Taiyuan Ligong Daxue xuebao (Sep 2021)
Tracing of Nitrate Sources in Groundwater of Loess Area Based on Hydrochemistry and Stable Isotopes
Abstract
In order to clarify the sources of nitrate in groundwater of the loess area, a typical loess area in northern Shaanxi Province was selected as the study area in this paper. On the basis of sampling and analysis, multiple environmental tracers such as characteristic ions and stable isotopes (δ(D), δ(18O-H2O), δ(15N-NO-3), δ(18O-NO-3)) were combined to study the spatial distribution characteristics, sources and transformation process of nitrate in groundwater, and the proportional contribution of different sources to nitrate in groundwater was quantitatively evaluated by using isotope mixing model (SIAR). The results show that the percentages of samples with NO-3 concentrations exceeding the WHO drinking water standard (50 mg/L) in shallow groundwater and deep groundwater were 73.7% and 51.9%, respectively, and the overall nitrate contamination level in shallow groundwater (average 91.40 mg/L) was higher than that in deep groundwater (average 78.24 mg/L). The spatial distribution of NO-3 content in shallow groundwater and deep groundwater was influenced by land use types as follows: forest>cropland>grassland>orchard>urban land, and urban land>forest>orchard>grassland>cropland, respectivtly. The main sources of NO-3 in groundwater were soil organic nitrogen, chemical fertilizers, and manure and sewage. Nitrification was the dominant transformation process of nitrogen nutrients in groundwater, while the denitrification was not obvious. The analysis results of SIAR model show that nitrate in groundwater was significantly affected by agricultural activities and urbanization. The contribution rates of soil organic nitrogen, chemical fertilizers, manure and sewage, and atmospheric precipitation to nitrate were 46.9%, 27.3%, 22.8%, and 3.0% for shallow groundwater, and 31.1%, 21.8%, 43.8%, and 3.3% for deep groundwater, respectively.
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