Factors controlling the interactions of 137Cs with suspended and deposited sediments in estuarine and coastal environments

Field studies and laboratory experiments have been carried out to evaluate the role of pertinent environmental factors in controlling the adsorption and to some extent desorption of 137Cs by marine sediments. The sedimentary environments used in these studies were the coastal and estuarine regimes of the eastern Irish Sea. Factors which have been considered in the field and laboratory studies include grainsize distribution, time, salinity, temperature, and the chemical activities of the stable cations Na+, K+, Ca2+ and Mg2+. Within realistic environmental ranges (e.g. 1-220C) temperature has no consistent influence on the distribution coefficient (Ksub(d)) of 137Cs between sea or brackish waters and Irish Sea sediments. Equilibrium Ksub(d) values were found to be in the range of approximately 300 to 500. In the estuarine environment, salinity has an important influence on 137Cs interaction with suspended sediments. A five-fold suppression of the equilibrium Ksub(d) was observed over the increasing salinity range from 5 to 30 per mille. Experimental studies using environmentally realistic chemical activities of the major cations in sea water demonstrated an effective order of influence on the suppression of 137Cs adsorption onto sediments. This influence was found to be K+> Na+> Mgsup(2+)> Casup(2+). K+ ions can account for > 90% of the observed influence of salinity on equilibrium Ksub(d) values. Extensive studies on the distribution of 137Cs in surface sediments and sediment cores in the eastern Irish Sea have shown that three factors are dominant in the control of 137Cs distribution. (i) the grainsize characteristics of sediments at different localities; (ii) the temporal fluctuations in the composition of the low level discharges; and (iii) the time elapsed during transport to and after the deposition of the sediments at a particular locality