Laboratory studies of the diagenesis and mobility of sup(239,240)Pu and 137Cs in nearshore sediments

Controlled laboratory experiments have been used to study the diagenetic chemistry of sup(239,240)Pu, 137Cs and 55Fe. Experiments using Buzzards Bay sediments in small tanks show that sulfate reduction is accompanied by the production of large pore water concentration gradients of alkalinity, phosphate, ammonia and dissolved organic carbon and the formation of subsurface maxima in Fe and Mn. These pore water profiles demonstrate that bacterially-mediated processes of organic matter degradation and redox reactions can be simulated in the laboratory. A vertical profile of 55Fe in pore waters is reported for the first time: it follows the profile of stable Fe and as such has a large subsurface maximum between 2 to 4 cm depth. Comparison of 55Fe/Fe ratios in sediments and pore waters shows that there is preferential solubilization of 55Fe over stable Fe. The pore water activities of sup(239,240)Pu show no gradients within the large uncertainties of the counting statistics, but are two to four times higher than Buzzards Bay seawater. The activity of 137Cs in the pore water profile is constant within the large counting uncertainties and is twice that of Buzzards Bay seawater. Cs-137 does not appear to be involved in diagenetic chemistry but may increase in pore waters as a result of ion exchange reactions. (author)