The stability and transport of radiolabeled Fe2O3 particles were studied using laboratory batch and column techniques. Core material collected from a shallow sand and gravel aquifer was used as the immobile column matrix material. Variables in the study included flow rate, pH, ionic strength, electrolyte composition, particle concentration, and particle size. Transport was highly dependent upon colloidal stability. Iron oxide colloids were not only mobile to a significant extent, but under some hydrogeochemical conditions were transported faster than tritiated water, a conservative tracer. The extent of colloid breakthrough was dependent upon a variety of parameters; however, the highest statistical correlation was observed with particle size and anionic composition of the supporting electrolyte. Arsenate was utilized for assessment of colloid-contaminant and contaminant-aquifer interactions and comparison of dissolved and colloidally associated transport in dynamic model systems. The rate of colloid-associated arsenate transport was over 21 times that of the dissolved arsenate. (Copyright (c) 1992 American Chemical Society.)