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, modem 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 (δ15N) for nitrate within the unconfined and semi-confined zones range between + 4‰ and + 10‰, characteristic of nitrified soil organic nitrogen. However, many Chalk groundwaters possess high N2/Ar ratios (39-72) indicating a significant contribution to dissolved N2 from denitrification. Denitrification is believed to be occurring within the overlying glacial deposits, providing a mechanism for naturally improving groundwater quality. δ15N values of low-nitrate groundwaters from the confined zone are isotopically light (-3‰ to + 4‰), inconsistent with an origin from denitrification: it is suggested that these waters have a pre-anthropogenic nitrate signature.
- Nitrogen isotopes