Microorganisms play a key role in the redox transformation of arsenic in aquifers. In this study, the impact of indigenous bacteria, especially the prevailing nitrate respirers, on arsenite oxidation was explored during groundwater filtration using granular TiO2 and subsequent spent TiO2 anaerobic landfill. X-ray absorption near edge structure spectroscopy analysis showed As(III) oxidation (46% in 10 days) in the presence of nitrate in the simulated anaerobic landfills. The Fe phase showed no change during the anaerobic incubation. Incubation experiments implied that the indigenous bacteria completely oxidized arsenite to arsenate in 10 days using nitrate as the terminal electron acceptor under anaerobic conditions. Phylogenetic tree analysis revealed that Proteobacteria was the dominant phylum, with Hydrogenophaga (34%), Limnohabitans (16%), and Simplicispira (7%) as the major bacterial genera. The nitrate respirers especially from the Hydrogenophaga genus anaerobically oxidized As(III) using nitrate as an electron acceptor instead of oxygen. Our study implied that microbes can facilitate the groundwater As oxidation using nitrate on the adsorptive media.