Deciphering natural and anthropogenic nitrate and recharge sources in arid region groundwater

Recently, the subsoils of ephemeral stream (arroyos) floodplains in the northern Chihuahuan Desert were discovered to contain large naturally occurring NO3- reservoirs (floodplain: similar to 38,000 kg NO3-N/ha; background: similar to 60 kg NO3-N/ha). These reservoirs may be mobilized through land use change or natural stream channel migration which makes differentiating between anthropogenic and natural groundwater NO3- sources challenging. In this study, the fate and sources of NO3- were investigated in an area with multiple NO3- sources such as accidental sewer line releases and sewage lagoons as well as natural reservoirs of subsoil NO3-. To differentiate sources, this study used a large suite of geocheinical tools including delta N-13[NO3], delta O-18[NO3], delta N-18[N-2], delta C-13[DIC], C-14, tritium ((II)-I-3), dissolved gas concentrations, major ion chemistry, and contaminants of emerging concern (CEC) including artificial sweeteners. NO3 at sites with the highest concentrations (25 to 229 mg/I. NO3-N) were determined to be largely sourced from naturally occurring subsoil NO3 based on delta N-15(NO3-) (<8 parts per thousand) and mass ratios of Cl-/Br- (<100) and NO3-/Cl- (>1.5). Anthropogenic NO3- was deciphered using mass ratios of Cl-/Br- (>120) and NO3-/Cl- (<1), delta N-18 [NO3) (>8 parts per thousand), and CEC detections. Nitrogen isotope analyses indicated that denitdflcation is fairly limited in the field area. CEC were detected at 67 % of sites including H-3 dead sites (<1 pCi/L) with low percent modern carbon-14 (PMC; <30 %). Local supply wells are H-3 dead with low PMC; as H-3 does not re-equilibrate and C-14 is very slow to re-equilibrate during recirculation through infrastructure, sites with low PMC, H-3 < 1 pCi/L, and CEC detections were interpreted as locations with substantial anthropogenic groundwater recharge. Neotame was used to identify locations of very recent (<15 years before present) or ongoing wastewater influxes to the aquifer. This work shows the important influence of naturally occurring subsoil NO3- reservoirs on groundwater in arid regions and the major contribution of artificial recharge.

Automated summary

This study investigates the fate and sources of NO3- in the northern Chihuahuan Desert and reveals the significant impact of naturally occurring subsoil NO3- reservoirs on groundwater. By using a range of geochemical tools, it is possible to differentiate between anthropogenic and natural sources of NO3-. The study also highlights the major contribution of artificial groundwater recharge.