Sediment heterogeneity drives divergent arsenic transformation pathways through organic matter-microbial coupling in aquifers.

Arsenic (As) speciation in groundwater is controlled not only by aqueous redox chemistry but also by sedimentary matrices that preserve organic matter, structure metabolism, and regulate Fe-S-As coupling. However, how sedimentary organic matter (SOM) and microbial functional differentiation jointly direct arsenic toward thioarsenate formation and methylation remains insufficiently constrained. This study combined sedimentological characterization, X-ray diffraction mineralogical analysis, Fe/As sequential extraction, excitation-emission matrix fluorescence spectroscopy, FT-ICR-MS molecular characterization of SOM, and metagenomic sequencing across three hydrogeochemical zones. Sediments shifted from coarse-grained alluvial deposits in the low-As recharge zone (ALZ) to fine-grained, organic-rich lacustrine sediments in the thioarsenate-enriched zone (HGD) and the methylation zone (SHX), with clay enrichment in HGD and carbonate enrichment in SHX. Along this gradient, ALZ showed open recharge conditions with labile SOM and dynamic redox environments, whereas As in HGD and SHX shifted from surface-bound forms to poorly crystalline and crystalline Fe-associated fractions, alongside SOM evolution toward humified, aromatic, highly unsaturated, and sulfur-containing molecules. The HGD exhibited enrichment of polyphenols and CHOS/CHONS compounds, providing substrates and redox-active ligands for Fe-S-As coupling. Metagenomics revealed zone-specific functional differentiation. The ALZ was dominated by Proteobacteria supporting heterotrophic metabolism, sulfur oxidation, and arsenic resistance. The HGD showed enhanced sat-aprAB-dsrAB pathways and weakened soxABCDXYZ-mediated sulfur oxidation, favoring reduced sulfur accumulation, Fe-As mineral sulfidation, and thioarsenate formation. The SHX displayed enrichment of arsC and arsenic resistance/efflux genes, supporting As(V) reduction and methylated As transformation. These results demonstrate that sediment heterogeneity governs As speciation and migration through coupled SOM evolution and microbial functional reorganization.