A dual isotope approach to identify denitrification in groundwater at a river-bank infiltration site

The identification of denitrification in the Torgau sand and gravel aquifer, Germany, was carried out by a dual isotope method of measuring both the delta 15N and delta 18O in NO3-. Samples were prepared by an anion exchange resin method (Silva et al., J. Hydrol. 228 (2000) 22) with a modification to the AgNO3-drying process from a freeze-drying to an oven-drying method. The occurrence of denitrification in the aquifer was confirmed by comparing the reduction of dissolved oxygen, dissolved organic carbon and NO3- concentrations with the dual isotope signatures. High nitrate concentrations were associated with low delta 15N and delta 18O values, and vice versa. The denitrification accords with a Rayleigh equation with calculated enrichment factors of epsilon = -13.62@1000 for delta 15N and epsilon = -9.80@1000 for delta 18O. The slope of the straight-line relationship between the delta 15N and delta 18O data demonstrated that the enrichment of the heavy nitrogen isotope was higher by a factor of 1.3 compared with the heavy oxygen isotope. It is concluded that the identification of this factor is a useful means for confirming denitrification in future groundwater studies.