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WWRC 85-17(u)
An Example of Solute Transport in Low Permeability Aquifers - Abandoned Oil-Shale Retort Near Rock Springs, Wyoming


Many areas of possible ground-water contamination in Wyoming are associated with low permeability aquifers. Sites for hazardous-waste disposal or sanitary landfill commonly are located within or overlying low permeability aquifers in attempts to minimize migration of contaminants. An in-situ retorting experiment within oil shale near Rock Springs, Wyoming, was conducted by the Laramie Energy Technology Center in 1976. Subsequent solute migration has provided the U.S. Geological Survey with an opportunity to study solute transport in low permeability rocks. Large-scale oil-shale development is unlikely to occur in Wyoming, but many of the techniques used in studying solute transport in oil shale can be applied to other ground-water contamination problems.

Study of the retort site shewed the need for careful attention to small changes in lithology when investigating ground-water flow and solute transport in low permeability aquifers. Accurate description of information, such as potentiometric surfaces and the distribution of solute, was only possible after mapping thin tuff and sandstone layers with relatively large values of hydraulic conductivity compared to the adjacent rocks. These thin, permeable strata constitute less than 10 percent of the total formation thickness, yet are the principal paths for water movement.

The pattern of solute transport from the retort snows the importance of these thin permeable strata. Movement of solute fran the retort site was expected to be slow because of the small, average values of hydraulic conductivity of the oil-shale formation. However, solute has migrated primarily within a thin sandstone layer overlying the oil shale. The greater permeability of the sandstone has resulted in rates of solute movement that greatly exceeded initial expectations.

Ground-water flow and solute-transport modeling of the retort site showed that accurate simulation required complex three-dimensional models. Three-dimensional models were needed because the hydraulic-head distribution within the system showed large vertical gradients. The hydraulic-conductivity distribution was estimated only after detailed measurements of ground-water discharge were obtained and flow-model calibration was completed. A quasilinear-regression technique was used to calibrate the flow model. The simulated distribution of solute was dependent on estimates of ground-water velocities. As a result, a flow model using regression techniques that quantify the uncertainty in estimates of hydraulic conductivity had an advantage over more simple models. The use of regression techniques at the retort site greatly simplified subsequent development of a solute-transport model. As a result, it was concluded that tha rate of sclute transport at the retort was well understood.

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