Leachate migration from an in situ oil-shale retort near Rock Springs, Wyoming

Abstract

Geohydrologic factors influencing leachate movement from an in situ oil shale retort near Rock Springs, Wyoming, were investigated by developing models of groundwater flow and solute transport. Leachate, indicated by the conservative ion thiocyanate, has been observed 1/2 mi downgradient from the retort. The contaminated aquifer is part of the Green River Formation and consists of thin, permeable layers of tuff and sandstone interbedded with oil shale. Most solute migration has occurred in an 8-ft sandstone at the top of the aquifer. Groundwater flow in the study area is complexly 3-D and is characterized by large vertical variations in hydraulic head. The solute transport model was used to predict the concentration of thiocyanate at a point where groundwater discharges to the land surface. Leachates with peak concentrations of thiocyanate--45 mg/L or approximately one-half the initial concentration of retort water--were estimated to reach the discharge area during January 1985. Advantages as well as the problems of site specific studies are described. Data such as the distribution of thin permeable beds or fractures may introduce an unmanageable degree of complexity to basin-wide studies but can be incorporated readily in site specific models. Solute migration in the study area primarily occurs in thin permeable beds rather than in oil shale strata. Because of this behavior, leachate traveled far greater distances than might otherwise have been expected. The detail possible in site specific models permits more accurate prediction of solute transport than is possible with basin-wide models. A major problem in site specific studies is identifying model boundaries that permit the accurate estimation of aquifer properties. If the quantity of water flowing through a study area cannot be determined prior to modeling, the hydraulic conductivity and groundwater velocity will be estimated poorly. (Author's abstract)