Abstract <p>The extent of groundwater pollution due to colloids can be assessed using formulas that can estimate colloid deposition parameters in terms of various physicochemical conditions. Ionic strength and pore-water velocity are known to affect colloid transport and retention significantly. This study deals with understanding the synergetic effects of ionic strength and pore-water velocity on particle deposition for a wide range of ionic strengths (0–100 mM) and pore-water velocities (0.55–11 cm/min) through column experiments and mathematical modelling. Colloid retention increased with decreasing pore-water velocity and increasing ionic strength. However, the effect of increasing pore-water velocity on reducing colloid retention increased with increasing ionic strength. In contrast, the effect of increasing ionic strength on enhancing colloid retention was found to decrease with increasing pore-water velocity. Breakthrough curves exhibited significant tailing at pore-water velocities of 0.55 and 2.75 cm/min, representing slow detachment. Colloid deposition was found to be irreversible, and colloids were observed to be retained in the concave regions on grain surface at pore-water velocities of 5.5 and 11 cm/min. A mathematical model consisting of two sites, one characterized by equilibrium and the other being a kinetic one, was found to simulate the experimental data reasonably well. Further, formulas were developed for the attachment and detachment rate coefficients of colloids in terms of ionic strength and pore-water velocity.</p> Research Highlights <p><UnorderedList Mark="Bullet"> <ItemContent> <p>Coupled effects of ionic strength and pore-water velocity on colloid transport were studied under a broad range of ionic strengths and pore-water velocities.</p> </ItemContent> <ItemContent> <p>Two-site sorption model, with site-1 being an equilibrium site and site-2 being a kinetic site, successfully fitted the experimental results.</p> </ItemContent> <ItemContent> <p>Ionic strength played a dominant role in colloid retention at low velocities and pore-water velocity significantly influenced colloid transport at high ionic strengths.</p> </ItemContent> <ItemContent> <p>Formulas were developed for the attachment and detachment rate coefficients of colloids vis-à-vis ionic strength and pore-water velocity.</p> </ItemContent> </UnorderedList></p>