Removal of radioisotopes from the Columbia River by natural processes

Some radioisotopes are introduced into the Columbia River by return of the water used to cool the Hanford reactors. Trace amounts of both fission product and activation product radioisotopes are present and measurable by sensitive radiochemical methods to allow determination of the self-purification processes occurring as the water moves downstream. By comparing the radioisotope input at given times with the amounts present in the river after traveling downstream, a measure of the self-purification rate was obtained. Removal in the first 40 miles downstream from the reactors was about 35% for 15 of the most prominent radioisotopes. The removal rate seemed to decrease with subsequent distance traveled downstream for three radioisotopes for which measurements were made. Radiochemical analyses of water, filterable solids, and sediment samples taken at downstream locations indicate that incorporation in sediments accounts for most of the removal. Because of the intense scouring action of the Columbia River, sediments are deposited in significant amounts only in slack water sections. The first extensive slack water section downstream from Hanford is the McNary Dam reservoir, Lake Wallula. Radiochemical analyses of some of the long-lived radioisotopes, including Mn/sup 54/, Co/sup 60/, Zn/sup 65/, and Cs/sup 147/ on sectioned sediment core samples indicate a widely varying rate of deposition of radioisotopes. Dating of the various sections of the cores was accomplished using isotope ratio measurements