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

Benjamin S. Linhoff

doi:10.1016/j.scitotenv.2022.157345

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

Benjamin S. Linhoff

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Abstract

Recently, the subsoils of ephemeral stream (arroyos) floodplains in the northern Chihuahuan Desert were discovered to contain large naturally occurring NO 3 − reservoirs (floodplain: ~38,000 kg NO 3 -N/ha; background: ~60 kg NO 3 -N/ha). These reservoirs may be mobilized through land use change or natural stream channel migration which makes differentiating between anthropogenic and natural groundwater NO 3 − sources challenging. In this study, the fate and sources of NO 3 − were investigated in an area with multiple NO 3 − sources such as accidental sewer line releases and sewage lagoons as well as natural reservoirs of subsoil NO 3 − . To differentiate sources, this study used a large suite of geochemical tools including δ 15 N[NO 3 ], δ 18 O[NO 3 ], δ 15 N[N 2 ], δ 13 C[DIC], 14 C, tritium ( 3 H), dissolved gas concentrations, major ion chemistry, and contaminants of emerging concern (CEC) including artificial sweeteners. NO 3 − at sites with the highest concentrations (25 to 229 mg/L NO 3 -N) were determined to be largely sourced from naturally occurring subsoil NO 3 − based on δ 15 N[NO 3 ] (<8 ‰) and mass ratios of Cl − /Br − (〈100) and NO 3 − /Cl − (>1.5). Anthropogenic NO 3 − was deciphered using mass ratios of Cl − /Br − (>120) and NO 3 − /Cl − (<1), δ 15 N[NO 3 ] (>8 ‰), and CEC detections. Nitrogen isotope analyses indicated that denitrification is fairly limited in the field area. CEC were detected at 67 % of sites including 3 H dead sites (<1 pCi/L) with low percent modern carbon-14 (PMC; <30 %). Local supply wells are 3 H dead with low PMC; as 3 H does not re-equilibrate and 14 C is very slow to re-equilibrate during recirculation through infrastructure, sites with low PMC, 3 H < 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 NO 3 − reservoirs on groundwater in arid regions and the major contribution of artificial recharge.

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Science of the Total Environment Article

Source-reported bounding extent: 31.332301° to 37.000232° latitude; -109.050173° to -103.001964° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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Benjamin S. Linhoff. 2022. Deciphering natural and anthropogenic nitrate and recharge sources in arid region groundwater. https://doi.org/10.1016/j.scitotenv.2022.157345

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