin p roduce high levels of nitrate (> 15 mg/l NO3-N) in the unconfined valley ar eas, whereas in confined regions the levels of nitrate are low and commonly below detection limits (< 0.04 mg/l NO3-N). To understand the source and f ate of nitrate within this aquifer system, a detailed hydrochemical samplin g programme has been completed in the River Bure catchment. Nitrogen isotop e values (delta(15)N) for nitrate within the unconfined and semi-confined z ones range between +4 parts per thousand and +10 parts per thousand, charac teristic of nitrified soil organic nitrogen. However, many Chalk groundwate rs possess high N-2/Ar ratios (39-72) indicating a significant contribution to dissolved N-2 from denitrification. Denitrification is believed to be o ccurring within the overlying glacial deposits, providing a mechanism for n aturally improving groundwater quality. delta(15)N values of low-nitrate gr oundwaters from the confined zone are isotopically light (-3 parts per thou sand to +4 parts per thousand), inconsistent with an origin from denitrific ation: it is suggested that these waters have a pre-anthropogenic nitrate s ignature. (C) 1998 Elsevier Science B.V. All rights reserved.