An engineering assessment was performed of the problems resulting from the existence of radioactive uranium mill tailings at the Riverton, Wyoming site. The soil, water and other sample analyses; radiometric measurements to determine areas with radium-contaminated materials; the evaluation of resulting radiation exposures of individuals and nearby populations; the investigation of site geology, hydrology, and meteorology; and the evaluation and costing of alternative corrective actions. Radon gas release from the 900,000 tons of tailings at the Riverton site constitutes the main environmental impact. The two alternative actions presented are fencing and maintenance of the site and off-site remedial action, and decontamination of the millsite and ore storage areas and additional stabilization cover to a minimum of 2 ft. The cost estimates for the options are $460,000 and $1,140,000, respectively. Estimated costs for moving the tailings and all contaminated materials to unspecified sites 5 and 10 mi from the present location are $6,000,000 and $6,400,000, respectively.