Using isotope fractionation of nitrate-nitrogen and nitrate-oxygen for evaluation of microbial denitrification in a sandy aquifer

The nitrate input into the groundwater from forest and agricultural land in the 'Fuhrberger Feld' catchment area is virtually completely degraded by microbial denitrification in the aquifer. Five multilevel groundwater sampling wells show characteristic depth functions of nitrate, proving the denitrification process. A series of 10 samples of 'arable land groundwater' was taken from different depths at these multilevel wells and isotope ratios of 15N/14N and 18(O)/16(O) in the nitrate were measured. The sampling was based on the two-dimensional vertical-plane groundwater flownet and on the knowledge of the landuse upgradient of the multilevel wells. High nitrate concentrations are associated with low delta 15N and low delta 18(O) values, and vice versa. The delta 15N/delta 18(O) ratio indicates a linear increase with a slope of 1:2.1. The microbial dentrification accords with a Rayleigh process with calculated enrichment factors of epsilon = -15.9 per thousand for 15N and epsilon = -8.0 per thousand for 18(O). Three samples taken from groundwater recharged under coniferous forest show deviating characteristics: low nitrate concentrations with low delta 15N values (1-4 per thousand) but very high delta 18(O) values (40-50 per thousand). Using the previously quantified enrichment factors for 15N and 18(O), an attempt was made to ascertain the significance of denitrification in specific depth sections at two other multilevel wells (7 samples). In two cases quantitative assessments on the degree of nitrate concentration decrease owing to denitrification were possible. However, in other cases the specific nitrate isotope fractionation was masked by the variability of nitrate concentration and of nitrate isotope ratios in the corresponding groundwater recharge preventing definite interpretations.