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

摘要

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 SO42-/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.Plain Language Summary Reductive dissolution of Fe (III) oxides via organic matter degradation and sulfide (H2S) oxidation, and As desorption induced by elevated pH and coexisting anions (e.g., HCO3- and PO43-) should be responsible for As mobility in anoxic aquifers in inland basins, where weakly alkaline pH and sulfidic conditions prevail. Quantifying these As mobility-related processes and their relative importance may improve our understanding of the geochemical processes causing As enrichment in aquifer systems in inland basins. We established a 1-D multicomponent redox process-controlled reactive transport model to quantify As release-related processes along an approximate flow path from the mountain to the flat plain in the Hetao Basin, China. The results showed that the reduction of Fe (III) oxides was mainly induced by organic matter degradation (75%), followed by via H2S oxidation (25%). Arsenic concentrations were depressed under sulfidic conditions due to As sequestration by mackinawite precipitation and adsorption, although abiotic H2S reduction of Fe (III) oxides contributed to the release of As. Overall, As was released through Fe (III) oxide reduction (70%) and desorption under elevated pH and HCO3- and PO43- concentrations (30%). This study provides an insight into the mechanisms of groundwater As mobilization in inland basins.