Arsenic fractioning in natural solid matrices sampled in a deep groundwater body

- Large As release can be expected in presence of vegetal-matter/paleo-waters contact.
- Lignin occurrence can play a significant role in cations' adsorption such as arsenic.
- Similar matrices taken at different depths can show different As content/fractioning.
- Amorphous and crystalline phases are key to control As partitioning and dynamics.
- Similar mechanisms of As release can be observed from fine and vegetal matrices.

We investigate arsenic (As) fractioning in four natural solid matrices collected in a deep alluvial aquifer body which constitutes a typical example of an alluvial system characterized by large values of local As concentrations in groundwater. Samples are collected at diverse depths within the aquifer and the underlying aquitard. Their composition ranges from sand to silt, including vegetal matter, the latter being particularly abundant within the analyzed host porous formation. Selective sequential extractions (SSE) are performed and allow quantifying and comparing As partitioning within all tested natural solid matrices. SSE evidenced that As content in vegetal-rich sediments is one order of magnitude larger than that observed in the other matrices; solid matrices with similar composition in terms of sand texture and collected at different depths may display diverse As content and partitioning; and amorphous and crystalline phases constitute the most critical fractions governing As partitioning for all tested matrices. Among all the collected matrices the key dynamics of As release from the silty matrix to groundwater are then experimentally evaluated by subjecting the solid matrix to alternating aerobic/anaerobic settings under a range of redox and pH conditions consistent with field values. Arsenic mobilization from the silty matrix was triggered by abrupt changes in redox conditions and displays a clear relationship with the temporal dynamics of the oxidation/reduction potential. Our study suggests that the environmentally critical As concentrations detected in the investigated aquifer are consistent with the occurrence of hot-spots of high natural As content associated with vegetal matter and the potentially significant role of the silty matrix on As dynamics.