Groundwater is an important source of drinking water in Belgium. In some regions, locally produced groundwater is the source of 100% of the drinking water supply. One of the most important aquifers in the eastern part of Belgium is the Chalk Aquifer. This aquifer is phreatic in the northern part of Wallonia, but dips down towards the north, in Flanders, where it quickly reaches large depths. This largely confined aquifer is of strategic societal importance because it is well protected against negative influences from the surface (nitrates, pesticides) on the water quality.
However, geological and hydrogeological information is scarce, leading to important uncertainties regarding sustainable yields. Due to the large depth of the aquifer in its confined part, relatively little borehole information is available. Furthermore, the Chalk Aquifer is characterized by a double porosity system which results in a strong heterogeneity and spatial variability of the hydrogeological properties. The goal of the CHARM project is to analyse the capacity of the Chalk Aquifer on a regional scale, and to deliver a management instrument that can be used for decision-making with regards to the quantitative use of this strategic aquifer for drinking water purposes. Special attention will be given to the characterization of all sources of uncertainty and its incorporation in a groundwater flow model.
In a first step, the geology and hydrogeological parameters of the Chalk aquifer are characterized in detail. Based on gamma-ray logs, flow measurements, pumping tests and literature data, the horizontal and vertical variability of the hydrogeological parameters are identified. Exploitation results are linked to geological and hydrogeological data providing insights why some exploitations have a higher yield than others. Next, a regional groundwater model (MODFLOW) is set-up. The integrated Bayesian multi-model approach of Mustafa et al. (2018) is adapted, so that input, parameter and conceptual model uncertainty can be quantified. This is done by coupling the MODFLOW model with the DiffeRential Evolution Adaptive Metropolis (DREAM) algorithm (Vrugt, 2016) and by applying Bayesian Model Averaging (BMA). Based on the results of this approach, well-founded decisions can be made regarding the quantitative use of this aquifer considering all different sources of uncertainty, which is of strategic importance for long-term drinking water purposes.