The transport and deposition behavior of pathogenic Escherichia coli O157:H7 was studied under unfavorable electrostatic conditions in saturated quartz sands of various sizes (710, 360, 240, and 150 ¿m) and at several flow rates. At a given velocity, column effluent breakthrough values for E. coli tended to decrease in magnitude, and concentration curves became more asymmetric with decreasing sand size. In a given sand, experiments conducted at a higher velocity tended to produce higher effluent concentrations, especially for finer (240 and 150 ¿m) textured sands. The shape of the deposition profiles for E. coli was also highly dependent on the sand size and velocity. Coarser-textured sands and higher flow rates were associated with less deposition and gradually decreasing concentrations with depth. Conversely, finer-textured sands and lower flow rates tended to produce greater deposition and nonmonotonic deposition profiles that exhibited a peak in retained concentration. This deposition peak occurred nearer to the column inlet for finer-textured sands and at low flow rates. Microscopic observations of E. coli retention in these finer-textured sands (micromodel experiments) clearly indicated that straining was the dominant mechanism of deposition. Batch experiments also indicated minor amounts of E. coli attachment for the selected sands and solution chemistry. A conceptual and numerical model was developed and successfully used to describe the observed E. coli transport and deposition data. Our conceptual model assumes that E. coli can aggregate when large numbers of monodispersed E. coli are deposited at pore constrictions or straining sites. When the deposited E. coli reach a critical concentration at the straining site, the aggregated E. coli O157:H7 can be released into aqueous solution as a result of hydrodynamic shearing forces.