Colloidal clay particle transport under saturated conditions is believed to be controlled by its interactions with the surrounding environment. The dominating forces among these interactions are electrostatic forces that are determined by colloidal clay particle and porous medium surface charge density and Lifshitz-van der Waals forces that are determined by colloidal clay particle and porous medium surface thermodynamic properties. Electrostatic forces are greatly affected by solution chemistry in terms of solution ionic strength and pH. In this research, electrostatic and Lifshitz-van der Waals forces of natural colloidal clay particles with a model porous medium of silica sand were quantified at different ionic strength and pH conditions. At the same time, colloidal clay particle transport in the model medium of silica sand was conducted in a laboratory column. The maximum electrostatic forces, FEL (max), which occurred when the separation distance between colloidal clay particles and the porous medium was in the range of the sum of the double layer thicknesses of the colloidal clay particles and the porous medium, was found to be the determinant factor for colloidal clay particle deposition in the porous medium. Colloidal clay particle desorption in the porous media was related to the net effect of attractive Lifshitz-van der Waals forces and repulsive electrostatic forces, evaluated at the equilibrium distance where physical contact between the colloidal clay particle and silica sand actually occurred (i.e., affix force). Higher colloidal clay particle desorption was found to coincide with smaller affix force values. (C) Koninklijke Brill NV, Leiden, 2009