Nitrate Transport Modeling in Deep Aquifers. Comparison between Model Results and Data from the Groundwater Monitoring Network

Nitrate measurements from the Netherlands Groundwater Monitoring Network and model simulations were compared for deep aquifers in the eastern part of the Netherlands. The area studied measured 40 x 30 km2. The model describes advective-dispersive solute transport in groundwater and utilizes a first-order decay process in a simplified approach to denitrification. On the basis of preliminary model runs several modifications of the solute transport model were proposed; the model was evaluated and discussed after a second series of runs. A major factor in the results appears to be the vertical distribution of the nitrate content and the processes that affect it, such as the vertical mixing by dispersion and the distribution of the downward groundwater velocity. With respect to the denitrification parameter, the best results were obtained with a half-lifetime (T50) between 3 - 5 years and a local refinement underneath the river valleys and brooks, and in a zone above and below the clay layers. The refinement consisted of a further reduction of the half-lifetime based on the presence of organic matter increasing the denitrification capacity. The report further discusses the representativeness in space of the monitoring data and the simulation results