Assessing the resilience of the Carboniferous limestone transboundary aquifer (Belgium/France) to recharge deficit events and groundwater abstraction

The carboniferous aquifer of the international hydrographic district of the Scheldt river extends across three countries and administrative regions: France, Wallonia (South Belgium), Flanders (North Belgium) covering 1420 km². More than 75 million cubic meters of water are pumped every year in the considered hydrogeological system for drinking water distribution, agriculture, industry, and quarry dewatering. Stresses on groundwater resources in the aquifer are therefore important and pumping operations need to be managed adequately. Groundwater levels have been decreasing significantly due to the overexploitation of the aquifer caused by the industrial and demographic development of the region during the 20^th century. In some area the piezometric level has dropped by 90 meters between 1910 and 2010.

The transboundary aquifer is mainly composed of fractured carboniferous limestone. The aquifer is considered as unconfined in eastern part and confined below marls and chalk in the northwest area. Recharge is thus mainly observed within the unconfined area, with important lateral groundwater flows to the confined area.

Groundwater flow in the aquifer has been modelled in 3D using the finite volume calculation code MARTHE, in collaboration between the different involved entities, and using data officially exchanged between administrations. The model has been calibrated for the 1900-2017 period considering abstracted groundwater volumes, recharge calculated from precipitation and evapotranspiration data, observed piezometric levels and river flow rates, collected or reconstructed since 1900.

The model is used for predictive purpose. Simulations are performed for the next decades following several scenarios including the possible evolution of groundwater abstraction as a function of the demographic and economic development of the region, the expected climate evolution and related groundwater recharge change, the evolution of dewatering operations in stone quarries. Recently, recharge scenarios based on historical meteorological data were applied to the model to determine the impact of rainfall deficit on groundwater resources and the resilience of the aquifer. The impact has been quantified and time recovery map of the aquifer were built. Results show a recovery time difference up to twenty years between the confined and unconfined area of the aquifer.

All these simulations constitute a scientific support for the decision-makers involved in the management of this transboundary aquifer.