Analysis of pumping tests in a sandy aquifer in Northern Belgium

During a 2008 data acquisition campaign performed in the framework of the surface disposal project of ONDRAF/NIRAS for short-lived low- to intermediate-level radioactive waste, multiple pumping tests were conducted in the Mol-Dessel region in Northern Belgium. The goal of these tests was to determine the values of the hydraulic parameters for the different hydrostratigraphical units of the shallow Neogene aquifer system in the area. Recent studies refined the initial hydrostratigraphy, evoking the need for a reinterpretation of the pumping tests. To do so, an axisymmetric approach for the numerical groundwater modeling software MODFLOW was used. This allows the implementation of a very fine spatial resolution, in particular in the vicinity of the pumped well where head gradients are large, whilst increasing computational speed. Additionally, to estimate the hydraulic parameters, an automated calibration process was used based on a non-linear regression technique. In this work, the multi-well pumping tests at the Dessel-3 and Retie-1 sites in the Mol-Dessel area were reanalyzed. The aims of this work are 1) to assess the performance of the axisymmetric pumping test models relative to the initial rectangular models, 2) to estimate hydraulic parameters for the units of the refined stratigraphy and provide additional relevant statistics and 3) to upscale quasi-continuous hydraulic conductivity logs of small-scale measurements taken from the sites in previous research, allowing to analyze the variability of hydraulic conductivity within the hydrostratigraphical units and assess the relation between small-scale and pumping test-scale measurements. The results show that the axisymmetric modeling approach combined with the automated calibration consistently improves the model fit in terms of RMSE when compared to the manually calibrated rectangular models. This is in great part due to the finer spatial discretization near the pumped well. In addition, the axisymmetric models were up to ten times faster than their rectangular counterparts. The updated hydraulic parameter estimates for the refined stratigraphy fall within reasonable ranges defined by literature. The results show a clear vertical anisotropic behavior of the Upper Mol unit at pumping test-scale. Moreover, these results were supported by 95% confidence intervals for calibrated parameters. The upscaling of small-scale hydraulic conductivity measurements showed that the scale effect exists: the small-scale measurements underestimated hydraulic conductivity when used on the pumping testscale without proper upscaling. After upscaling, a larger spatial variability of hydraulic conductivity than initially accounted for was suggested at the Retie-1 site. Furthermore, the bottom of the Lower Mol unit showed upscaled hydraulic conductivity values more akin to the underlying Kasterlee Sands at both test sites. Near the base of the Kasterlee Clay and the Diest Clayey Zone, an increase of this parameter was observed. These variabilities in hydraulic conductivity can be of importance when reviewing the refined hydrostratigraphy in the Mol-Dessel region. The results represent important prior information for local groundwater flow and solute transport models that are developed in the framework of radioactive waste disposal.