Nitrate distribution under the influence of seasonal hydrodynamic changes and human activities in Huixian karst wetland, South China

Karst wetland is a special, complex and fragile type of wetland. With increasing economic and agricultural activities, the negative impacts of human activities on water quality particularly on karst wetland is also increasing. In this paper multiple datasets and methods including hydrological, hydrochemical and stable isotope data were used to elucidate the distribution and transport of nitrate pollution in Huixian karst wetland under the influence of seasonal hydrodynamic changes and human activities. Hydrodynamic conditions around the wetland during both wet and dry seasons show large differences due to the complex exchange between surface water and groundwater. The northern recharge area receives rainfall recharge with rapid responses in groundwater level, temperature and electrical conductivity, while those responses are relatively stable in the central area where the surface water interacts with groundwater frequently. However, their diurnal variations are significant. In the western drainage area where groundwater always discharges to rivers, the groundwater levels show sharp responses after rainfall, but their temperatures are stable. In addition, δ15N and δ18O of nitrate suggest that the nitrate pollution mainly comes from NH4+ in fertilizer and rainfall, soil nitrogen, animal feces and livestock manure. The nitrate concentrations of most surface waters are lower during the wet season, affected by the dilution and mixing effect of heavy rainfall and agricultural activities. Nitrate distributions show considerable spatial variability during the dry season, especially in the wells located in residential areas which are mainly polluted by nearby residents. This study highlights the vulnerability of karst wetlands and presents methods that are significant for ecological restoration as well as development of karst water resources in karst areas.