Determining hydrogeological and anthropogenic controls on N pollution in groundwater beneath piedmont alluvial fans using multi-isotope data

Abstract

Piedmont alluvial fans have, for a long time, supported clean groundwater for human development worldwide. Rapid urbanization in China has over-exploited groundwater and resulted in severe groundwater nitrate pollution in piedmont alluvial fans, which has led to both human health concerns and clean water shortages, especially in rural residential areas where drinking water is mainly derived from shallow groundwater. However, sources of nitrate in groundwater and their pathways are still unclear. In this study, we hypothesized that untreated manure and domestic sewage are the primary nitrate sources in a residential area, and that an unconfined aquifer would be more vulnerable to nitrate pollution than a confined aquifer due to nitrate infiltration via the vadose zone. To verify this hypothesis, groundwater nitrate pollution in a well-characterized piedmont alluvial fan was studied through hydrochemical composition and multi-isotope (δ15NNO3, δ34SSO4, δ18OSO4, δ13CDIC) data over four seasons. Seasonal variations for both NO3− concentrations and δ15NNO3 values in unconfined groundwater (UGW) and confined groundwater (CGW) were observed. Mixing of nitrate sourced from manure/sewage, precipitation, soil nitrogen, and synthetic fertilizer controlled the groundwater nitrate level, and manure/sewage contributed 52.4%, 32.4%, and 33.7% nitrate to UGW, CGW, and surface water (SFW), respectively, based on Stable Isotope Mixing Models in R (SIMMR) analysis. Denitrification had a minor effect on groundwater nitrate levels in a residential area, and nitrate pollution in CGW was partly due to nitrate migration through geological faults and fracture zones in a piedmont alluvial fan. Nitrate pollution sources and pathways in residential areas were confirmed through multiple isotopes (δ15NNO3, δ13CDIC, δ34SSO4, and δ18OSO4). Collectively, these results highlight the roles of the hydrogeological setting and anthropogenic activities in groundwater N pollution beneath piedmont alluvial fans.