Sources and transformations of nitrate constrained by nitrate isotopes and Bayesian model in karst surface water,...

Surface water suffering from nitrate (NO₃^-) contamination in karst area is not only harmful to human health as drinking water but can also affect the process of carbonate rock weathering, so it is crucial to trace the sources and transformations of NO₃^- in karst surface water. In this study, an investigation of water chemical data and NO₃^- isotopes (δ¹⁵N and δ¹⁸O) was used to elucidate the transformations of NO₃^- and quantify a proportional apportionment of NO₃^- sources of individual potential sources (incl. soil organic nitrogen (SON), atmospheric precipitation (AP), manure and sewage wastes (M&S), and chemical fertilizer (CF)) in the Lijiang River (typical karst surface water), Guilin, Southwest China. δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- values of water samples from the Lijiang River range from 2.14 to 13.50‰ (mean, 6.59‰) and from - 2.44 to 6.97‰ (mean, 3.76‰), respectively. A positive correlation between Cl^- and NO₃^- but no correlations between NO₃^- and δ¹⁵N-NO₃^- or δ¹⁸O-NO₃^- are found and the δ¹⁸O-NO₃^- values fitted the theoretical δ¹⁸O-NO₃^- values produced from nitrification, suggesting that the genesis of NO₃^- in waters of the Lijiang River is affected by nitrification processes and the mixing process has a major effect on NO₃^- transportation. Results of the Bayesian stable isotope mixing model show that the M&S and SON are the main NO₃^- source through the whole year (accounting for ~ 61% and 65% of the total NO₃^- in the wet and dry season, respectively), followed by CF (~ 29%). Furthermore, we find that nitrification of nitrogen in fertilizers, soil, and manure and sewage can promote the carbonate rock weathering. The estimated contribution of such nitrification to the weathering of carbonate rocks accounts for about 11% of the total carbonate rock weathering flux (calculated by HCO₃^-) in the Lijiang River. This finding indicates that the weathering of carbonate rock is probably affected by nitrogen nitrification processes in karst catchment.