Nitrate sources and nitrogen dynamics in a karst aquifer with mixed nitrogen inputs (Southwest China): Revealed by multiple stable isotopic and hydro-chemical proxies.

The nitrate (NO) contamination of karst aquifers as an important drinking water reservoir is increasing globally. Understanding the behavior of nitrogen (N) in karst aquifers is imperative for effective groundwater quality management. This study combined multiple stable isotopes (δH-HO, δO-HO, δC-DIC, δN-NO, and δO-NO), including hydro-chemical data, with a tracer test and a Bayesian isotope mixing (SIAR) model to elucidate the NO sources and N cycling within the Babu karst aquifer in Guizhou Province, Southwest China. Nitrate isotopes and SIAR model revealed that manure and sewage, nitrogen fertilizer, and soil organic nitrogen were the three dominant NO sources in winter, contributing to 37%, 32%, and 31% to spring NO, and 38%, 31%, and 31% to surface water NO, respectively. The δO-NO values of sampled waters ranging from 0.3‰ to 13.7‰ (mean of 7.7 ± 3.0‰; N = 63) and the significant negative correlations between δN-NO and δC-DIC in the spring waters (P < 0.01) revealed that nitrification was the primary N transformation process in the Babu watershed. Whereas, denitrification might still occur locally, confirmed by the enriched values of δN-NO (14.3 ± 7.6‰; N = 6) and high denitrification extent (46.6 ± 22.2%; N = 6) in the springs from residential areas, and by elevated δC-DIC (-11.2 ± 0.6‰; N = 26) and δN-NO values (18.9 ± 5.2‰; N = 26) in the boreholes. During the base flow period, point-inputs of the AMD-impacted stream and sewage waters, and short transit time (<5 days) were conducive to nitrification processes in the karst conduit, resulting in elevated NO concentration and NO/Cl ratio at the watershed outlet. Approximately 50% of NO flux at the outlet was derived from nitrification, indicating that a significant extent of nitrification occurred in the NH-contaminated karst conduit, which may be a new NO source to receiving rivers and lakes. This study provided an integrated method for exploring the N dynamics in contaminated karst aquifers. Moreover, the study highlighted that the point N sources control required particular attention for groundwater protection and restoration.