EFFECT OF IRON (III) OXYHYDROXIDE FRACTURE COATINGS AND CELL MORPHOLOGY ON THE TRANSPORT OF BACTERIA THROUGH SATURATED FRACTURED CRYSTALLINE ROCK

Due to such environmental issues as ground-water contamination by pathogenic bacteria and bioremediation of contaminated aquifers, there is a need to understand and accurately predict bacterial transport through aquifers. Extensive research has been conducted on bacterial transport through porous media, yet experiments in fractured rock at the field or laboratory-scale are rare. In May 2000, forced-gradient tracer tests were conducted in fractured crystalline bedrock at the Mirror Lake Fractured Rock Research Site to investigate the transport behavior of various bacterial cell morphologies. Tracers used for the field investigation included cultured, indigenous, aerobic bacteria (nonmotile coccoid, nonmotile gram-negative rod, motile gram-negative rod, and nonmotile gram-positive rod), positively and negatively-charged polystyrene microspheres, and deuterated water. Ground-water chemistry at the site suggests the fractures are coated with iron (III) oxyhydroxide. Since bacteria cells generally have a negative surface charge and iron oxyhydroxide has a positive surface charge, the fracture coatings, if present, could significantly affect bacterial transport by increasing filtration. To determine the effect of these coatings on transport, lab-scale column tests are being conducted using clean glass beads and glass beads coated with iron oxyhydroxide. Tracers include bacteria species used in the field investigation, positively- and negatively-charged microspheres, and bromide. Using breakthrough data from column tests, the effect of iron coatings on bacteria transport will be modeled using a one-dimensional advection-dispersion model with a filtration coefficient. This model can then be applied to field data to determine the effect of the fracture coatings on transport at Mirror Lake. Preliminary analysis of tests conducted in the clean glass column show microsphere arrival before bromide, with negative preceding positive microspheres. More negative microspheres were recovered than positive, but they also exhibited increased tailing. These results suggest that negative microspheres are undergoing reversible filtration, possibly due to anion exclusion, while positive microsphere filtration is somewhat irreversible due to electrostatic attraction.