15N isotope biogeochemistry and natural denitrification process in groundwater: Application to the chalk aquifer of northern France

The use of 15N natural isotope tracing in an aquifer contained within chalk rocks in northern France indicates that, under certain hydrogeological conditions, major denitrification occurs. At the boundary where the aquifer becomes confined, the nitrate concentrations decrease in the direction of groundwater flow accompanied by an exponential increase in 15N (expressed in δ15N) of the residual nitrate. This is characteristic of kinetic isotope effects, which accompany the reduction of the nitrate ion during denitrification. Hydrogeochemical and bacteriological observations confirm this process. Natural isotope tracing also permits this process to be distinguished from local dilution with nitrate-free surface water, which would entail a major drop in nitrate values without 15N isotopic enrichment. A model is proposed to explain the relatively small observed magnitude of the isotopic fractionation effect.