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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 in France and also in much of northwestern Europe (large areas between the Paris Basin, the London Basin, Denmark and the northwest of Germany) as it is intensively exploited for human use, with a large proportion reserved for drinking water. The improvement of the knowledge on Chalk aquifer hydrodynamical functioning and groundwater geochemistry is essential for the management of this resource. To achieve this aim, we developed a methodology based on a combination of hydrodynamic, hydrochemistry and groundwater dating tools. A study site of about 500 km2 was selected in NE France where Chalk outcrops. At 15 sampling points including boreholes and rivers (mainly from Chalk aquifer drainage), groundwater geochemistry were monitored monthly for 2 years from June 2017 to June 2019. Groundwater level in boreholes were monitored hourly with in-situ water loggers. In addition, two sampling campaigns for the analysis of CFCs and SF6 have been conducted in boreholes (in high and low water level periods) to define the groundwater residence time. The aquifer at the majority of sampling sites has an inertial behavior with respect to recharge as no impact of individual rainfall event was observed. Rapid piezometric response following rainfalls was observed at one site, suggesting the presence of locally highly developed fracture network. Contrariwise, high spatial and temporal heterogeneities of geochemistry was observed on the study area. According to the mixing process (piston flow, exponential or binary mixing) defined at each sampling site, groundwater dating indicated rather heterogeneous ages ranging from less than 20 years to more than 50 years. A combination of hydrogeological setting, residence time and land use information was used to explain the hydrochemical heterogeneities, highlighting the main factors governing the Chalk groundwater geochemistry including water level fluctuation, thickness of the unsaturated zone, superficial formations, distribution of fracture network, aquifer-river relations and human activities. A conceptual model was proposed to clarify the hydrogeosystem functioning of unconfined Chalk aquifers. The proposed methodology can be used for the study of other worldwide aquifers (Chalk or not) with the aim to provide appropriate recommendations in terms of water management.
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Feifei Cao, J. Jaunat, Virginie Vergnaud-Ayraud, Nicolas Devau, Thierry Labasque, et al.. Heterogeneous behaviour of unconfined Chalk aquifers infer from combination of groundwater residence time, hydrochemistry and hydrodynamic tools. Journal of Hydrology, Elsevier, 2020, 581, pp.124433. ⟨10.1016/j.jhydrol.2019.124433⟩. ⟨insu-02395723⟩

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