Pathways of arsenic from sediments to groundwater in the hyporheic zone: Evidence from an iron isotope study

Ssulfide, Fe content and heavy Fe isotopic signatures of the bulk core sediments all indicate anoxic and sulfidic conditions in the hyporheic zone. The relationship between Ssulfide and Fe contents suggests that Fe(III) oxides/hydroxides are transferred between non-sulfidic Fe(II) minerals and Fe(II)-sulfides under anoxic and sulfidic conditions, respectively. The Fe isotope composition provides further evidence that microbial dissimilatory reduction of Fe(III) and the formation of Fe(II)-sulfides and non-sulfidic Fe(II) minerals are the dominant Fe geochemical pathways and take place at different depths in the hyporheic zone. In the upper sections of the Core A and B (with depth less than ≈10 m), microbial Fe(III) reduction and non-sulfidic Fe(II) minerals formation govern the Fe cycling and the Fe isotope composition in hyporheic water and bulk sediments. Microbial Fe(III) and image reduction and interaction between produced Fe(II)aq and Fe(II)-sulfides precipitate control δ56Fe values of sediments and water sample in the midsections (≈13–19 m) of the Core A. Conversely, abiotic Fe(III) reduction by HS− determines the bulk δ56Fe values of core sediments and water in the midsections (≈13–19 m) of the Core B. Microbial image reduction is limited and microbial Fe(III) reduction controls the δ56Fe values of water and sediments at the bottom of both cores. The variation of δ56Fe values and the As concentration in hyporheic water are similar at each depth, indicating that As enrichment in the water is strongly associated with the microbial reduction of Fe(III) oxides/hydroxides and the formation of Fe(II)-sulfides and non-sulfidic Fe(II) minerals. The enriched-δ56Fe values of high As water concentrations suggest that microbial reduction of Fe(III) oxides/hydroxides is the dominant process that promotes As mobility in the hyporheic zones.