Evaluating the sources and fate of nitrate in riparian aquifers under agricultural land using in situ-measured noble gases, stable isotopes, and metabolic genes

Abstract

Riparian zones with their buffering ability and abundant water supply are often subjected to intensive agricultural activities. We investigated a riparian aquifer located near a stream in South Korea that recently experienced sharply decreasing groundwater levels and elevated nitrate (NO₃^-) concentrations, which were attributed to local agricultural activities. Our goal was to identify the predominant nitrogen sources and NO₃^- removal processes. Multiple approaches including geochemical and isotopic tracers, land-use analysis, metabolic gene quantification, and inert gas tracers were used to elucidate groundwater and nutrient dynamics in stream-side granitic aquifers. The dual isotopic composition of NO₃^- identified manure and sewage as the major sources of NO₃^- contamination. Denitrification was the dominant NO₃^- removal process in the aquifer, as demonstrated by the negative relationship between δ¹⁵N and δ¹⁸O values in NO₃^-and NO₃^-/Cl^-. Denitrification and anammox genes were also observed in microbial communities of the aquifer throughout the study site, suggesting that these processes support effective natural NO₃^- attenuation in groundwater. A mixing model constructed using a catchment-scale dataset including SiO₂ concentrations and δ¹⁸O-H₂O suggested that mixing with paddy soil water was the major driver of denitrification in the aquifer at the study site, where impervious layers provided anaerobic conditions for natural NO₃^- attenuation. Denitrification reduced the NO₃^- flux into the nearby stream by up to 114.4 NO₃^- kg/ha/y (26 kg N/ha/y). The N₂ generated by denitrification did not accumulate in the groundwater, but mostly escaped from groundwater to the atmosphere, as demonstrated by the degassed signature of dissolved inert gases below the air saturated water level. This study identified the predominant NO₃^- sources and conceptualized N cycling in the heavily developed agricultural riparian aquifer using multiple tracers, demonstrating that NO₃^- is partially removed through denitrification and possibly anammox while N₂ mostly escapes into the atmosphere.

Keywords: Anammox; Denitrification; Natural attenuation; Nitrate; Noble gas; Riparian aquifer.