Elevated arsenic (As) levels in groundwater pose a risk to the local population that relies on this resource for their living needs. Here we analyzed samples collected from a sediment core recovered from Jianghan Plain, China, a region experiencing high As concentrations in groundwater. We evaluated the vertical distribution of As forms, fluorescent dissolved organic matter (DOM), microbial assemblages, and a suite of anions and cations. Our aim was to elucidate the As species in the local groundwater and their migration and transformation in the aquifer. As(III), Fe, Mn, TOC, SO₄²⁻, and NO₃⁻ were negatively correlated with depth, whereas As(V) and As(Total) did not show any correlation with depth, nor was As correlated with the amount and characteristics of DOM. High concentrations of As inhibited the metabolic activity of bacteria and thereby limited the production of fluorescent DOM. Sulfate-reducing bacteria transformed free-sulfate radicals to S²⁻ and promoted the dissolution and fixation of arsenic. We identified three main forms of arsenic that varied with depth: FeOOH (absorption), COO (absorption), and FeAsS. Microbial communities also varied with depth, and Novosphingobium and Brevibacillus showed high relative abundances in the sediment profile. TOC and DOM were strongly depleted in strongly As-enriched depths.