INVESTIGATING THE POTENTIAL FOR MICROBIAL IRON REDUCTION IN A HYDROLOGICALLY DYNAMIC RESERVOIR SYSTEM, VILLA PARK DAM, VILLA PARK, CA

Sediment samples obtained from two locations within the Villa Park reservoir in Villa Park, CA were evaluated for their potential to support microbial iron reduction. Results from geochemical analyses of the reservoir water have shown concentrations of iron to be above maximum contaminant levels. These high concentrations are thought to be a result of microbial iron reduction occurring in the reservoir sediments, but the conditions under which these sediments are able to support an iron reducing population is unknown. The Villa Park reservoir system is dynamic, in that it experiences extreme variability in hydrologic state (0 – 30+ ft of standing water) and quantity of organic material. This variability results in ever-changing redox conditions and availability of electron donor. Understanding how the environmental conditions in this system impact its capacity to support microbial iron reduction is key to making future predictions of iron concentrations in the reservoir water.

The potential for microbial iron reduction in these sediments was evaluated using physical, geochemical and microbial analyses of core sediments and experimental microcosms. Two shallow sediment cores (~3 ft) were collected from the Villa Park reservoir; one in an area with little visible organic material, and one in an area with abundant visible organic material. These sediment cores were sub-sampled for grain size, carbon content, magnetic susceptibility, and major cations and anions. Sediments were also analyzed using the most probable number method for quantification of iron reducing bacteria. Additionally, microcosm experiments were conducted using sediment and water from the core locations. Experiments were conducted in three groups: one control (no supplements), one supplemented with electron donor (acetate and lactate), and one supplemented with oxygen. Each experiment was done in duplicate. Iron reducing bacteria were found to be abundant in most of the reservoir sediments analyzed. Results from microcosm experiments indicate that the concentration and type of electron donor plays an important role in controlling the rate of iron reduction in this system. The results from this study indicate that iron reducing bacteria are responsible for the high concentrations of iron observed in the reservoir water.