Groundwater nitrate contamination in an agroecosystem in Zhangye Oasis, Northwest China

A study assessed the risk of nitrate (NO3--N) pollution in groundwater was conducted in Zhangye Oasis, Northwest China. The mean value of NO3--N concentrations of groundwater samples was 10.66 +/- 0.19 mg/L, NO3--N concentrations exceeding 10 mg/L (the threshold of drinking water set by World Health Organization) was found in 32.4% of 71 wells, and were 13%, 33.3%, 52.4%, and 50.0% in the groundwater from drinking wells, irrigation wells, hand-pumping wells and groundwater table observation wells, respectively. The result showed that the samples which NO3--N concentrations above the dinking water standards nearly were collected from the depth of lower than 20 meter. Groundwater NO3--N concentrations under vegetable area, seed maize area and intercropped maize area were significantly higher than that under urban area or paddy area. Groundwater NO3--N pollution in sandy soil area was more severely than that in loam soils area.