Reduction of iron (hydr)oxide-bound arsenate: Evidence from high depth resolution sampling of a reducing aquifer in Yinchuan Plain, China.

Sediment in fluvial-deltaic plains with high-As groundwater is heterogenous but its characterization of As and Fe oxidation states lacks resolution, and is rarely attempted for aqueous and solid phases simultaneously. Here, we pair high-resolution (> 1 sample/meter) Fe extended fine-structure spectroscopy (EXAFS, n = 40) and As X-ray absorption near-edge spectroscopy (XANES, n = 49) with groundwater composition and metagenomics measurements for two sediment cores and their associated wells (n = 8) from the Yinchuan Plain in northwest China. At shallower depths, nitrate and Mn/Fe reducing sediment zones are fine textured, contain 9.6 ± 5.6 mg kg-1 of As(V) and 2.3 ± 2.7 mg kg-1 of As(III) with 9.1 ± 8.1 g kg-1 of Fe(III) (hydr)oxides, with bacterial genera capable of As and Fe reduction identified. In four deeper 10-m sections, sulfate-reducing sediments are coarser and contain 2.6 ± 1.3 mg kg-1 of As(V) and 1.1 ± 1.0 mg kg-1 of As(III) with 3.2 ± 2.6 g kg-1 of Fe(III) (hydr)oxides, even though groundwater As concentrations can exceed 200 μg/L, mostly as As(III). Super-enrichment of sediment As (42-133 mg kg-1, n = 7) at shallower depth is due to redox trapping during past groundwater discharge. Active As and Fe reduction is supported by the contrast between the As(III)-dominated groundwater and the As(V)-dominated sediment, and by the decreasing sediment As(V) and Fe(III) (hydr)oxides concentrations with depth.