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Groundwater Nitrate Pollution in the Oasis Agricultural Areas in the Middle Reaches of Heihe River,Northwest China

YANG Rong,SU Yong-zhong(Laboratory of Watershed Hydrology and Ecology,CAREERI,CAS,Linze Inland River Basin ComprehensiveResearch Station,CERN,Lanzhou Gansu 730000,China)  
Nitrate concentration in groundwater was monitored and analyzed to assess the nitrate contamination of groundwater in the middle reaches of Heihe River,Northwest China.Water samples,from 71 wells,were marked with type of well,soil type,land using type and sampling depth.The mean value of NO-3-N contamination in groundwater for the 71 wells was(10.66±0.19) mg·L-1.32.4% of the wells have the nitrate concentrations above the dinking water standards established by World Health Organization(10 mg·L-1 NO-3-N),and 16.9% of them exceed 20 mg·L-1.The wells in the study areas are grouped into four according to the depth and usage: drinking wells,irrigation wells,hand-pumping wells and wells for monitoring groundwater table.The mean NO-3-N concentrations in the groundwater from drinking wells,irrigation wells,hand-pumping wells and wells for monitoring groundwater table are(5.75±0.20) mg·L-1,(11.44±1.70) mg·L-1,(17.41±0.96) mg·L-1 and(11.53±0.92) mg·L-1,respectively,that is,13.0%,33.3 %,52.4 % and 50.0 % exceeding 10 mg·L-1,and 6.3 %,16.7 %,28.6 % and 25.0 % exceeding 20 mg·L-1,respectively.The result shows that nitrate concentration in groundwater is inversely related to depth of the well.The wells with nitrate concentrations above the dinking water standards mostly have the depth less than 20 m.Groundwater contamination by nitrate is affected by land using.NO-3-N concentrations in groundwater under vegetable fields,seed maize and intercropped maize are significantly higher than that under urban land or paddy fields.It is considered that the vegetable fields are the principal sources of nitrate contamination in groundwater.Soil type is also a main factor affecting nitrate concentration in groundwater.Groundwater nitrate pollution in sandy soil areas is more severe than that in loam soils.
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