La problématique environnementale du sélénium est principalement liée à son domaine de concentration très étroit entre toxicité et carence, et à sa présence diffuse au-delà des limites de potabilité dans de nombreux aquifères. Ainsi, des teneurs en sélénium supérieures à la norme européenne (10 ppb) ont été détectées dans les eaux souterraines du Site Expérimental Hydrogéologique (SEH) de Poitiers, dont le comportement hydrodynamique est particulièrement bien caractérisé. Le couplage d'études géochimiques, minéralogiques et pétrographiques a mis en évidence que les matériaux de remplissage (« argiles noires ») des structures karstiques subhorizontales présentes dans l'aquifère du Dogger au niveau du SEH, riches en sélénium et considérés comme la source potentielle du sélénium dissous dans ce réservoir, étaient issus de dépôts continentaux ayant subi des phases réductrices. Des fractionnements granulométriques et chimiques d'échantillons d'« argiles noires » ont montré que la fraction séléniée était principalement sous forme de sélénites dans la matrice et "associée" à la fraction organique soluble en milieu alcalin de nature majoritairement aliphatique. Des tests de lixiviation ont mis en évidence l'impact très important des conditions acido-basiques sur la quantité de sélénium dissous et sur la nature des espèces séléniées libérées dans les eaux. Les cinétiques de libération du sélénium tendent à montrer l'existence de trois processus principaux et les suivis de dissolution d'espèces majoritaires suggèrent que les mécanismes de libération du sélénium mis en jeu dans le contexte de l'étude sont principalement régis par des processus d'échange ou de complexation de surface. / The environmental issue of selenium is mainly related to the narrow range between its deficiency and toxicity concentrations, and to its widespread presence beyond the limits of potability in many aquifers. Selenium concentrations above the European standard (10 ppb) were detected in the groundwater of the Hydrogeological Experimental Site (HES) of Poitiers, whose hydrodynamic behavior is particularly well characterized.The coupling of geochemical, mineralogical and petrographic investigations showed that black argillaceous sediments that fulfill subhorizontal karstic cavities present in the Dogger aquifer at the SEH and considered as the potential source of dissolved selenium in the groundwater, are continental deposits that have been transformed in reduced environments.Grain-sized and chemical fractionations of these "black clays" showed that selenium is mainly present as selenite in the geological matrix and is "associated" with the alkali soluble organic fraction which is predominantly aliphatic.Leaching tests have highlighted the important impact of acid-base conditions on the amount of dissolved selenium and on the nature of selenium species released into the water. The release kinetics of selenium tends to show the existence of three main processes. The major species dissolutions suggest that the selenium release mechanisms are mainly governed by exchange process or surface complexation.

Indiana University-Purdue University Indianapolis (IUPUI) / Subglacial chemical processes in Antarctica are potentially significant contributors to global geochemical cycles, but current understanding of their scale and nature is limited. A sequential chemical extraction procedure was developed and tested to investigate the utility of meteoric 10Be as a tracer for chemical weathering processes beneath the East Antarctic Ice Sheet. Subglacial meltwater is widely available under the Antarctic Ice Sheet and chemical constituents within it have the potential to drive geochemical weathering processes in the subglacial environment. Meteoric 10Be is a cosmogenic nuclide with a half-life of 1.39×106 years that is incorporated into glacier ice, therefore its abundance in the subglacial environment in Antarctica is meltwater dependent. It is known to adsorb to fine-grained particles in aqueous solution, precipitate with amorphous oxides, and/or be incorporated into authigenic clay structures during chemical weathering. The presence of 10Be in weathering products derived from beneath the ice therefore indicates chemical weathering processes in the subglacial environment. Freshly emerging subglacial sediments from the Mt. Achernar blue ice moraine were subject to chemical extractions where these weathering phases were isolated and 10Be concentrations therein quantified. Optimization of the phase isolation was developed by examining the effects of each extraction on the sample mineralogy and chemical composition. Experiments on 10Be desorption revealed that pH 3.2-3.5 was optimal for the extraction of adsorbed 10Be. Vigorous disaggregation of the samples before grain size separations and acid extractions is crucial due to the preferential fractionation of the nuclide with clay-sized particles. 10Be concentrations of 2-22×107 atoms g-1 measured in oxides and clay minerals in freshly emerging sediments strongly indicate subglacial chemical weathering in the catchment of the Mt. Achernar moraine. Sediment-meltwater contact in the system was calculated to be on the order of thousands of years, based on total 10Be sample concentrations, local basal melt rates, and 10Be ice concentrations. Strong correlation (R = 0.96) between 10Be and smectite abundance in the sediments indicate authigenic clay formation in the subglacial environment. This study shows that meteoric 10Be is a useful tool to characterize subglacial geochemical weathering processes under the Antarctic Ice Sheet.