The bioavailability of arsenic and other trace elements in soils is strongly influenced its speciation. The redox potential of soil environments affects bioavailability by affecting the redox state of trace elements (affecting the strength of their retention) and the soil mineralogy and porewater chemistry. In particular, aqueous arsenic concentrations are affected by the redox cycling of iron minerals that are strong sorbents of As, and sulfur, which indirectly affects the fate of As. Typically reduction of Fe(III) mineral phases leads to the release of As from insoluble solid-phase forms to bioavailable aqueous forms. Sulfate reduction has an opposite effect, creating hydrogen sulfide that can precipitate with As and other metals into a variety of insoluble sulfide minerals. Although Fe(III) and sulfate reduction yield different amounts of energy, they often overlap in real soil solutions. This overlap makes it difficult to accurately predict the fate of As in soil solutions under typical conditions. Here, we examine the effects of iron mineralogy, organic matter and sulfate amendments on natural microbial communities within mine waste impacted soils. The activity of in situ microbial consortia was stimulated in a suite of variably amended natural sediment microcosms and their effect on groundwater geochemistry and mineralogy was monitored over time.  Sediments were also removed periodically for iron and arsenic solid phase speciation analysis using X-ray absorption spectroscopy, and these data were used to develop a quantitative model to examine the biological and chemical processes most important the release and sequestration of As in these microcosms.  Arsenic appears to be most bioavailable when both Fe(III) and sulfate reduction occur together at comparable rates.