EFFECT OF MICROBIAL SULFATE-REDUCTION RATES ON ARSENIC MOBILITY IN IRON-RICH ENVIRONMENTS

Anoxic, low-gradient aquifer systems, such as those found on Southeast Asian deltas, often exhibit arsenic contamination at dangerous levels. In these environments, arsenic is most commonly released from iron oxide-rich geologic media into anoxic groundwaters through the reductive dissolution of arsenic-bearing iron oxides. Dissimilatory sulfate reduction often occurs in conjunction with iron reduction; here we examine how sulfide produced by dissimilatory sulfate reduction constrains the chemistry and mineralogy of iron-rich systems and thus controls arsenic partitioning. Dissolved sulfide may cause either arsenic sequestration (in sulfide minerals) or arsenic mobilization (by inducing reductive iron dissolution); thus, it is important to identify the factors that determine the effect of sulfide in arsenic-bearing sediments. Here we consider the effect of sulfate concentration, and thus the rate of sulfate reduction, on arsenic mobility. Static and flow-through systems containing arsenic-bearing iron oxides were innoculated with the sulfate-reducing bacterium Desulfovibrio desulfuricans. Sulfate reduction by D. desulfuricans causes significant release of arsenic into solution in dynamic flow systems, across a wide range of sulfate concentrations (and thus rates of bacterial metabolism), even as the mineralogical transformations occurring vary dramatically. This is in contrast to stagnant incubation systems, in which arsenic is retained in the solid phase even while the mineralogy of the system changes dramatically. These results indicate that biological sulfate reduction may lead to arsenic contamination in dynamic systems, and suggest a complex relationship between groundwater composition, changing mineralogy, bacterial populations, and arsenic.