Heterogeneous behaviour of unconfined Chalk aquifers infer from combination of groundwater residence time, hydrochemistry and hydrodynamic tools

Abstract

Chalk groundwater is an important aquifer resource. It is intensively exploited for human use, with a large proportion utilized for drinking water. The improvement of the knowledge on Chalk aquifer hydrogeological functioning is essential for the management of this resource. Here, we developed a methodology based on a combination of hydrodynamic, hydrochemistry and groundwater dating tools. A study site with Chalk outcrops was selected in NE France where groundwater geochemistry and water level were monitored continuously for 2 years from 2017 to 2019 and groundwater residence time was estimated using CFCs and SF6. Overall, the aquifer has an inertial behaviour with respect to recharge. Nevertheless, a rapid water level response following rainfall events was observed at one site, suggesting the presence of highly developed fracture network at the local scale. According to the mixing process (piston flow, exponential or binary mixing) defined at each sampling site, groundwater dating indicated rather heterogeneous ages ranging from modern to about 50 years. High spatial and temporal heterogeneities were observed and interpreted by a combination of hydrogeological setting, residence time and land use information, highlighting main factors governing the Chalk groundwater geochemistry including water level fluctuation, thickness of the Unsaturated Zone (UZ), superficial formations, distribution of fracture network, aquifer-river relations and human activities. A conceptual model was proposed accordingly to explain the hydrogeological functioning of unconfined Chalk aquifers.