Colloidal Fe2O3 transport studies in laboratory model systems using shallow aquifer material

The stability and transport of radiolabeled Fe2O3 colloids were studied using laboratory batch and column techniques. Core material collected from a shallow sand and gravel aquifer near Globe, Arizona was used as the column matrix material. Scintillation counting and laser light scattering with photon correlation spectroscopy (PCS) were used as means of colloid detection. PCS and scanning electron microscopy (SEM) were used to verify the size of the colloids and their stability. Variables in the study included flow rate, pH, ionic strength, electrolyte composition (anion/cation), colloid concentration, and colloid size. Transport was highly dependant upon colloidal stability. Iron oxide colloids in the 100-900 nm particle diameter range were not only mobile to a significant extent, but under some hydrogeochemical conditions were transported faster than a conservative tracer, tritium. The rate of colloid transport was over 21 times that of the dissolved arsenate. Extent of breakthrough was dependant upon a complex variety of parameters, however the highest statistical correlation was observed with particle size and anionic composition of the supporting electrolyte