The mobilization/immobilization of colloidal-sized particles which have high surface areas per unit mass is an important process occurring in groundwater flow systems. Association of contaminants with mobile colloidal particles may enhance the transport of adsorbed pollutants, or deposition of colloidal particles in porous media may decrease permeability and reduce contaminant transport. The general objective of this work was to elucidate physical and chemical factors affecting colloidal particle (Brownian and non-Brownian) transport in porous media under typical groundwater flow velocities. The most critical chemical factor influencing Brownian particle (0.1 and 1.0 mu m) transport in a packed column was found to be pH. The next most critical factor was electrolyte concentration (calcium ion and sodium ion concentration). Gravitational force was found to be an important factor for non-Brownian particle (10 mu m) transport. The non-Brownian particle transport was observed to be independent of solution chemistry. Increases in superficial velocity (from 0.9 to 2.7 m/day) resulted in different types of behavior for Brownian and non-Brownian particle transport under different conditions. The Brownian particle throughputs at a superficial velocity of 0.9 m/day were mainly controlled by the surface interaction forces, that is, hydrodynamic action was not important. The difference in Brownian diffusivity between 0.1 and 1.0 mu m particles caused opposite results in particle throughputs in all experimental columns regardless of solution chemistries. Particles of 0.1 mu m produced the maximum transport in the column billed with the smallest glass beads, while 1.0 mu m particles produced the maximum transport in the column packed with the largest glass beads.