Evaluating the Sources and Fate of Nitrate in the Alluvial Aquifers in the Shijiazhuang Rural and Suburban Area, China: Hydrochemical and Multi-Isotopic Approaches

The identification of the sources and fate of [GRAPHICS] contaminants is important to protect the water quality of aquifer systems. In this study, [GRAPHICS] contaminated groundwater from the drinking water wells in the Shijiazhuang area, China, was chemically ( [GRAPHICS] /Cl- ratio) and isotopically (delta N-15(NO3), delta O-18(NO3) and delta C-13(DOC); delta H-2(H2O), delta O-18(H2O)) characterized to identify the sources of [GRAPHICS] and address subsequent biogeochemical processes. The positive correlations between dominant anions and cations suggested that the dissolution of calcium carbonate and gypsum minerals was the most effective process in the groundwater. Elevated concentrations of [GRAPHICS] , Cl- and Mg2+ could be related to the wastewater irrigation and usage of fertilizers. The natural water in the study area originated primarily from precipitation and experienced a limited extent of evaporation, as demonstrated by measurements of delta H-2(H2O) and delta O-18(H2O). A cross-plot of delta N-15(NO3) vs. delta O-18(NO3) gave an enrichment of the 15N isotope relative to the 18O isotope by a factor of 2. A further insight into the denitrification process was obtained by the synergistic changes in delta C-13(DOC) and delta N-15(NO3) values, confirming that a low extent of denitrification occurred. Nitrification processes were evaluated by means of delta O-18(NO3) and delta O-18(H2O). The initial delta N-15(NO3) value(s) of the [GRAPHICS] source(s) were roughly estimated between 2 parts per thousand and 5 parts per thousand. Based on the level of natural [GRAPHICS] , anthropogenic activities were considered the main reason for the elevated [GRAPHICS] concentration of the shallow groundwater. [GRAPHICS] fertilizers were the major source of [GRAPHICS] in the non-wastewater irrigated area, while wastewater was regarded as the primary source of [GRAPHICS] in the wastewater-irrigated area. A low content of [GRAPHICS] in deep groundwater might mainly be influenced by precipitation and soil organic N that was involved in denitrification reactions. Some of the deep groundwater samples could have been contaminated by wastewater. The mixing process of multiple [GRAPHICS] sources was identified as another important factor affecting the [GRAPHICS] concentration of the groundwater in the study area. The combined use of delta N-15(NO3), delta O-18(NO3) and delta C-13(DOC) results and hydrochemical data ( [GRAPHICS] /Cl- ratios) gives an insight into the mixing effect of different [GRAPHICS] sources and processes affecting [GRAPHICS] concentration under conditions of intensive land-use activities.