Determination of denitrification parameters in deep groundwater. A pilot study for several pumping stations in the Netherlands

Little information is available on the rate of denitrification and its spatial distribution. In this report a model concept is proposed to simulate denitrification. The unknown parameters in this concept are determined by a calibration procedure using the program PEST. For calibration nitrate measurements from the Monitoring Network are used as well as nitrate data from the abstracted water in a number of public drinking water pumping stations. Calibration is carried out for nine groundwater-pumping stations in the central and eastern part of the Netherlands. In zones where the presence of organic substances is likely, denitrification may be described as an exponential decay with a half-life time of around 500 days. In zones where organic material is absent, the half-life time is much higher (2750 days). At or near the phreatic surface an instantaneous nitrate reduction is assumed. Here, an average reduction of 50% is found. This figure, however, may also represent a compensation for over- or underestimation of the nitrate input at the water table. The parameter values found in this report appear to have a high measure of uncertainty. Several explanations are suggested and discussed.