Using ¹⁷O to investigate nitrate sources and sinks in a semi-arid groundwater system

We apply a triple isotope approach for nitrate that utilizes Δ ¹⁷O as a conservative tracer, in combination with δ ¹⁸O and δ ¹⁵N, to assess source/sink dynamics of groundwater nitrate beneath alluvial washes in a semiarid urban setting. Other studies have used δ ¹⁸O and δ ¹⁵N to determine nitrate sources and cycling, but the atmospheric δ ¹⁸O signature can be overprinted by biogeochemical processes. In this study, δ ¹⁸O and δ ¹⁵N values of nitrate were coupled with δ ¹⁷O values of nitrate to quantify atmospheric nitrate inputs and denitrification amounts. Results show generally low groundwater nitrate concentrations (<0.2 mmol/L) throughout the basin; high nitrate concentrations (up to 1 mmol/L) with evidence for some denitrification were detected in areas where effluent was the predominant source of recharge to groundwater. Furthermore, the denitrification was inferred from elevated δ ¹⁸O and δ ¹⁵N values which were reinforced by increases in observed δ ¹⁷O values. Finally, relatively low, but significant atmospheric nitrate concentrations were measured in groundwater (up to 6% of total nitrate). This study concludes that the triple isotope approach improves determination of the proportion of atmospheric nitrate and the significance of denitrification in natural waters, allowing us to develop a conceptual model of the biogeochemical processes controlling nitrogen in an urban setting.