USING MICROCOSMS TO EVALUATE IRON AND SULFATE REDUCTION RATES UNDER VARYING BIOGEOCHEMICAL CONDITIONS

The important role of redox reactions in the natural attenuation/bioremediation of contaminated aqueous environments has been well established. However, there is still much to be teased out as to the role of simple electron donors (ex. lactate and acetate) in regulating redox processes in relatively pristine and donor-limited environments. This study sought to evaluate initial results from four series of microcosms (five per series) amended to determine redox processes and rates of reactions in uncontaminated, donor-limited, iron/sulfate-rich sediments. Rates of iron and sulfate reduction were based on change in concentrations over time relative to initial concentrations. The microcosm series included: a control, which contained no manipulation to establish baseline geochemical conditions; a sterilized series, where samples were autoclaved to eliminate biological reactions; and two series which each contained a different simple carbon substrate (lactate and acetate). This was done to evaluate the role of both microbial mediation and the role of electron donor availability on the rates of reactions. Over the course of 75 days, results showed that maximum iron reduction rates were achieved in both the lactate and acetate amended series of microcosms. Rates in the donor-amended series were an order of magnitude faster than those observed in the control series. Lactate stimulated sulfate reduction whereas sulfate reduction was not observed in the acetate series. Results from the sterilized series indicated both iron and sulfate reduction but with a significant lag time (35 days) and overall slower rates when compared to the non-sterilized microcosms. This highlights probable competition between microbial populations for preferred electron donors, an area of research requiring further investigation. The findings from this study show the significance of variability in both electron donor availability and microbial activity on regulating reactions in aqueous systems. Understanding these controls will lead to improved natural attenuation/bioremediation of contaminated systems. Results of this initial study will be compared to results from newly constructed microcosms with varying electron donor combinations and the role of microbial activity will be further explored.