Abundant Fe(III) Oxide‐Bound Arsenic and Depleted Mn Oxides Facilitate Arsenic Enrichment in Groundwater From a Sand‐Gravel Confined Aquifer

Abstract

AbstractEnrichment mechanisms of high‐arsenic (>10 μg/L) groundwater in confined aquifers with sand‐gravel sediments were rarely documented. Both groundwater and sediment were sampled to investigate mechanisms of groundwater arsenic mobility in sand‐gravel confined aquifers from the Songnen Basin, China. The results show that, although high arsenic groundwater was present in both shallow sandy (<60 m) and deep sand‐gravel aquifers (60–100 m), relatively higher arsenic groundwater was hosted in the deep confined aquifers. The significantly higher proportion of biologically degradable organic carbon (BDOC) in deep confined groundwater than those in shallow unconfined ones (p = 0.03) suggests that higher arsenic concentrations were associated with higher DOM biodegradability. High‐arsenic groundwater was the result of Fe(III) oxide reduction facilitated by BDOC degradation. Arsenic‐rich pyrite framboids were observed in shallow aquifers, which would lower groundwater arsenic concentrations. Sand‐gravel sediments in deep aquifers had remarkably higher contents of extracted Fe(III) oxides and the bound arsenic, HCl‐extractable Fe(II) minerals, and adsorbed arsenic, but considerably lower contents of Mn oxides and Mn(II) minerals than the shallow sandy sediments. Siderite, which is not as efficient as pyrite in sequestering arsenic, was the dominant Fe(II) mineral in the deep aquifer. In the deep aquifer, Mn oxide reduction was almost completed and Fe(III) oxide reduction was predominated, which resulted in high concentrations of groundwater arsenic due to high arsenic/iron molar ratio in the Fe(III) oxides. This study proposed that geogenic high‐arsenic groundwater can occur in sand‐gravel aquifers being high in Fe(III) oxide‐bound arsenic but poor in Mn oxides under reducing conditions.