| Bacterial mobility under unsaturated soils plays an important role in the transport of pathogenic microorganisms. The effect of swelling soils upon the transport of bacteria was investigated under unsaturated flow conditions at 22°C and 4°C, These investigations involved using repacked laboratory columns and a tracer bacterium, streptomycin-resistant Aeromonas hydrophila . The soil swelling effect was simulated in columns containing 5% and 10% bentonite-sand mixture, with homogeneous silica sand as a control column. The columns were dosed at rainfall intensities of 5, 10 and 20 mm/h. At the completion of the experiments, the soil columns were extruded and sampled for cell retention. The results indicated that deeper vertical penetration of bacteria occurred in clean sand compared to the swelled soil mixtures. In addition, rainfall intensities affected the bacterial front in the soil columns, with deepest vertical penetration under 20 mm/h. The bacterial movement was found to be retarded compared to chloride movement in all fractions of the porous materials examined. Bacteria moved furthest in the silica sand compared to the swelling soils. Swelling of the soil hindered the transport of bacteria by the effects of filtration and reduction in the permeability of the porous media. The colloidal-filtration model predicted the effect of filtration. The filtration coefficient increased when flow occurred in the bentonite-sand mixtures.; Bacterial recovery was influenced by the air-surface interface with 93%, 95%, and 85% recovery occurring at the top layers under 5, 10 and 20 mm/h in 10% bentonite-sand, respectively. Temperature differences also affected the recovery of bacteria in the soil column. At 4°C, bacteria were better retained at the upper portions of the columns, indicated by higher bacterial recovery, compared to 22°C. |
