『Abstract
　Geographical information system (GIS) technology was used to investigate NO3^- concentration of groundwater in the Zhangye Basin, Northwest China. Thematic information and NO3^- data of groundwater from the Zhangye Basin were analyzed in a GIS environment to study the extent and variation of NO3^- concentration and to establish spatial relationships with corresponding land use types. About 38.8％ of groundwater samples showed NO3^- concentration above the human affected value (13 mg/l NO3^-), while more than 10.2％ exceeded the maximum acceptable level (50 mg/l NO3^-) according to WHO regulations. The spatial distribution of NO3^- of groundwater in the Zhangye Basin showed an obvious character of zonation; the high NO3^- concentration was mainly located in the city seat areas, and Zhangye City was the high-value center. The probability of over 90％ for NO3^- concentration exceeding 13 mg/l was found in urban seat areas. The spatial analyses indicated that groundwater contamination by NO3^- was closely related to one specific land use class, the urban. The NO3^- concentration of groundwater under urban was significantly higher than that under irrigation land or sand dune. Most of the unacceptable NO3^- levels were found in groundwater samples assigned to urban area, but a few were also found allotted to irrigation land class. Therefore, urban source was considered the principal source of NO3^- contamination of groundwater in the Zhangye Basin, while irrigation land source was still a possible source of NO3^- contamination.

Keywords: GIS; NO3^- concentrations; Land use; Zhangye Basin; northwest China』

Introduction
Study area
Hydrogeology
Materials and methods
Results and discussion
　Variogram modeling
Spatial distribution of NO3^-
Groundwater NO3^- contamination and land use
Summary and conclusions
Acknowledgments
References