We report the effects of salt type and concentration on the change in attachment kinetics when bacteria are pumped through a column of water-saturated clean sand over relatively long periods of time (up to 35 pore volumes). The species Pseudomonas fluorescens UPER-1 was found to exhibit three different kinds of attachment kinetics: first order, second order, and an intermediate order. The attachment kinetics of bacteria was modeled by using the advection-dispersion equation coupled with a set of equations for each kind of attachment kinetics while using colloid filtration theory to predict collector efficiencies. At low or zero salt concentrations (≤10-4M) a second-order kinetics model (blocking), a first-order kinetics model, and an intermediate-order kinetics model (ripening), were all found to fit the data equally well. At intermediate and high salt concentrations (≥10-3M) the ripening model was found to fit the data best. We report values for collision efficiencies of bacteria in the range 0.01--0.2, depending upon the salt type and concentration. This study points out the importance of long-term experiments to study the effect of ionic strength on bacteria attachment kinetics in saturated porous media and the phenomenon of cell-to-cell attachment at high ionic strength. This study further points out the range of kinetics to expect when bacteria attach to natural porous media and suggests a modeling framework. ¿ 1999 American Geophysical Union