The Occurrence and Retention of Radionuclides in the Sediments of White Oak Lake

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Abstract
As a result of the discharges of large volumes of low-level radioactive liquid wastes to surface streams at the Oak Ridge National Laboratory, large quantities of radionuclides have accumulated in the bottom sediments of White Oak Lake. Ruthenium-106 (1038 c) and cesium-137 (704 c) account for more than 90% of the total activity now present at the site, while Co60 (152 c), the rare earths (17 c, exclusive of Y90), and Sr90 (15 c) make up the remainder. More than half of the activity is associated with the upper 6-in. sediment layer, while progressively smaller quantities of activity are found with depth. The ruthenium,which is restricted to a small area in the now dry upper lake bed, is partially water soluble; however, its rate of movement through the soil is slow enough so that radioactive decay reduces the concentration of that reaching surface streams to insignificant levels. Most of the Cs137 occupies highly selective exchange sites on the illitic fraction of the clay in the sediment and can be desorbed only by disruption of the lattice structure. Only a small fraction of the CO60 in the soil was found to be exchangeable. It is, therefore, unlikely that any large fraction of the Cs137 or Co60 would move from the area except through erosion of the sediment. About one-half of the Sr90and the rare earths in the sediment appears to be exchangeable, while the other half is in the form of slightly soluble salts. Through leaching by ground water, a slow depletion of strontium from the dry part of the lake bed occurs. The accumulation of radionuclides in the sediments of White Oak Lake illustrates the effectiveness of relatively quiescent bodies of water in concentrating activity in stream beds and in retarding the downstream movement of these materials.

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