Integrated assessment on groundwater nitrate by unsaturated zone probing and aquifer sampling with environmental tracers

By employing chemical and isotopic tracers (15N and 18O in NO3−), we investigated the main processes controlling nitrate distribution in the unsaturated zone and aquifer. Soil water was extracted from two soil cores drilled in a typical agricultural cropping area of the North China Plain (NCP), where groundwater was also sampled. The results indicate that evaporation and denitrification are the two major causes of the distribution of nitrate in soil water extracts in the unsaturated zone. Evaporation from unsaturated zone is evidenced by a positive correlation between chloride and nitrate, and denitrification by a strong linear relationship between δ18ONO3δ18ONO3 and ln(NO3−/Cl). The latter is estimated to account for up to 50% of the nitrate loss from soil drainage. In the saturated zone, nitrate is reduced at varying extents (100 mg/L and 10 mg/L at two sites, respectively), largely by dilution of the aquifer water.

► The transport of nitrate in unsaturated zone and saturated zone was integrated in a single investigation. ► Oxygen isotope enrichment factor for denitrification in the unsaturated zone was calculated. ► Denitrification loss in the aquifer was evaluated.