Benjamin S. Linhoff 2022 <p id="sp0045"><span>Recently, the&nbsp;subsoils&nbsp;of&nbsp;ephemeral stream&nbsp;(arroyos) floodplains in the northern Chihuahuan Desert were discovered to contain large naturally occurring NO</span>₃⁻<span>&nbsp;</span>reservoirs (floodplain: ~38,000 kg NO₃-N/ha; background: ~60 kg NO₃<span>-N/ha). These reservoirs may be mobilized through&nbsp;land use change&nbsp;or natural stream channel migration which makes differentiating between anthropogenic and natural groundwater NO</span>₃⁻<span>&nbsp;</span>sources challenging. In this study, the fate and sources of NO₃⁻<span>&nbsp;</span>were investigated in an area with multiple NO₃⁻<span>&nbsp;sources such as accidental sewer line releases and sewage&nbsp;lagoons&nbsp;as well as natural reservoirs of subsoil NO</span>₃⁻. To differentiate sources, this study used a large suite of geochemical tools including δ¹⁵N[NO₃], δ¹⁸O[NO₃], δ¹⁵N[N₂], δ¹³C[DIC],<span>&nbsp;</span>¹⁴<span>C,&nbsp;tritium&nbsp;(</span>³<span>H), dissolved gas concentrations, major ion chemistry, and contaminants of emerging concern (CEC) including artificial&nbsp;sweeteners. NO</span>₃⁻<span>&nbsp;</span>at sites with the highest concentrations (25 to 229 mg/L NO₃-N) were determined to be largely sourced from naturally occurring subsoil NO₃⁻<span>&nbsp;</span>based on δ¹⁵N[NO₃] (&lt;8 ‰) and mass ratios of Cl⁻/Br⁻<span>&nbsp;</span>(〈100) and NO₃⁻/Cl⁻<span>&nbsp;</span>(&gt;1.5). Anthropogenic NO₃⁻<span>&nbsp;</span>was deciphered using mass ratios of Cl⁻/Br⁻<span>&nbsp;</span>(&gt;120) and NO₃⁻/Cl⁻<span>&nbsp;</span>(&lt;1), δ¹⁵N[NO₃<span>] (&gt;8 ‰), and CEC detections.&nbsp;Nitrogen isotope&nbsp;analyses indicated that&nbsp;denitrification&nbsp;is fairly limited in the field area. CEC were detected at 67 % of sites including&nbsp;</span>³H dead sites (&lt;1 pCi/L) with low percent modern carbon-14 (PMC; &lt;30 %). Local supply wells are<span>&nbsp;</span>³H dead with low PMC; as<span>&nbsp;</span>³H does not re-equilibrate and<span>&nbsp;</span>¹⁴C is very slow to re-equilibrate during recirculation through infrastructure, sites with low PMC,<span>&nbsp;</span>³<span>H &lt; 1 pCi/L, and CEC detections were interpreted as locations with substantial anthropogenic&nbsp;groundwater recharge. Neotame was used to identify locations of very recent (&lt;15 years before present) or ongoing wastewater influxes to the aquifer. This work shows the important influence of naturally occurring subsoil NO</span>₃⁻<span>&nbsp;reservoirs on groundwater in arid regions and the major contribution of&nbsp;artificial recharge.</span></p> application/pdf 10.1016/j.scitotenv.2022.157345 en Elsevier Deciphering natural and anthropogenic nitrate and recharge sources in arid region groundwater article