Regional groundwater model of north-east Belgium

Since 1975, the possibility to dispose of high-level radioactive waste in the Boom Clay formation is investigated in Belgium at the test site in Mol. This research involves detailed studies of the hydrogeological system at various scales. The regional groundwater model is the largest-scale model, providing boundary conditions for smaller-scale flow and transport models and for long-term predictions (e.g. climate change). The major challenges for its newest update were as follows: (1) to better understand regional groundwater flow, (2) to better calibrate the regional hydraulic parameters of the Boom Clay and the underlying aquifers, (3) to apply new data from the site investigation, and (4) to simulate the transient behaviour of the overexploited aquifers underlying the Boom Clay. The new regional groundwater model using the MODFLOW 2000 code includes an updated geometry, new estimates of parameter values and a new conceptualisation of the hydrogeological system. Two steady states are simulated: the hypothetical original state (prior to pumping) and the equilibrium state of the aquifers assuming continuation of the present day pumping. The partly observed period of the transition between these states is simulated using a specific transient model. The information included in the current set of observations is not sufficient to estimate the hydraulic properties of the major regional aquitard (the Boom Clay) using inverse modelling. However, low value of its hydraulic conductivity, based on large number of measurements, appears to be compatible with the model. Future data acquisition campaigns should also focus on increasing the confidence in the regional validity (transferability) of the measured values of the hydraulic conductivity.