A dual-isotope approach to the nitrogen hydrochemistry of an urban aquifer

The potential for exploitation of urban aquifers is partly dependent on understanding the distribution and fate of urban N sources, such as sewage and fertilisers, that can limit the use of groundwater for public supplies. To investigate the application of the dual-isotope approach to understanding the N hydrochemistry of urban groundwater, this paper presents δ¹⁵N–NO₃^? and δ¹⁸O–NO₃^? data collected from two multi-level piezometers in the Sherwood sandstone aquifer beneath Nottingham in the English Midlands, UK. At one multi-level piezometer (Old Basford), depth sample measurements of δ¹⁵N–NO₃^? in the range +9.2 to +11.4 ‰ and δ¹⁸O–NO₃^? in the range +8.2 to +10.9‰, together with NO₃^? nitrate concentrations from 31.7 to 66.7 mg/l, are evidence for nitrification of sewage-derived inputs. In contrast, at the other multi-level piezometer (the Meadows), isotopically enriched samples (δ¹⁵N–NO₃^? in the range +24.3 to +42.2 ‰ and δ¹⁸O–NO₃^? in the range +20.5 to +29.4‰) are evidence for denitrification, although the compositional range of δ¹⁵N–NO₃^? does not identify the N source without corroborating data. For the Meadows location, a cross-plot of δ¹⁵N–NO₃^? versus δ¹⁸O–NO₃^? gave an enrichment of the ¹⁵N isotope relative to the ¹⁸O isotope by a factor of 1.9, within the range of 1.3–2.1 reported for denitrification in other studies. This study has shown that the dual-isotope approach provides improved understanding of N sources and fate in the urban environment but further work is required to identify nitrification pathways to provide more confidence in the application and interpretation of δ¹⁸O–NO₃^? measurements.