The transport and deposition behavior of Escherichia coli O157: H7 was investigated in saturated packed-bed columns and micromodel systems over a range of ionic strength (IS) (1, 10, and 100 mM) and pH (5.8, 8.4, and 9.2) conditions. At a given IS, elevated solution pH resulted in decreased deposition as a result of the increase in the measured zeta potential of the quartz sand. This deposition trend was consistent with predictions from classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Conversely, the E. coil O157:H7 deposition was inversely proportionalto IS (1-100 mM) at high pH conditions (8.4 and 9.2), whereas no effect of IS was observed at pH 5.8. This deposition trend was not consistent with DLVO theory, but could be explained by pH-associated electrosteric stabilization. This phenomenon is driven by the pH-dependent protonated state of functional groups on E. coil O157:H7 surface macromolecules and the corresponding conformational state of the bacterial polymers. Results from this study demonstrate that retention of E. coil O157:H7 cells in porous media is a complex process that depends on the solution chemistry, cell-cell interactions, and pore structure. The findings in this study also imply that previous work conducted at lower pH and IS conditions may underestimate E. coli O157:H7 travel distance in higher salt and pH groundwater environments.