Nitrate fate and origin in epikarst springs in Jinfo Mountain area, Southwest China

摘要

Concentration of dissolved nitrate (NO3-) and stable nitrogen isotopic composition of nitrate in water samples from 19 epikarst springs in the Jinfo Mountain area were analysed in order to understand the temporal-spatial distribution of nitrate in karst spring waters and trace nitrogen sources for the springs that have suffered from nitrate contamination. The results show that hydrochemistry of epikarst spring water is Ca.Mg-HCO3 facies, reflecting the control of water-carbonate rock interaction. NO3- concentrations in water of springs within the national nature reserve (NNR) in the Jinfo Mountain area range from 0 to 8.16 mg/L, suggesting that these springs within the NNR have good water quality. However, most springs outside the NNR have high NO3 concentrations (c: 10.5-46.18 mg/L) that have exceeded the value of Chinese national drinking water quality standard (NO3-N <= 10 mg/L), revealing that these springs have suffered from nitrate contamination to a significant degree. delta N-15-NO3 values of these contaminated springs outside the NNR range from -1.2 % to 11.27%, higher than those of unpolluted springs within NNR (-5.2 % -8.6%). Two main sources of nitrate in these contaminated springs have been identified, including animal manure/sewage and chemical nitrogen fertilisers that are from anthropogenic activities. Our study also shows that the nitrate fate and origin are associated with the land use in the recharge catchment of individual epikarst springs, and the establishment of national nature reserve can protect the water quality of epikarst spring.