Colloid Mobilization and Transport Through the Vadose Zone

NON-TECHNICAL SUMMARY: Soil is vital for supporting civilization. Soil carries and stores water, acts as filter for contaminants, and is used as a storage location for waste. This transmission, storage, and filter function of the soil, however, is not perfect. For instance, it has been found that the soil filter can be leaky. A leaky filter can be dangerous if contaminants that leak through the filter are highly toxic, such as pesticides, pathogenic microorganisms, or radionuclides. In such cases groundwater resources can be negatively affected. Two mechanisms, in addition to simple solute transport, that can cause a leaky filter in soils are preferential flow and colloid-facilitated transport. There is indeed strong evidence that contaminants can be transported in preferential flow channels and via colloids. Most experimental evidence for colloid-facilitated transport of contaminants has been obtained from saturated porous media, either saturated laboratory column studies or groundwater studies. Important aspects of colloid-facilitated contaminant transport are colloid mobilization and colloid transport. Moving air-water interfaces likely play a dominant role in controlling these processes. In this project, we will focus on these topics, and systematically investigate water movement, colloid mobilization, and colloid transport in variably-saturated soil and sediment systems. It is anticipated that the results of this project will lead to a better fundamental understanding of these important aspects of subsurface processes.

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APPROACH: The objectives will be achieved through a series of column experiments using packed and undisturbed soil and sediment materials. Experiments will be conducted with vertically oriented columns and inflow applied using a sprinkler head and a peristaltic pump. Column outflow will be collected with a fraction collector. We will measure eluent pH, electric conductivity, and concentrations and electrophoretic mobilities of colloids released from the columns. Colloidal particles in the outflow will be characterized by a variety of means as follows: x-ray diffraction will be used to determine mineralogical composition; scanning and transmission electron microscopy will be used to determine size and morphology; contact angle measurements will be used to obtain surface thermodynamic properties; and light scattering will be used to determine average particle size and electrophoretic mobility. We will use a Geocentrifuge (at the DOE Idaho National Laboratory's Subsurface Science Laboratory) to elucidate the effects of water flow rate and water content on colloid mobilization. We have developed a theory that allows us to predict the critical centrifugal acceleration at which colloid filtration under saturated flow is altered relative to that under the acceleration of gravity. Different concepts have to be applied for colloid attachment under favorable and unfavorable conditions. Our previous experiments have indicated that colloid transport at Hanford occurs under unfavorable conditions. To assess the effect of acceleration on colloid transport under unfavorable conditions for attachment, our theory considers the relative importance of sedimentation and diffusion. The experimental data will allow us to relate in situ colloid release patterns and release quantities to the flow rate and water content inside the columns. It is expected that the colloid release quantities will be inversely related with flow rates and water contents, but not necessarily with a linear relationship. We will use the experimental data to derive empirical relationships between colloid release patterns/quantities and flow rates/water contents and develop a mechanistic understanding of water flow, colloid mobilization and transport in unsaturated soils and sediments. To mechanistically interpret the colloid release experiments described above, we consider interactions that act on colloidal particles, including electrostatic forces, Lifshitz-van der Waals interactions, shear forces, and capillary forces. Electrostatic, Lifshitz-van der Waals, and capillary interactions act perpendicular to the sediment plane, whereas shear forces act parallel to the sediment plane. We will measure the forces between the particles and the air-water interface by using a tensiometer. We will use both model particles (with well-defined geometries) as well as natural colloidal particles for the experiments.