A filtration model commonly used to describe removal of colloids during packed-bed filtration in water treatment applications was modified for describing downgradient transport of bacteria in sandy, aquifer sediments. The modified model was applied to the results of a small-scale (7 m), natural-gradient tracer test and to observations of an indigenous bacterial population moving downgradient within a plume of organically contaminated groundwater in Cape Cod, MA. The model reasonably accounted for concentration histories of labeled bacteria appearing at samplers downgradient from the injection well in the tracer experiment and for the observed 0.25-jum increase in average cell length for an unlabeled, indigenous bacterial population, 0.6 km downgradient from the source of the plume. Several uncertainties were apparent in applying filtration theory to problems involving transport of bacteria in groundwater. However, adsorption (attachment) appeared to be a major control of the extent of bacterial movement downgradient, which could be described, in part, by filtration theory. Estimates of the collision efficiency factor, which represents the physicochemical factors that determine adsorption of the bacteria onto the grain surfaces, ranged from 5.4 X 10'3 to 9.7 X 10~3.

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The transport and filtration behavior of Cryptosporidium parvum oocysts in columns packed with quartz sand was systematically examined under repulsive electrostatic conditions. An increase in solution ionic strength resulted in greater oocyst deposition rates despite theoretical predictions of a significant electrostatic energy barrier to deposition. Relatively high deposition rates obtained with both oocysts and polystyrene latex particles of comparable size at low ionic strength (1 mM) suggest that a physical mechanism may play a key role in oocyst removal. Supporting experiments conducted with latex particles of varying sizes, under very low ionic strength conditions where physicochemical filtration is negligible, clearly indicated that physical straining is an important capture mechanism. The results of this study indicate that irregularity of sand grain shape (verified by SEM imaging) contributes considerably to the straining potential of the porous medium. Hence, both straining and physicochemical filtration are expected to control the removal of C. parvum oocysts in settings typical of riverbank filtration, soil infiltration, and slow sand filtration. Because classic colloid filtration theory does not account for removal by straining, these observations have important implications with respect to predictions of oocyst transport.