Effect of salinity on particle release and hydraulic conductivity in sediments Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/pv63g232j

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  • Fine particles and colloids, attached to grain surfaces, are abundant in the earth's subsurface. Under certain conditions these particles can be released from the matrix and transported with the mobile phase. One of the mechanisms for sudden particle release is a decrease in groundwater salt concentration below the critical salt concentration (CSC), where repulsion forces between fine particles and matrix surfaces exceed binding forces. Typically, CSCs are determined with column experiments, where salt solutions with specific concentrations are applied to the matrix of interest. In this study it was attempted to determine the CSC with batch experiments as well as columns. Two types of sediment were tested: (a) pure, mineralogically homogeneous silica sand; and (b) mineralogically heterogeneous sandy sediment, taken from the Hanford formation in southeast Washington. Stepwise decreasing concentrations of salt solution (NaNO₃) were applied until fine particles were released from the sediments and the CSC was determined. CSCs from batch experiments were compared to those obtained from column experiments, showing that CSCs were determined successfully with this method. It was also found that the amount of particle release, and also the CSC, of the mineralogically heterogeneous Hanford Sediment was generally an order of magnitude higher than for the Silica Sand. The CSC for the Hanford Sediment was found to be 0.1 mol/l NaNO₃, which was higher than expected. Particle release can cause a change in hydraulic conductivity of the matrix, either by washing out the fines and thus increasing the pore sizes, or by plugging of pore constrictions. The phenomenon of permeability changes as a result of particle detachment was investigated in a series of column experiments using coarse and fine sediments from the Hanford Formation in southeast Washington. Columns were subject to a pulse of highly saline solution (NaNO₃) followed by a freshwater shock causing particle release. No permeability decrease occurred within the coarse matrix alone. However, when a thin layer of fine sediment was imbedded within the coarse material (mimicking field conditions at the Hanford Site), permeability decreased significantly during the freshwater shock down to 10 percent of the initial value. The reduction in permeability was shown to be due to occlusion of the fine layer.
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