Mobility of trace elements in deep-sea sediments: the use of in-situ Kd values

In modelling the transport of radionuclides through aquatic sediments one needs to know the distribution of the element between the sediment and the dissolved phase. Distribution coefficients are often used for this purpose, but they have no general validity and should preferably be determined in-situ or under in-situ conditions. Results of such measurements in two deep-sea sediments from the dumpsite for radioactive waste in the NE Atlantic demonstrate the large influence of dissolved oxygen on the in-situ Kd of some trace metals: The transition of oxic to suboxic conditions is associated with decreases in the Kd of Mn, Fe, and Co with factors of 8000, 220, and 180, respectively. The decreases for Ni, Cu, and Cd are less pronounced, but still amount to factors of 8, 6, and 3, respectively. Solubility and adsorption equilibria are compared in their influence on dissolved concentrations, and the suitability of in-situ Kd values for the modelling of transport behaviour is discussed. (author)