How hydrogeological and geochemical approaches can contribute to the effective management of water resources in a confined aquifer? Example of the Beauce multilayer aquifer system (Centre region, France)

Confined aquifers, generally more protected from anthropogenic pressure, present potential alternative water resources of good quality to surface and subsurface water resources. However, their exploitation, potentially affecting the resource in a long-term way, requires a good understanding of their functioning to ensure their sustainable management. In order to gain some understanding of the processes involved in confined aquifers, we present here results obtained from the multilayered Beauce limestone aquifer, in the Centre region of France. This aquifer is already heavily exploited for drinking water supply and agriculture.

The developed methodology implies 1) a new interpretation of potential recharge areas from the newly acquired aquifer geometry through geological data analysis, 2) an extensive analysis of the piezometric and geochemical (major and trace elements) and isotopic groundwater database, and 3) new data obtained from groundwater sampled from a nested piezometer plateform.

The main results are synthesized below:

• Several potential recharge zone have been identified. The first one corresponds to a fault zone that could allow exchanges between the surface, the Beauce limestone and deeper aquifers. The second one is a local outcrop of the limestone caused by an anticline. The third one corresponds to local, diffuse recharge from the unconfined water table where the overlying aquitard become thinner or non-existent;
• The Beauce limestone formation comprises two main aquifer sub-units, locally separated, when existing, by a semi-permeable aquitard. The two sub-aquifers nevertheless may have distinct geochemical signatures, even though they are largely interconnected;
• Piezometric data indicate that the groundwater regionally flows from east to west, originating in an area where the aquifer does not outcrop and indicating indirect recharge from another aquifer;
• Dissolved inorganic carbon isotopes (¹³C, ¹⁴C) show an apparent ageing of the groundwater in the opposite direction to the flow, with more ancient groundwater upstream (up to 30 ka B.P.) and younger groundwater downstream (< 10 ka B.P.). Depleted groundwater in ²H and ¹⁸O, in agreement with the radiocarbon residence time, confirm the paleoclimatic effect recorded in the confined Beauce limestone aquifer although the confined aquifer is relatively shallow (< 150 m deep).

These results allow to better understand the hydrodynamic behaviour of this aquifer by highlighting potential recharge zones, groundwater origin as well as the possible exchanges between inter- and intra-aquifer systems. Combining all the available information (geological, hydrogeological, chemical and isotopic) is thus essential for establishing robust conceptual model of multi-layer confined systems and will provide useful information to managers for the sustainable management of the resource.