Use of nitrogen isotopes to identify nitrogen contamination of the Sherwood Sandstone aquifer beneath the city of Nottingham, United Kingdom

The city of Nottingham is underlain by the unconfined Sherwood Sandstone aquifer, which is vulnerable to contamination from various sources arising from urban and industrial activities of the region. In this study, samples of aquifer recharge, both artificial and natural, and of shallow and deep groundwater were collected to determine the sources and level of contamination from nitrogen species. Deep groundwater contains low concentrations of ammonium (less than 0.3 mg-N/L) throughout, but much higher nitrate concentrations (less than 1.0 mg-N/L to 28.0 mg-N/L). Shallow groundwater shows distinct evidence of surface contamination; ammonium concentration is as much as 1.2 mg-N/L and nitrate as much as 20 mg-N/L. Two-thirds of the deep groundwater samples have measured δ¹⁵N values that range from +4‰ to +8‰, indicative of a predominantly soil organic nitrogen source for dissolved nitrate. Most remaining groundwater samples (δ¹⁵N<5‰) have a nitrogen fertiliser source, possibly derived from an influent river draining a rural catchment; some samples have δ¹⁵N values >8‰, suggestive of point-source inputs from sewer leakage. In the future, contaminated shallow groundwater is likely to penetrate deeper into the aquifer, although a gradual dilution with older groundwater is occurring toward the confined zone of the aquifer.