The purpose of the Saturated Zone Colloid-Facilitated Transport Analysis and Modeling Report (AMR), as outlined in its Work Direction and Planning Document (CRWMS M & O 1999a), is to provide retardation factors for colloids with irreversibly-attached radionuclides, such as plutonium, in the saturated zone (SZ) between their point of entrance from the unsaturated zone (UZ) and downgradient compliance points. Although it is not exclusive to any particular radionuclide release scenario, this AMR especially addresses those scenarios pertaining to evidence from waste degradation experiments, which indicate that plutonium and perhaps other radionuclides may be irreversibly attached to colloids. This report establishes the requirements and elements of the design of a methodology for calculating colloid transport in the saturated zone at Yucca Mountain. In previous Total Systems Performance Assessment (TSPA) analyses, radionuclide-bearing colloids were assumed to be unretarded in their migration. Field experiments in fractured tuff at Yucca Mountain and in porous media at other sites indicate that colloids may, in fact, experience retardation relative to the mean pore-water velocity, suggesting that contaminants associated with colloids should also experience some retardation. Therefore, this analysis incorporates field data where available and a theoretical framework when site-specific data are not available for estimating plausible ranges of retardation factors in both saturated fractured tuff and saturated alluvium. The distribution of retardation factors for tuff and alluvium are developed in a form consistent with the Performance Assessment (PA) analysis framework for simulating radionuclide transport in the saturated zone. To improve on the work performed so far for the saturated-zone flow and transport modeling, concerted effort has been made in quantifying colloid retardation factors in both fractured tuff and alluvium. The fractured tuff analysis used recent data and interpretation from the C-wells reactive tracer testing complex in the saturated zone of Yucca Mountain. As no data regarding colloid transport have been developed by the Yucca Mountain Site Characterization Project (YMP) for the alluvial system, a theoretical analysis based on studies performed in other alluvial systems is developed. The parameters derived in this AMR are developed in a manner consistent with the PA methodology and can be readily integrated into that analysis.

The main purpose of this project was to advance the basic scientific understanding of colloid and colloid-facilitated Cs transport of radionuclides in the vadose zone. We focused our research on the hydrological and geochemical conditions beneath the leaking waste tanks at the USDOE Hanford reservation. Specific objectives were (1) to determine the lability and thermodynamic stability of colloidal materials, which form after reacting Hanford sediments with simulated Hanford Tank Waste, (2) to characterize the interactions between colloidal particles and contaminants, i.e., Cs and Eu, (3) to determine the potential of Hanford sediments for \textit{in situ} mobilization of colloids, (4) to evaluate colloid-facilitated radionuclide transport through sediments under unsaturated flow, (5) to implement colloid-facilitated contaminant transport mechanisms into a transport model, and (6) to improve conceptual characterization of colloid-contaminant-soil interactions and colloid-facili\-tated transport for clean-up procedures and long-term risk assessment. We have previously shown that upon contact with simulated waste tank solutions, Hanford sediments change their mineralogical composition. Certain minerals, i.e., quartz, smectite, and kaolinite, are partially dissolved, and new mineral phases, i.e., the feldspathoids cancrinite and sodalite, are formed. We have characterized these mineral transformations and clarified the mineral transformation pathways. The new minerals were mainly in the colloidal size fraction (diameter less than 2 mum), had a negative surface charge, and were microporous, meaning they contained small pores. When Cs was present during the formation of the minerals, contaminants, like Cs, could be trapped inside the mineral structure. Transport experiments under water saturated and unsaturated conditions showed that the colloids were mobile in Hanford sediments. As the water saturation of the sediments decreased, the amount of colloids transported also decreased. The colloids had the ability to enhance the migration of the radionuclide Cs; however, Cs initially sorbed to colloids was desorbed during transport through uncontaminated Hanford sediments. The finding that Cs was stripped off the colloids during the transport through uncontaminated sediments implies that colloids will likely not be an effective carrier for Cs, unless the Cs is incorporated into the mineral structure of the colloids such that the radionuclide cannot desorb from the colloids. Nevertheless, it appears that the amount of Cs that can be transported by mobile colloids beneath Hanford waste tanks is limited. Colloids will not be able to move the bulk mass of Cs through the vadose zone at Hanford. Colloid stability studies indicate that Hanford sediment form stable colloidal suspensions when suspended in Hanford sediment pore waters. Colloid stability was assessed by determination of the critical coagulation concentration, i.e., the chemical electrolyte concentration at which colloidal suspensions flocculate and settle out (become unstable). Although in the stable mode, Hanford colloids will settle out of solution after extended periods of time (months to years). Given the low recharge rates at Hanford range, which from near 0 to more than 100 mm/year, and the long travel times for rainwater to reach the groundwater of more than 40 years, it appears that colloidal transport is unlikely to occur if colloids are initially to be suspended close to the soil surface by infiltrating rainwater. However, if preferential flow or transient flow occurs, then colloidal transport may become more important. The results of this project have also led to improvements of our fundamental understanding of colloid transport and mobilization under unsaturated flow conditions in porous media. We have found that colloid attachment to the liquid-gas interface is not that relevant and that colloids rather attached near the triple phase interface where air, water, and solid phases meet. We have also found that capillary forces are the most dominant forces governing colloid release in unsaturated porous media. These results help to advance our understanding of colloid fate and transport in unsaturated porous media.

Although a wide variety of studies have been conducted to understand the numerous processes responsible for the transport of solutes and particulates through soils in order to prevent groundwater contamination, many gaps remain. This thesis presents the findings of two mechanisms (colloid facilitated transport and development of preferential flow infiltration) by which contaminants are able to expedite their transport through unsaturated soils (i.e., the vadose zone), easily reach deeper groundwater, and lower the filtering capacity of soils. The first study of this thesis bridges the gap between changes in polymeric characteristics of dissolved organic matter-colloid complexes induced by solution composition, and the effect these have on colloid transport through unsaturated soils. The second study presents a semi-empirical approach to improve existing models that predict attachment efficiency ([alpha]) of electrosterically stabilized suspensions moving through a porous medium using direct measurements of polymeric characteristics. The fourth study elucidates the capillary forces responsible for the transition between pinning or allowing particles to slip when they approach an air-water-solid interface. Lastly, bioclogging and soil-water repellency from dehydrated microbial exudates are studied in terms of biotic changes in structure and surface properties that generate points of wetting instability that can result in the formation of persistent preferential flow paths.

The overarching goal of this study was to improve understanding of colloid-facilitated transport of radioactive cations through unsaturated soils and sediments. We conducted a suite of laboratory experiments and field experiments on the vadose-zone transport of colloids, organic matter, and associated contaminants of interest to the U.S. Department of Energy (DOE). The laboratory and field experiments, together with transport modeling, were designed to accomplish the following detailed objectives: 1. Evaluation of the relative importance of inorganic colloids and organic matter to the facilitation of radioactive cation transport in the vadose zone; 2. Assessment of the role of adsorption and desorption kinetics in the facilitated transport of radioactive cations in the vadose zone; 3. Examination of the effects of rainfall and infiltration dynamics and in the facilitated transport of radioactive cations through the vadose zone; 4. Exploration of the role of soil heterogeneity and preferential flow paths (e.g., macropores) on the facilitated transport of radioactive cations in the vadose zone; 5. Development of a mathematical model of facilitated transport of contaminants in the vadose zone that accurately incorporates pore-scale and column-scale processes with the practicality of predicting transport with readily available parameters.

This project seeks to improve the basic understanding of the role of colloids in facilitating the transport of contaminants in the vadose zone. We focus on three major thrusts: (1) thermodynamic stability and mobility of colloids formed by reactions of sediments with highly alkaline tank waste solutions, (2) colloid-contaminant interactions, and (3) in situ colloid mobilization and colloid-facilitated contaminant transport occurring in both contaminated and uncontaminated Hanford sediments. The specific objectives that will be addressed are: (1) Determine the liability and thermodynamic stability of colloidal materials, which form after reacting Hanford sediments with simulated Hanford Tank Waste. (2) Determine the potential of Hanford sediments for in situ mobilization of colloids for different types of sediments and different leaching scenarios. (3) Characterize the interactions between initially-formed colloids, their dissolution/alteration products, and native colloidal particles with contaminants in batch experiments under various ionic strength and pH conditions. (4) Evaluate colloid-facilitated radionuclide transport through sediments under different degrees of water saturation in packed and undisturbed sediment columns. (5) Implement colloid-facilitated contaminant transport mechanisms and thermodynamic stability constants into a reactive chemical transport model, and verify model simulations with experimental transport data. Results of this project will help to understand the fundamental mechanisms of Cs transport under the leaking Hanford tanks, and thus contribute to the long-term clean-up strategies at the Hanford site.