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 nitrate in drinking water, there is a real issue of the costs associated with removing nitrate from drinking water supplies. In the Chalk aquifer system of north-east Norfolk, modern contaminants of a mainly agricultural origin produce high levels of nitrate (> 15 mg/l NO3-N) in the unconfined valley areas, 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 fate of nitrate within this aquifer system, a detailed hydrochemical sampling programme has been completed in the River Bure catchment. Nitrogen isotope values (delta(15)N) for nitrate within the unconfined and semi-confined zones range between +4 parts per thousand and +10 parts per thousand, characteristic of nitrified soil organic nitrogen. However, many Chalk groundwaters possess high N-2/Ar ratios (39-72) indicating a significant contribution to dissolved N-2 from denitrification. Denitrification is believed to be occurring within the overlying glacial deposits, providing a mechanism for naturally improving groundwater quality. delta(15)N values of low-nitrate groundwaters from the confined zone are isotopically light (-3 parts per thousand to +4 parts per thousand), inconsistent with an origin from denitrification: it is suggested that these waters have a pre-anthropogenic nitrate signature. (C) 1998 Elsevier Science B.V. All rights reserved.