Dept. of Environmental Science
Oregon Graduate Center
Solute transport in a fractured porous clay aquifer has been investigated in a study which included field, laboratory, and mathematical modeling components. The presence of a fracture network in 'low' permeability materials is important because it results in a reduction in their ability to retard groundwater flow and contaminant migration. Central to the investigation was the conduction of a large-scale natural-gradient tracer test. The test site was the Alkali Lake chemical disposal site located in southeastern Oregon on a 5 km diameter playa in the Alkali Lake basin. A 200 ppm fluorescein dye solution with an initial injection volume of 60 liters was used as the tracer. Injection took place between 1 and 2 meters below the water table at a location approximately 300 meters downgradient of the disposal site. Monitoring of the advancing tracer was accomplished through a network of multi-level piezometers which was installed over a 60 by 35 meter field and was sampled periodically for over one year. An equivalent porous media model has been found to be effective for simulating the tracer plume as it migrated through the fractured porous aquifer. The high density of fractures at this site is responsible for the validity of the equivalent porous media approach. The results of the model simulations indicate the principal factors controlling tracer migration are matrix diffusion and the high variability in hydraulic conductivity. The strong influence of K [subscript h] variability and matrix diffusion on the tracer migration demonstrates that understanding transport through a fractured porous media requires characterization of the fracture specific and porous media properties of the system. The results of the investigation will be useful for understanding solute migration in other fractured systems.
DeCesar, Richard T., "Natural gradient tracer tests in a highly fractured soil" (1987). Scholar Archive. 240.