Technical Abstract: Experimental and theoretical studies were undertaken to explore the coupling effects of chemical conditions and pore space geometry on bacteria transport in porous media. The retention of Escherichia coli D21g was investigated in a series of batch and column experiments with solutions of different ionic strength (IS) and ultra pure quartz sand. DLVO calculations and results from batch experiments suggested that bacteria attachment to the sand surface was negligible when the IS was less than or equal to 50 mM. Breakthrough data from column experiments showed significant cell retention was strongly depending on the IS. This finding indicates that cell retention was dependent on the depth of the secondary energy minimum which increased with IS. When the IS of the influent solution was decreased to 1mM only a small fraction of the retained bacteria were released from the column. The remaining retained bacteria, however, were recovered from the sand that was excavated from the column and then placed in excess amounts of solution having the original IS. These observations suggest that the solution chemistry is not the only mechanism controlling bacteria retention in the porous media. Computational simulations of flow around several collector grains revealed another retention mechanism, which is dependent on both the solution chemistry and the pore space geometry. Simulations demonstrate that the pore space geometry created low velocity regions. The number of bacterial cells that may be transported to these relatively ¿immobile¿ regions will theoretically be dependent on the depth of the secondary energy minimum (the IS). Once the bacteria are trapped in these immobile regions, reduction of the secondary energy minimum does not necessarily release them due to hydrodynamic constraints.