Abstract:
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.
