Resumen
The identification of the sources and fate of
NO
3
-
NO
3
-
contaminants is important to protect the water quality of aquifer systems. In this study,
NO
3
-
NO
3
-
contaminated groundwater from the drinking water wells in the Shijiazhuang area, China, was chemically (
NO
3
-
NO
3
-
/Cl- ratio) and isotopically (d15NNO3, d18ONO3 and d13CDOC; d2HH2O, d18OH2O) characterized to identify the sources of
NO
3
-
NO
3
-
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
NO
3
-
NO
3
-
, 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 d2HH2O and d18OH2O. A cross-plot of d15NNO3 vs. d18ONO3 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 d13CDOC and d15NNO3 values, confirming that a low extent of denitrification occurred. Nitrification processes were evaluated by means of d18ONO3 and d18OH2O. The initial d15NNO3 value(s) of the
NO
3
-
NO
3
-
source(s) were roughly estimated between 2? and 5?. Based on the level of natural
NO
3
-
NO
3
-
, anthropogenic activities were considered the main reason for the elevated
NO
3
-
NO
3
-
concentration of the shallow groundwater.
NH
4
+
NH
4
+
fertilizers were the major source of
NO
3
-
NO
3
-
in the non-wastewater irrigated area, while wastewater was regarded as the primary source of
NO
3
-
NO
3
-
in the wastewater-irrigated area. A low content of
NO
3
-
NO
3
-
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
NO
3
-
NO
3
-
sources was identified as another important factor affecting the
NO
3
-
NO
3
-
concentration of the groundwater in the study area. The combined use of d15NNO3, d18ONO3 and d13CDOC results and hydrochemical data (
NO
3
-
NO
3
-
/Cl- ratios) gives an insight into the mixing effect of different
NO
3
-
NO
3
-
sources and processes affecting
NO
3
-
NO
3
-
concentration under conditions of intensive land-use activities.