『Abstract
　Nitrate (NO3^-) is major pollutant in groundwater worldwide. Karst aquifers are particularly vulnerable to nitrate contamination from anthropogenic sources due to the rapid movement of water in their conduit networks. In this study, the isotopic compositions (δ¹⁵N-NO3^-, δ¹⁵N-NH4⁺) and chemical compositions (e.g., NO3^-, NH4⁺, NO2^-, K⁺) were measured in groundwater in the Zunyi area of Southwest China during summer and winter to identify the primary sources of contamination and characterize the processes affecting nitrate in the groundwater. It was found that nitrate was the dominant species of nitrogen in most of the water samples. In addition, the δ¹⁵N-NO3^- values of water samples collected in summer were lower than those collected in winter, suggesting that the groundwater received a significant contribution of NO3^- from agricultural fertilizer during the summer. Furthermore, the spatial variation in the concentration of nitrate and the δ¹⁵N-NO3^- value indicated that some of the urban groundwater was contaminated with pollution from point sources. In addition, the distribution of δ¹⁵N-NO3^- values and the relationship between ions in the groundwater indicated that synthetic and organic fertilizers (cattle manure) were the two primary sources of nitrate in the study area, except in a few cases where the water had been contaminated by urban anthropogenic inputs. Finally, the temporal and spatial variation of the water chemistry and isotopic data indicated that denitrification has no significant effect on the nitrogen isotopic values in Zunyi groundwater.

Keywords: Nitrate; Groundwater; Nitrogen isotope; Agriculture fertilizer』

Introduction
Materials and methods
　Site description
　Sampling and analytical methods
Results and discussion
　Chemical composition of groundwater
　Concentration and distribution of nitrogen species
　Co-variation of NO3^- with water chemistry in groundwater
　δ¹⁵N of nitrate in groundwater
　Processes affecting nitrate in groundwater
Conclusion
Acknowledgments
References