Using nitrogen-15, oxygen-18, and oxygen-17 to determine nitrate sources and removal processes from groundwater, Tucson, Arizona

Nitrate is a common groundwater contaminant. Due to adverse health effects, waters above the Maximum Contaminant Level (MCL) of 10 mg NO3-N/L or 0.71 mmols/L, are banned from domestic consumption by the EPA. Studies have measured elevated nitrate concentrations in arid land soils and groundwater around the world. These elevated concentrations could be detrimental to the environment and to human health. Thus, it is important to consider the different sources and processes affecting nitrate concentrations Here, a novel triple isotope system approach was employed, coupling delta 17O with delta18O and delta15N of nitrate to determine the sources (atmospheric, terrestrial, fertilizer, wastewater) and removal processes influencing nitrate concentrations in the Tucson basin groundwater system. Results show low groundwater nitrate concentrations (0.2 mmols/L) where wastewater was not a predominant source of water, versus high concentrations (1 mmols/L) above the MCL in groundwaters where wastewater was the dominant water source. Furthermore, groundwater up to 1.6 Km away from the wastewater stream was contaminated with effluent recharge waters. In addition, denitrification was inferred from delta18O and delta 15N data with this inference reinforced by delta17O data and delta15N enrichments up to 26‰. Finally, low atmospheric nitrate was measured in groundwater, representing up to 6% of total nitrate. The triple isotope approach studied here is ideal for determining the proportion of atmospheric nitrate versus other terrestrial nitrate sources and the significance of nitrate removal processes.