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‰ for (delta)15N and (epsilon)=9.80‰ 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.