Sulfur cycling processes and its effect on arsenic mobility in groundwater from typical areas of the Beijing Plain

The widespread occurrence of naturally high-arsenic (As) groundwater in Beijing severely restricts the utilization of regional groundwater resources. Sulfur cycling in groundwater is one of the important hydrogeochemical processes, yet its role in controlling As mobility remains unclear. In this study, groundwater samples from different depths in Shunyi and Tongzhou districts were analyzed for geochemical composition, sulfur and oxygen isotopes in \mathrmSO_4^2- , and the speciation of sulfur-containing minerals in sediments to reveal the influence of sulfur cycling on As enrichment in groundwater. Results suggest that high As and high \mathrmSO_4^2- groundwater are mainly enriched in shallow aquifers, with the As and \mathrmSO_4^2- concentrations gradually decreasing with depth. The positive correlation between δ³⁴S-SO₄ and δ¹⁸O-SO₄, together with the increase in δ³⁴S-SO₄ with the decrease in \mathrmSO_4^2- /Cl⁻ ratio, indicates the occurrence of microbial \mathrmSO_4^2- reduction process in the groundwater. The positive correlation between δ³⁴S-SO₄ and As concentration and the negative correlation between \mathrmSO_4^2- /Cl⁻ molar ratio and As indicate that \mathrmSO_4^2- reduction promoted the release of As. The generated H₂S from \mathrmSO_4^2- reduction triggered the chemical reductive dissolution of As-bearing Fe(III) oxides and the release of As. The presences of acid volatile sulfide (AVS), Cr(II)-reducible sulfide (CRS), and elemental sulfur (ES) in sediments at different depths provide evidence for the sulfur cycling process and its effect on groundwater As mobility. The study reveals how the sulfur cycling drives the enrichment and transformation of As in groundwater system, and provides theoretical basis for the coupled S-As treatment on As in groundwater.