Plutonium solubility in sediment pore waters

Using in situ porous cup samplers, dissolved Pu concentrations have been measured over a year in the pore waters from two contrasting sites in the valley of the River Esk, North West England. In saltmarsh sediments, dissolved Pu represents approximately 1 part in 106 of the total inventory. The Pu concentration in solution is in the range 1.1–3.5 mBq l−1, varying by a factor of 3 in the course of the year. Most of the changes in dissolved Pu coincide with changes in dissolved Fe and Mn concentrations, with Pu being low in the summer months when Fe and Mn are high. Nevertheless, there are a number of factors which make it unclear as to whether these patterns might be related to seasonal redox changes in the saltmarsh. At the highly organic, reducing reedbed site, the proportion of Pu in solution is typically around 1 part in 103, proportionately much higher than in the saltmarsh, giving concentrations ranging between 9.0 and 28.5 mBq l−1, and are apparently maintained by complexation to dissolved organic matter. There is no obvious seasonal pattern at the reedbed site nor is there any relation to any of the dissolved species measured (Fe, Mn, Na, DOC).