Brief description

In the Schaugraben catchment the transport pathway of the nitrogen loads runs from the farmland through soil and groundwater into the surface water of the Schaugraben stream. Therefore, the shallow groundwater can be considered as important nitrogen source for the surface waters. To determine the amount of nitrogen degradation taking place in this compartment, groundwater observation wells were installed to provide a measuring field site (MFS) for tracer analysis. Isotope labelled potassium nitrate and potassium bromide were used as tracers. Nitrate degradation was determined summarily via the mass ratio of nitrate and bromide. In addition to the field experiments, reactor experiments were carried out in the laboratory. These experiments allowed more precise analysis of the chemical process of nitrate degradation in comparison to the field experiments. The reactor experiments were modelled with the geochemical model PHREEQC to verify the agreement of nitrate degradation with the model expectations.
The field experiments showed that nitrate degradation takes place in the groundwater close to the surface. The differences in groundwater temperatures found in the field experiments have shown that nitrate degradation depends strongly on the temperature. During the summer field experiment the intensity of nitrate degradation was 4.4 times higher than during the winter experiment. The Arrhenius activation energy for nitrate degradation was estimated to 310 kJ*mol-1. In the five reactor experiments which were carried out in the laboratory nitrate degradation in the measuring field was found to proceed by the process of heterotrophic denitrification. Two reactor experiments in which temperature was changed in the course of the experiment revealed the dependence of nitrate degradation on temperature to be similar to that in the flied experiments. Degradation rates in the laboratory experiments differed widely. The mean variation coefficient for the five experiments was calculated with 125%. This gave rise to the assumption that the rate of nitrate degradation is strongly dependent on micro-heterogenities in the sediment. The extraordinary high temperature dependence requires further investigations which includes examination of the quality of organic matter and its influence on nitrate degradation.