MOBILIZATION OF CESIUM IN ORGANIC-RICH SOILS - CORRELATION WITH PRODUCTION OF DISSOLVED ORGANIC-CARBON

A study of the downward movement of Cs-137 in an undisturbed forest so il is presented. Seasonal variations and depth profiles of Cs-137 acti vities were measured in seepage water, which is the transport medium f or the downward movement of anthropogenic substances in soils. Further more the correlation of Cs-137 mobilization and production of dissolve d organic carbon (DOC) was investigated. Seasonal variations of both C s-137 and DOC fluxes in the seepage water in a depth of 5 cm depth wer e observed, where the maximum fluxes in the summer months were about o ne order of magnitude higher than the minimum fluxes in the winter mon ths. Cs-137 fluxes are found to be correlated with DOC fluxes with a c orrelation coefficient of r = 0.63, and both are highly correlated wit h soil temperature. This indicates that cesium is bound to soil organi c material. The production of DOC is controlled by microbial decomposi tion of soil organic matter and we assume that this holds true for the Cs-137 release as well. The actual transport velocity (0.2 +/- 0.14 m m/a) of Cs-137 (calculated by the weighed mean of Cs-137 concentration in the seepage water and the total Cs-137 content of the soil) is abo ut one order of magnitude less than the mean transport velocity (1.2 /- 0.3 mm/a) over the past 25 years (calculated from the Cs-137 depth profile). It is possible that the transport velocity of Cs-137 in undi sturbed soils decreases with time as it binds to aged organic material which is less easily decomposable than fresh organic material.