Multilevel hydrogeochemical monitoring of spatial distribution of arsenic: A case study at Datong Basin, northern China

• Spatial distribution characteristics of aqueous As species were investigated.
• Reduction of Fe minerals and As(V) can enhance As(III) enrichment in groundwater.
• Microbial sulfate reduction may promote As(V) transformation into As(III).

To elaborate primary geochemical factors controlling the enrichment and spatial distribution of arsenic (As) species in groundwater systems, multilevel hydrogeochemical monitoring was conducted at an As-contaminated site in central part of Datong Basin, northern China. Aqueous As concentration was highly variable, ranging from 5.47 to 2690 μg/L. High As groundwater was characterized by elevated HCO3−, Fe(II), HS−, NH4+ and dominated by As(III) species. The positive correlation between As and Fe contents was observed in easily-reducible Fe pool, indicating that poorly crystalline Fe minerals served as an important As sink via chemical adsorption and/or co-precipitation. Under moderately reducing and weakly alkaline conditions (Eh ≥ 100 mV and pH between 7.69 and 8.04), As mobilization could be controlled by pH-dependent desorption process. The relationships of aqueous As(V) and As(III) versus Fe(II) concentration confirm that reductive dissolution of As-bearing Fe oxides/hydroxides and reductive desorption of As(V) could be responsible for As enrichment and As(III) predominance in groundwater under more reducing conditions (Eh ≤ 100 mV). The co-presence of elevated As(III) and sulfide indicates that microbial sulfate reduction may promote As(V) transformation into thioarsenate and/or thioarsenite and further into As(III). However, the correlations of As(V) and As(III) concentration versus saturation indices of mackinawite suggest that decrease in aqueous As concentration may be due to re-sequestration of both As(V) and As(III) by Fe(II)-sulfide precipitates.