One of the major issues that must be addressed during the evaluation of the efficiency of tailings embankment designs for compliance with groundwater standards is the estimation of source concentrations and the change in these concentrations with time. Because any effort to predict concentrations of contaminants in the uppermost aquifer requires a source concentration, data from these analyses are essential. Thetechnical approach of this study was twofold. The first approach was to investigate the rates of natural flushing of contaminants. Two sets of tailings samples were collected at two sites on the Old Rifle tailings pile at the Rifle UMTRA Project site in Colorado. One set of samples was collected at a site where the lower portion of the profile is continuously inundated with water and the other set was collected in anarea that only receives water from precipitation. The tailings samples were refluxed in strong acid (nitric acid) and the leachate was analyzed for hazardous constituents. The results of this investigation indicate that many of the hazardous constituents have been leached from the tailings at the wet site and that there has been little redistribution of elemental hazardous constituents at the dry site. The second approach involved a laboratory investigation of contaminant removal from tailings by doubly distilled water and two lixiviants. Tailings samples from the Gunnison, Colorado, UMTRA Project site were subjected to leaching by doubly distilled water, and by the lixiviants sodium bicarbonate and disodium ethylenediaminetetraacetic acid. The resulting data were tabulated and plotted with concentration as a function of pore volume. Evaluation of the data indicates that pore fluids should show a decrease in concentration after very few pore volumes of liquid have eluted through the tailings. It is also demonstrated that lixiviants significantly increase the solubility and rate of elution of all of the hazardous constituents.