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(17)O as a conservative tracer, in combination with δ(18)O and δ(15)N, to assess source/sink dynamics of groundwater nitrate beneath alluvial washes in a semiarid urban setting. Other studies have used δ(18)O and δ(15)N to determine nitrate sources and cycling, but the atmospheric δ(18)O signature can be overprinted by biogeochemical processes. In this study, δ(18)O and δ(15)N values of nitrate were coupled with δ(17)O 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(18)O and δ(15)N values which were reinforced by increases in observed δ(17)O 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.