Transport of Escherichia coli and F-RNA Bacteriophages in a 5-M Column of Saturated, Heterogeneous Gravel

The relative transport and attenuation of bacteria, bacteriophages, and bromide was determined in a 5 m long × 0.3 m diameter column of saturated, heterogeneous gravel. The average pore velocity (V), longitudinal dispersivity (α x ), and total removal rate (λ) were derived from the breakthrough curves at 1, 3, and 5 m, at a flow rate of 24.8 L h−1. The experiments largely confirmed the differences in transport and attenuation patterns among bacteria, phages, and bromide, and between colloid-associated and “free” microorganisms, previously observed in a study using homogeneous pea gravel. Cultured Escherichia coli J6-2 cells were transported faster than phage MS2 and bromide, consistent with velocity enhancement of the larger particles. The evidence for velocity enhancement of phage MS2 compared with bromide was less conclusive, with some evidence of retardation of the phage as a result of adsorption–desorption processes in the finer media. On average, phage in sewage and adsorbed to kaolin particles were transported faster than free phage, suggesting that most sewage phage are adsorbed to colloids. However, average velocities of cultured and sewage E. coli differed far less, suggesting that most E. coli in sewage exist as individual (non colloid-associated) cells. There was no conclusive evidence that the wider pore size range in the heterogeneous mixture compared with pea gravel increased velocity enhancement effects. Removal rates of free phage were far higher than in the pea gravel, and were attributed to adsorption in the finer materials. Equivalent increases in removal of cultured and sewage E. coli and colloid-associated phage were attributed to straining in finer materials and settling in quiescent zones. Inactivation (μ) rates (determined in the pea gravel study) indicated little contribution to removal of either free or attached microorganisms. The results showed the importance of association with colloids in determining the relative transport of bacteria and viruses in alluvial gravels.