Relationship of nitrate isotopic character to population density in the Loess Plateau of Northwest China

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 δ15N-NO3- enrichment process is mainly caused by the intense anthropogenic activity in the city center. From the urban center to suburban areas, the mean δ15N-NO3- values varied from +16.4‰ to +5.4‰, and the mean NO3--N concentrations varied from 28.0 mg L−1 to 4.0 mg L−1. In particular, the δ15N-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 δ15N-NO3- values and population density show a significant logarithmic correlation in Xi'an city.