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.
ASJC Scopus subject areas
- Environmental Chemistry