Concomitant behavior of arsenic and selenium from the karst infillings materials of the fractured carbonate Dogger Aquifer (Hydrogeological Experimental Site, Poitiers, France).

Abstract Petrographic and mineralogical analyses combined with sequential extractions and leaching experiments as a function of pH were performed on black clayey sediments fulfilling karsts in the Hydrogeological Experimental Site (HES) of Poitiers (France) to investigate the behavior of arsenic and selenium in a fractured limestone aquifer. Sequential extractions showed that arsenic is mainly associated with pyrite (about 35%) and secondary iron oxyhydroxides (around 13%), along with a substantial exchangeable fraction (about 13%). The soluble and associated to organic matter fractions are SEM analyses revealed the presence of arsenic “hot spots” into euhedral pyrite crystals surrounded by a halo of iron oxyhydroxides resulting from their alteration both of which are enriched with arsenic. Selenium has a similar pyritic origin but after alteration, it is predominantly associated with organic matter. Despite different distributions, the leaching experiment as a function of pH showed that the mobilization of arsenic and selenium overlapped below pH 2 and above pH 8. The main differences are observed between pH 2 and pH 8 with a plateau at 5% of released selenium, whereas the amount of mobilized arsenic continuously decreases. The pH-dependence of both elements is attributed to the partial dissolution of pyrite in acidic conditions combined with desorption processes for higher pH values.