Subsurface colloids: Stability, sampling, and transport under gravitational and centrifugal accelerations

As a legacy of 40-year of Pu production, millions of liters of hazardous waste have been leaking to the vadose zone for the past four decades at the US DOE's Hanford reservation in south-central Washington State. Radionuclides, previously believed relatively immobile, are found deep in the subsurface. Submicron size underground particles, so-called colloids may be responsible for the enhanced mobility of the contaminants. The main objectives of this dissertation were: (1) To assess the stability of reference clay minerals and colloids extracted from Hanford sediments and to compare three experimental methods to determine critical coagulation concentration (CCC). (2) To determine the suitability of fiberglass wicks for in situ colloid sampling, and evaluate the effect of colloid type, flow rate, and pH on colloid recovery. (3) To examine the potential for geocentrifuges to study colloid transport.; Critical coagulation concentrations were sensitive to settling time, electrolyte type, and initial colloid concentration. Colloids at low ionic strength (7 mM) can be stable for the time frame of weeks. The lower the initial colloid concentration and shorter the settling times were, the larger the CCC was. Our stability results suggest, that colloids can form a stable suspension in the Hanford pore water for several weeks. However their potential for facilitating the transport of colloids is limited as travel time for the recharged water in the vadose zone is 40 to 100 years. Critical coagulation concentration values obtained by dynamic light scattering were usually higher than those obtained by batch turbidity experiments.; Fiberglass wicks are suitable for colloid sampling under certain conditions. We observed decreasing colloid recovery with decreasing flow rate. Lower recovery rate was observed at pH 7 than at pH 10, as the average surface charge shifted to less negative values at the lower pH. Native colloids at both pH values and ferrihydrite at pH 10 showed almost complete breakthrough. In two cases (kaolinite and ferrihydrite at pH 7) no colloids moved through the wicks.; For natural subsurface colloids, filtration will be different in centrifuge experiments as compared to normal gravity conditions. At typical subsurface flow rates, accelerations as low as 4 g can change the filtration behavior.