Leaching behavior of selenium from the karst infillings of the Hydrogeological Experimental Site of Poitiers

The selenium enrichment in groundwater may be caused by the contact with sedimentary formations. In many French regions, geogenic sedimentary sources of Se were identified as responsible of the Se concentrations exceeding the European limit in many wells designated to produce drinking water. This study focuses on the black argillaceous infillings of the Hydrogeological Experimental Site of Poitiers (HESP) and aims to better understand the Se release mechanisms at the interface between these infillings and aqueous media using batch experiments with variable pH and contact time. The Se behavior as function of pH was similar in the ultrapure water and in the HESP water and can be discriminated in three main domains. For very acidic domain the speciation of the released Se, that can be associated to the dissolution of some mineral phases, was majorly unidentified. For 2 < pH < 7.5, Se is mainly released as oxyanions which can be associated to interfacial processes. For basic pH, the maximal quantity of Se was released as oxyanions and unidentified species and is accompanied with the Fe and organic carbon mobilization. The kinetic data showed that the HESP water enhance the Se mobilization as function of time. The kinetic release of Se can be modeled as the release of three main fractions that cannot be directly correlated to the Se speciation: an immediate fraction followed by the release of two fractions corresponding to the pseudo first order kinetic. These results coupled with some in-situ data confirm the diversity and the complexity of the release mechanisms of Se that is not associated with a specific carrier phase but disseminated within the sedimentary matrix.