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

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 (NO) concentrations, which were attributed to local agricultural activities. Our goal was to identify the predominant nitrogen sources and NO 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 NO identified manure and sewage as the major sources of NO contamination. Denitrification was the dominant NO removal process in the aquifer, as demonstrated by the negative relationship between δN and δO values in NOand NO/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 NO attenuation in groundwater. A mixing model constructed using a catchment-scale dataset including SiO concentrations and δO-HO 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 NO attenuation. Denitrification reduced the NO flux into the nearby stream by up to 114.4 NO kg/ha/y (26 kg N/ha/y). The N 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 NO sources and conceptualized N cycling in the heavily developed agricultural riparian aquifer using multiple tracers, demonstrating that NO is partially removed through denitrification and possibly anammox while N mostly escapes into the atmosphere.