『Abstract
　In order to assess the extent of groundwater contamination by nitrate (NO3^--N) and to provide information that the deterioration of the groundwater quality in Zhangye Oasis, Northwest China, a study was conducted in this area. The mean value of NO3^--N concentrations in groundwater sample was 10.66±0.19 mg l^-1. NO3^--N concentrations exceeding 10 mg l^-1 (the threshold for drinking water set by the World Health Organization) were found in 32.4％ of 71 wells, and were 13, 33.3, 52.4 and 50.0％ in the groundwater samples from drinking wells, irrigation wells, hand-pumping wells and groundwater table observation wells, respectively. The result showed that the groundwater samples that had NO3^--N concentrations exceeding the threshold for drinking water were mostly collected from a depth of less than 20 m. Groundwater NO3^--N concentrations in areas used for the cultivation of vegetables, seed maize and intercropped maize were significantly higher than those in urban or paddy areas. NO3^--N contamination of groundwater in areas with sandy soil was more severe than in those with loam soil.

Keywords: NO3^--N; Soil; Land use systems; Irrigation』

Introduction
Materials and methods
　The study area
　Collection of groundwater samples
　Measure of groundwater NO3^--N and estimate standards
Results and discussion
　Status of groundwater NO3^--N pollution
　NO3^--N contamination in different well types
　NO3^--N concentrations of groundwater samples from different collection depths
　Influence of land use and soil texture on NO3^--N contamination of groundwater
Discussion
　Status of NO3^--N contamination of groundwater at the study area
　Factors that have an effect on the NO3^--N contamination of groundwater
　Strategies to mitigate NO3^--N contamination of groundwater
　　Optimizing N fertilizer application
　　Improved irrigation management
　　Improved efficiency of N use
　　Agricultural environmental policies and legislation
Conclusion
Acknowledgments
References