Spatial distributions of Cryptosporidium oocysts in porous media: evidence for dual mode deposition

Spatial distributions of Cryptosporidium parvum oocysts in columns packed with uniform glass-bead collectors were measured over a broad range of physicochemical conditions. Oocyst deposition behavior is shown to deviate from predictions based on classical colloid filtration theory (CFT) in the presence of repulsive (unfavorable) colloidal interactions. Specifically, CFT tends to predict greater removal of oocysts (less transport) than that observed in controlled laboratory experiments. Comparison of oocyst retention with results obtained using polystyrene latex particles of similar size suggests that mechanisms controlling particle deposition are the same in both systems. At a given ionic strength, the deposition of Cryptosporidium oocysts is generally greater than that of the microspheres; however, this discrepancy is partly attributable to large differences in oocyst and microsphere zeta potentials. A dual deposition mode (DDM) model is applied which considers the combined influence of "fast" and "slow" oocyst deposition due to the concurrent existence of favorable and unfavorable oocyst-collector interactions. Model simulations of retained oocyst profiles and suspended oocyst concentration at the column effluent are consistent with experimental data. Because classic CFT does not account for the effect of dual mode deposition (i.e., simultaneous "fast" and "slow" oocyst deposition), these observations have important implications for predictions of oocyst transport in subsurface environments, where repulsive electrostatic interactions predominate. Supporting elution experiments further suggest that specific surface interactions between oocyst wall macromolecules and the glass bead collectors could retard or even completely inhibit oocyst release upon perturbation in solution chemistry.