加速器质谱中心知识库

Nitrate pollution of groundwater is an increasingly serious anthropogenic problem. In this study, the hydrogeochemistry of major ions and stable isotope ratios of NO3- in groundwater were determined to identify the contamination sources and chemical transformation processes occurring in the shallow groundwater of Xi'an, the capital of Shaanxi province, NW China. Of a total of 32 groundwater samples, 31% had NO3--N concentrations exceeding the accepted drinking water limit of 10 mg-N L (1). Most of these samples were from the urban center of the study area, while samples with <10 mg-N L (1) were mainly from suburban areas. Combined with information on NO3- and Cl-, the variation in isotopes of NO3- in the groundwater suggest a mixing of multiple NO3- sources in areas on the urban/suburban border. By determining rainwater and river water NO3- isotopic values, the groundwater recharge mode can be deduced for Xi'an city. Chemical fertilizers and nitrification of N-containing organic materials contribute NO3- to suburban groundwater, while sewage effluent and nitrification dominate NO3- distribution in urban groundwater. Nitrification from organic soil N, manure and sewage was significant in some sampling areas, and NO3- isotopic values from groundwater in Xi'an indicated that the effects of denitrification were not an obvious contributor. Thus, the delta N-15-NO3- enrichment process is mainly caused by the intense anthropogenic activity in the city center. From the urban center to suburban areas, the mean delta N-15-NO3- values varied from +16.4 parts per thousand to +5.4 parts per thousand, and the mean NO3--N concentrations varied from 28.0 mg L (1) to 4.0 mg L (1). In particular, the delta N-15-NO3- value (r = -0.75, p < 0.01) correlated more significantly with distance from the urban center than did the NO3--N concentration data (r = -0.49, p < 0.01), which suggests that NO3- isotopic values are an effective indicator of contamination sources. In addition, the delta N-15-NO3- values and population density show a significant logarithmic correlation in Xi'an city.