Application of the hydrochemistry, stable isotopes and MixSIAR model to identify nitrate sources and transformations in surface water and groundwater of an intensive agricultural karst wetland in Guilin, China

Abstract

Karst water as the vital water supply source is generally suffered from NO3– contamination in intensive agricultural areas worldwide. Identifying NO3– sources and transformations is the key for understanding nitrogen pathways, and also for effectively controlling diffuse NO3– pollution. In this study, chemical variables and stable isotopes (δ2H-H2O, δ18O-H2O, δ15N-NO3– and δ18O-NO3–) were measured in 10 surface water (SW) samples and 13 groundwater (GW) samples collected from the Huixian karst wetland, with the application of a Bayesian stable isotope mixing model (MixSIAR) to identified NO3– sources and biogeochemical transformations. The results showed that the NO3– concentrations ranged from the below detection limit to 117 mg/L, with 30.8% of GW samples obtained from the north central part of the study area exceeding the maximum permissible limit for drinking water, and posing significant non-carcinogenic health risks for native people through drinking water pathway. Moreover, based on characteristics of the hydrochemistry and stable isotopes, different biogeochemical fates were evaluated in SW and GW: nitrification process was a dominant factor in GW, as a result of high NO3– levels, and this microbial process was unlikely occurred in SW associated with relatively anaerobic condition and low NO3– levels; however, the denitrification might not be a main process of degradation NO3– levels throughout the study area. The MixSIAR outputs revealed that the long-term application of synthetic NH4+ fertilizer (36.6%) and soil organic nitrogen (28.0%) were the main contributors to NO3– pollution, followed by synthetic NO3– fertilizer (16.8%) and domestic sewage and manure (15.1%), whereas NO3– in precipitation (3.44%) played a less important role. Additionally, NO3– concentration was significantly influenced by agricultural activities rather than NO3– source’s contribution between SW and GW. This work suggests that synthetic NH4+ fertilizer should be the primary target for control to prevent further NO3– pollution of the karst groundwater.