Nitrate contamination of groundwater in an agroecosystem in Zhangye Oasis, Northwest China

In order to assess the extent of groundwater contamination by nitrate (NO3--N) and to provide information about 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 samples 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.