Tracking selenium in the Chalk aquifer of northern France: Sr isotope constraints

International audienceGroundwater at the southern and eastern edges of France's Paris Basin has a selenium content that at times exceeds the European Framework Directive's drinking-water limit value of 10 µg/L. To better understand the dynamics of the Chalk groundwater being tapped to supply the city of Lille and the Se origins, we used a combination of geochemical and isotopic tools. Strontium isotopes, coupled with Ca/Sr, Mg/Sr and Se/Sr ratios, were used to identify the main groundwater bodies and their mixings, with the Mg/Sr ratio constraining a ternary system. Groundwater in the agricultural aquifer-recharge zone represents a first end-member and displays the youngest water ages of the catchment along with the highest strontium isotopic signature (0.7084) and low Se contents. Anaerobic groundwater constitutes a second major end-member affected by water-rock interactions over a long residence time, with the lowest strontium isotopic signature (0.7079) and the lowest Se content, its low SF6 content confirming the contribution of old water. Se-rich groundwater containing up to 30 µg/L of Se represents a third major end-member, with an intermediate Sr isotopic ratio, and is mainly constrained by the clayey Se-rich formation overlying the Chalk aquifer. The spatial and temporal Se variability in the groundwater is clearly linked to the presence of this formation identified as Tertiary and also to the hydrological conditions; saturation of the Se-rich clays by oxygenated groundwater enhances Se mobility and also Sr adsorption onto the clays. This multi-tool study including Sr isotopes successfully identified the Se origins in the aquifer and has led to a better understanding of the regional mixing and processes affecting the Chalk groundwater

Deep water circulation at the northern Pyrenean thrust: Implication of high temperature water-rock interaction process on the mineralization of major spring water in an overthrust area

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Origin of groundwater salinity (current seawater vs. saline deep water) in a coastal karst aquifer based on Sr and Cl isotopes. Case study of the La Clape massif (southern France).

International audienceOrigin of groundwater salinity (current seawater vs. saline deep water) in a coastal karst aquifer based on Sr and Cl isotopes. Case study of the La Clape massif (southern France) a b s t r a c t In this study a typical coastal karst aquifer, developed in lower Cretaceous limestones, on the western Med-iterranean seashore (La Clape massif, southern France) was investigated. A combination of geochemical and isotopic approaches was used to investigate the origin of salinity in the aquifer. Water samples were collected between 2009 and 2011. Three groundwater groups (A, B and C) were identified based on the hydrogeological setting and on the Cl À concentrations. Average and maximum Cl À concentrations in the recharge waters were calculated (Cl Ref. and Cl Ref.Max) to be 0.51 and 2.85 mmol/L, respectively). Group A includes spring waters with Cl À concentrations that are within the same order of magnitude as the Cl Ref concentration. Group B includes groundwater with Cl À concentrations that range between the Cl Ref and Cl Ref.Max concentrations. Group C includes brackish groundwater with Cl À concentrations that are significantly greater than the Cl Ref.Max concentration. Overall, the chemistry of the La Clape groundwater evolves from dominantly Ca–HCO 3 to NaCl type. On binary diagrams of the major ions vs. Cl, most of the La Clape waters plot along mixing lines. The mixing end-members include spring waters and a saline component (current seawater or fossil saline water). Based on the Br/Cl molar ratio, the hypothesis of halite dissolution from Triassic evaporites is rejected to explain the origin of salinity in the brackish groundwater. Groundwaters display 87 Sr/ 86 Sr ratios intermediate between those of the limestone aquifer matrix and current Mediterranean seawater. On a Sr mixing diagram, most of the La Clape waters plot on a mixing line. The end-members include the La Clape spring waters and saline waters, which are similar to the deep geo-thermal waters that were identified at the nearby Balaruc site. The 36 Cl/Cl ratios of a few groundwater samples from group C are in agreement with the mixing hypothesis of local recharge water with deep saline water at secular equilibrium within a carbonate matrix. Finally, PHREEQC modelling was run based on cal-cite dissolution in an open system prior to mixing with the Balaruc type saline waters. Modelled data are consistent with the observed data that were obtained from the group C groundwater. Based on several trac-ers (i.e. concentrations and isotopic compositions of Cl and Sr), calculated ratios of deep saline water in the mixture are coherent and range from 3% to 16% and 0% to 3% for groundwater of groups C and B, respectively. With regard to the La Clape karst aquifer, the extension of a lithospheric fault in the study area may favour the rise of deep saline water. Such rises occur at the nearby geothermal Balaruc site along another lithospheric fault. At the regional scale, several coastal karst aquifers are located along the Gulf of Lion and occur in Mezosoic limestones of similar ages. The 87 Sr/ 86 Sr ratios of these aquifers tend toward values of 0.708557, which suggests a general mixing process of shallow karst waters with deep saline fossil waters. The occurrence of these fossil saline waters may be related to the introduction of seawater during and after the Flandrian transgression, when the highly karstified massifs invaded by seawater, formed islands and peninsulas along the Mediterranean coast