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The mobilization of subsurface As poses a serious threat to human health, particularly in a region such as Florida where population is heavily dependent on highly porous karstic aquifers for drinking water. Injection water used in aquifer storage and recovery (ASR) or aquifer recharge (AR) operations is commonly high in dissolved organic matter (DOM) and OM can also be present in the subsurface carbonate rock. Using batch incubation experiments, this study examined the role of core preservation methods, as well as the influence of labile and more refractory DOM on the mobilization of As from carbonate rock. Incubation experiments used sealed reaction vessels with preserved and homogenized core materials collected via coring the Suwannee Formation in southwest Florida and treatment additions consisting of 1) source water (SW) enriched in sterilized soil DOM, 2) SW enriched in soil DOM and microbes, and 3) SW enriched in sodium acetate. During an initial equilibration phase in native groundwater (NGW) with low dissolved oxygen (DO; Phase 1), we found the greatest As release of the whole incubation. In the beginning of Phase 2 (N2 headspace) in which NGW was replaced with treatment solutions, there was little As release except in the vessel with Na-acetate added, which also had the lowest ORP. At the start of Phase 3, when incubations were exposed to air, most vessels saw more ion (including As) release into solution. Vessel with Na-acetate had less As release in Phase 3 than in Phase 2. During all experimental phases, treatments of DOM or microbe additions had no apparent effect on the amount of As release. The core materials was found contain significant amount of indigenous DOM (about 8 g OC/kg core) which was released during the incubation so DOC concentrations displayed no clear pattern among different treatments. At least three abiotic As mobilization mechanisms may play a role in As released during different stages of the experiment. Desorption of As from iron oxyhydroxides may have occurred, particularly at the beginning of each experimental phase. Reductive dissolution and oxidative dissolution likely lead to As release during phase 2 and 3, respectively. While not directly implicated, the presence of labile OM clearly fueled microbial alteration of redox conditions, leading to further As release. Addition of microbes had no effect as indigenous microbes were just as active in untreated cores. |