Project members: Thomas Demmel, Thomas R. Rüde, Roland Schindler

This study aimed at analyzing the hydraulic conditions and especially the interaction of multiple aquifers at a waterworks site of the city of Viersen (waterworks Aachener Weg) in the Niederrheinischen Tiefland. Thus a high resolution hydrogeological model was set up in cooperation with the NiederrheinWasser GmbH. Based on that, a numerical aquifer simulation was developed to verify the hydrogeological model representation and to assess the tasks of this study.

The investigation area includes parts of the Venloe block and the Krefeld block with a heterogeneous lithology. The most important tectonic element is the „Viersener Sprung“, a fault zone with a displacement of several hundreds of meters. Parallel fault zones show displacements up to 50 m. This results in a complex hydrogeological situation, which is highly affected by the tectonic displacements and the heterogeneous lithology with hydraulic shortcuts. Special attention was paid to the parameterization of the fault zones highly influencing the groundwater flow.

The following hydrostratigraphic units were derived from the interpretation of available drilling profiles: miocene and oligocene sands (GwL2b), clays of the Hauptrotton (GwGl2) and Oberer Rotton (GwGl2b), gravels/sands of the Hauptkies-Serie and Reuver-Serie (GwL2), clays of the ReuverB (GwGl1) and ReuverC (GwGl1b) and quaternary terrace sediments (GwL1). The lignite seam Frimmersdorf was set as lower model boundary.

The transient numerical model was built on the base of the hydrogeological model as a Finite Elements Model (FEM) for a given period of eleven years (1996-2006). The model discretization was done with 18 layers consisting of 163,368 nodes (305,354 cells) in total. Natural boundary conditions were set at the water divide between Nette and Niers in the west and the Niers as receiving stream in the north. As further boundaries, a constant hydraulic head in the south and a constructed no-flow boundary in the east were chosen. The seepage rate was calculated with the soil water balance model Theseus spatially distributed in monthly resolution. Due to the depth to groundwater table, the seepage water reaches the groundwater with delay and damped amplitudes of annual fluctuations. A moving average was applied to the seepage hydrograph curve to simulate time shift and damping.

The calibration was done by varying the hydraulic conductivity as the most uncertain parameter. An average RMSE of 0.35 m in the upper aquifer (GwL1) and of 0.44 m in the second aquifer (GwL2/2B) could be achieved. The RMSE inside the area of interest (water works and upstream area) was even smaller. Thus, the hydraulic conductivities could be determined with good degree of accuracy. For the fault zones, a vertical as well as a horizontal differentiation of hydraulic conductivities could be obtained. This led to new insights into the hydraulic conditions in the working area. In a sensitivity analysis, the main fault zones (Dülkener Sprung and Viersener Sprung) as well as GwL2b were identified as the most sensitive model elements.

High nitrate concentrations in the shallow aquifer forced the water extraction to proceed to the second aquifer. Actually, nitrate concentrations have been rising also in this aquifer for some years. With the help of a transport model, the existing conditions were simulated to reconstruct the development of the nitrate concentrations over the given period and to investigate the processes responsible for nitrate spreading on field-scale. The transport of nitrate in GwL2 was simulated in a transient model under simplified assumptions. It was assumed that the input in GwL1 is 130 mg/L on average (input under agricultural area and model influx) and that the initial concentration in GwL2 is 0.1 mg/L. The scenarios were simulated with different degradation rates. In the simulation, a zoning of microbial degradation in GwL2 without changes in time was assumed. The model area was separated into one area with degradation and one without. With the help of the scenarios, a range for the degradation rate λ (first order decay reaction) could be determined on a field scale. The assumption of ongoing denitrification has shown to be necessary for the successful fitting of simulated and measured concentrations. Thus, the conducted simulations corroborate the assumptions of nitrate degradation zonings, taken in the hydrogeological model.

The potential hazard of a planned commercial area on the water supply in the west of Viersen was investigated. In a path line based scenario analysis, the influx from the quaternary aquifer through hydraulic window structures in the area of the waterworks Viersen into the Tertiary wells was studied. The scenario analysis was realized with a steady-state model derived from the transient model for October 2004. In different scenarios, the influence of pumping rates of the Tertiary wells on the oncoming flow was analyzed.