REMOBILIZATION OF CESIUM FROM FRESH-WATER SEDIMENTS

A series of incubation experiments was performed to investigate the re lease of caesium from either undisturbed or resuspended freshwater sed iment which had not been previously dried. Ammonium and potassium ions readily released caesium from resuspended material, in keeping with t he results of other workers, but treatment of the overlying water with hydrochloric acid (pH 4) or nitrilo triacetic acid did not enhance re lease of caesium from undisturbed sediment. Caesium was released from undisturbed sediment when it was treated with 0.5 mol litre-1 NaCl, bu t the release of 0.5% (as dry weight of sediment) was low compared wit h the release which occurs from resuspended sediment treated with lowe r concentrations of sodium chloride. The smaller release observed in t he experiments with undisturbed rather than resuspended sediments was attributed to the interstitial waters being buffered by redox and soli d phase reactions. There was minimal release (0.05%) from undisturbed sediment after 40 days of either oxic or anoxic incubation, but the ma gnitude of the release was consistent with assumed ammonia concentrati ons in the pore-waters of the surface sediments and experiments which investigated the ammonia-induced release from resuspended wet sediment . It was suggested that the observed elevated contributions in the bot tom waters of lakes could be largely due to remobilization from sinkin g particles rather than release from the sediment. The results indicat e that the caesium present in lacustrine sediments is unlikely to be a ppreciably remobilized in response to acidification, eutrophication or occasional industrial accidents. However, inundation of coastal fresh water basins by seawater could reintroduce into the aqueous environmen t considerable burdens of formerly safely bound caesium.