Spatiotemporal variability and control factors of NO3− in a polluted karst water system of an agricultural wetland in South China

Nitrate (NO3−) pollution in karst water is an important environmental issue in intensive agricultural regions worldwide. The integrated understanding of the spatiotemporal variability and control factors of NO3− pollution in karst water is imperative for controlling the diffuse pollution caused by agricultural activities. In this study, 49 water samples were collected from surface water (SW) and groundwater (GW) in the Huixian karst wetland (HKW) and analyzed using hydrogeochemical and isotopic data (δ18O–NO3–, δ15N–NO3– and δ13CDIC) in combination with a Bayesian mixing model to investigate the spatiotemporal distribution and control factors in NO3−-polluted karst water. The results showed that approximately 40.82% of the karst water samples exceeded the natural threshold value of 3 mg/L for NO3−-N, and 32.14% of the GW samples exceeded the permissible limit for drinking water established by WHO (10 mg/L as NO3−-N), indicating that high levels of NO3− were mainly found in GW samples from the agricultural core area, especially in the dry season. The NH4+-synthetic fertilizer (NHF) and soil organic nitrogen (SON) were the dominant factors controlling pollution sources in the HKW, accounting for 36.13% ± 4.66% and 28.68% ± 4.75% of the karst GW NO3− concentration, respectively. However, the seasonal differences in NO3− pollution sources were not significant in GW. Microbial nitrification was the main process affecting the NO3− levels in GW, whereas the occurrence of denitrification did not significantly affect NO3− concentration in the HKW due to the relatively low rate. Moreover, the HNO3 produced from NH4+ via microbial nitrification facilitated carbonate weathering, thereby controlling NO3− enrichment in karst GW. Our results suggest that NHF should be controlled to prevent further GW pollution in the HKW. Our study also provides a scientific basis for understanding the factors controlling the NO3− concentrations in karst water systems.