The relative importance of dispersion, physical straining, nonequilibrium sorption, and cell density on the dispersal of bacteria was examined in saturated, flow-dynamic sand columns. The bacterial breakthrough as a result of different size distributions of sand particles was followed by measuring the effluent concentration of (3)H-adenosine-labelled cells of a Bacillus sp. and an Enterobacter sp. strain suspended in ground-water. The breakthrough curves were compared with theoretical curves predicted from an advective-dispersive equilibrium sorption model (ADS), an ADS model with a first order sink term for irreversible cell reactions, a two-site model (equilibrium and nonequilibrium sorption sites), and a filtration model. Bacterial sand:water isotherms were linear in the experimental concentration range but had positive intercepts. The partition coefficients ranged from 15 to 0.4 for the Bacillus sp., and 120 to 0.4 for a Pseudomonas sp., and decreased with increasing particle size of the dominant fraction. In a kinetic study, the partition coefficient for the Enterobacter sp. in the smaller particle sand was 63 after one hour, but had decreased to 9 after 19 hours. (Copyright (c) Springer-Verlag New York Inc. 1992.)