The Role of Partially-Saturated Conditions, Physical Heterogeneity, and Desorption Kinetics on Colloid-Facilitated Transport of Cesium and Strontium by Illite in Quartz Sand

In many soil and groundwater systems, colloids are present, mobile, and capable of enhancing the transport of some contaminants if the rate of contaminant desorption from the contaminants is slow relative to the rate of colloid and contaminant transport. The importance of contaminant desorption kinetics has been well illustrated in saturated porous media systems, but not in partially-saturated systems. The goal of this research project was to examine and quantify the effects of contaminant desorption kinetics on colloid-facilitated transport under partially-saturated conditions. Laboratory breakthrough experiments were conducted using a column (12.7 cm diameter and 33.5 cm length) packed with (1) quartz sand of uniform size and (2) quartz sand of two different sizes packed as a central tube simulating a macropore (coarse sand) and surrounding matrix (fine sand). Illite was used as the colloids. Cesium and strontium were chosen as model contaminants because their mechanisms of adsorption to illite result in slower desorption of cesium than of strontium and because these cations are found in their radioactive forms at contaminated Department of Energy sites. In the partially saturated columns of uniform sand, the illite colloids facilitated the transport of both cesium and strontium; however, the transport of cesium was facilitated to a greater extent than strontium because cesium binds strongly to the frayed edge sites on the illite colloids. Compared to saturated conditions, partially-saturated conditions increased the transport of cesium and strontium due to the decreased effective surface area of the quartz sand. At moisture contents near saturation, addition of a macropore increased transport of cesium and strontium due to increased porewater movement through the macropore. At lower moisture contents, less porewater was transported through the macropore and cesium and strontium transport was similar to the uniform sand experiments. A model for colloid-facilitated transport in saturated porous media was adapted to partially-saturated conditions. The partially-saturated colloid-facilitated transport model was adapted to account for physical heterogeneity (the central macropore). The experimental breakthroughs were used to validate the models and demonstrate the effects of partial saturation, physical heterogeneity, and desorption kinetics on colloid-facilitated transport of contaminants.