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.