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

S_sulfide, 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 S_sulfide 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 ≈10m), 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 SO42- reduction and interaction between produced Fe(II)aq and Fe(II)-sulfides precipitate control δ⁵⁶Fe values of sediments and water sample in the midsections (≈13-19m) of the Core A. Conversely, abiotic Fe(III) reduction by HS^- determines the bulk δ⁵⁶Fe values of core sediments and water in the midsections (≈13-19m) of the Core B. Microbial SO42- 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.