Quantifying Geochemical Processes of Arsenic Mobility in Groundwater From an Inland Basin Using a Reactive Transport Model.

High arsenic (As) groundwater is frequently found in inland basins, yet the contributions of different processes to aqueous As distributions remain unresolved. In the Hetao Basin, a typical inland basin, groundwater As concentrations generally increased from the alluvial fan through the transition area to the flat plain. A geochemical process‐based reactive transport model was established to evaluate and quantify the processes of As mobilization in the northwestern Hetao Basin. Thirty‐six groundwater samples and eight sediment samples were collected from the alluvial fan to the flat plain to investigate the geochemical characteristics of the groundwater system. Along the approximate flow path, groundwater evolved from oxic‐suboxic conditions to anoxic conditions, with increasing concentrations of As, Fe (II), and NH₄⁺, and decreasing Eh and SO₄ ²⁻/Cl⁻. Modeling results indicated that the observed concentrations of Fe (II) were caused by reductive dissolution of Fe (III) oxides and subsequent precipitation of mackinawite and siderite. Reductive dissolution of Fe (III) oxides was primarily driven by organic matter degradation (>75%), followed by H₂S oxidation (<25%). More As was sequestered by mackinawite precipitation and adsorption than that released via abiotic reduction of Fe (III) oxides by H₂S. Reductive dissolution of Fe (III) oxides was the dominant mechanism for liberating As in both the transition area and the flat plain (>70%), and As desorption under elevated pH and competitive adsorption by HCO₃⁻ and PO₄³⁻ made an important contribution to As enrichment (up to 30%). Overall, this study provides an insight into the relative contributions of different geochemical processes to As enrichment in inland basins. [ABSTRACT FROM AUTHOR]