Source-to-sink of natural arsenic in Neogene sedimentary systems of the south western Paris Basin

Natural arsenic concentrations in groundwater locally exceed drinking water standards, raising public health concerns. Preventing its release requires identifying its 3D distribution in rocks and water–rock interactions. We address this through a source-to-sink approach applied to predicting arsenic distribution in Miocene sedimentary deposits of the southwestern Paris Basin, where geogenic arsenic is present and monitored in aquifers (e.g., lacustrine deposits of Beauce Fm. – Aquitanian) underlying sedimentary cover (fluvial deposits of Sables et Argiles de Sologne Fm. – Burdigalian to Pliocene).

Our approach combines source-to-sink tools, chiefly geomorphology and sedimentology, with arsenic geochemistry in order to trace arsenic transfer from source areas to sedimentary reservoirs. It involves: (1) identifying sources coupling the reconstruction of paleoreliefs from planation surfaces and paleodrainage areas and characterizing arsenic concentrations in the drainage areas using pXRF ; (2) characterizing depositional environments, including sedimentary facies and their actual and past physicochemical conditions, and the associated arsenic content in terms of abundance, speciation, carrier phases within each facies (pXRF, sequential extractions, μXRF and ICP-MS), and mechanisms of arsenic retention (sorption, complexation and mineral precipitation).

Two source-to-sink systems have been identified in our study: (i) an Aquitanian endorheic lake system and (ii) a mid-Miocene terrigenous system. The Aquitanian system is fed by the surrounding reliefs, leading to the remobilization of arsenic contained in older sedimentary series. Arsenic is sequestered in deep-lake facies, where it is mainly trapped by framboidal pyrite and organic matter under reducing conditions. The mid-Miocene system reflects a reorganization of the drainage basins, resulting in the recycling of older formations and material derived from the northern Massif Central (MCF). Arsenic is predominantly carried by pyrite in reduced floodplain facies. Tectono-hydrogeological destabilizations since the Tortonian have promoted the oxidation of these reduced sedimentary reservoirs, the release of arsenic in groundwater and partial precipitation of secondary arsenic carrier phases.

This study highlights the role of geomorphology and sedimentology in controlling arsenic drainage and trapping, and establishes a link between initial trapping and current groundwater quality. (ANR-22-PEXO-0010 – PEPR One Water Eau bien commun research program).