Nitrogen isotope studies of nitrate contamination of the thick vadose zones in the wastewater-irrigated area

The objective of this study was to investigate natural abundance and the distribution of nitrogen isotopic compositions to assess denitrification in two ~30 m thick vadose zones beneath the different land uses in the wastewater-irrigated area located in southern Shijiazhuang, China. Sediment samples were collected from cores of boreholes drilled in the vegetable growth plot and the wastewater-irrigated farmland for analyses of nitrogen isotopes, physical and chemical properties, respectively. The profile of borehole A drilled in the vegetable growth plot only applied animal wastes had lower δ15N values of mean +7.5 ‰ in the upper vadose zone, but higher values of mean +10.9 ‰ in the lower vadose zone. δ15N values in each part varied little with depth, indicating no or little denitrification occurred in the deep vadose zone below the soil zone. The profile of borehole B drilled in the wastewater-irrigated farmland had low δ15N values of mean +5.7 ‰ below the soil zone and little variations of δ15N values with depth, indicating no or little denitrification occurred in the deep vadose zone below the soil zone. This was also verified by consistent variations of NO3 − and SO4 2− contents with Cl− contents. Our results suggested most of leachable nitrate from the soil zone was hardly subjected to biological attenuation into groundwater.