[Source and Transformation Process of Nitrate in Karst Groundwater within Guiyang City]

Urbanization has caused the groundwater environment to face varying degrees of threats， among which nitrate pollution of groundwater is one of the problems that cannot be ignored. To research the sources of nitrate pollution and biogeochemical processes in the karst groundwater system under the influence of urbanization， 86 karst groundwater samples were collected from the Guiyang urban area from July to August 2021， and their major water chemical indicators and dual nitrate isotope composition （δ¹⁵N-NO₃^- value and δ¹⁸O-NO₃^- value） were measured. The MixSIAR model was used to analyze the contribution rate of different pollution sources of nitrate in the groundwater. The results showed that the main hydrochemical facies of karst groundwater in the study area was HCO₃-Ca type. Nitrate concentrations ranged from 0.1 to 74.6 mg·L^-1， with an average value of （21.1±17.3） mg·L^-1； δ¹⁵N-NO₃^- values ranged from -1.1‰ to 37.7‰， with an average value of （10.8±5.1）‰； and δ¹⁸O-NO₃^- values varied from -10.1‰ to 17.8‰， with an average value of （3.3±3.7）‰， suggesting that nitrate in the groundwater mainly came from fecal sewage and soil organic nitrogen. The δ¹⁸O-NO₃^- values of the groundwater fell on both sides of the nitrification theoretical value line， indicating that a biogeochemical process dominated by nitrification occurred in the groundwater system. The ratio of （δ¹⁵N-NO₃^-）∶（δ¹⁸O-NO₃^-） for the groundwater was 0.15， which was much lower than the denitrification ratio ranging from 1.3 to 2.1， suggesting that no denitrification had occurred in groundwater in the study area. The contribution rate of manure and sewage to nitrate sources in groundwater in the study area was as high as 48.6%， and soil organic nitrogen， chemical fertilizers， and atmospheric precipitation were 22.7%， 21.6%， and 7.1%， respectively. The research results provide theoretical basis for the prevention and control of nitrate pollution in urban groundwater.