Accumulation of fallout cesium 137 in soils and sediments in selected watersheds

The fallout of certain radioisotopes produced in nuclear explosions and their subsequent dispersion over the earth provide tracers for following the movement of soil particles in the sedimentation cycle. Cesium 137, which is strongly adsorbed by most soil particles, was identified in sampled soils and sediments by gamma differential spectrometric analysis using a 1024‐channel pulse height analyzer ancl a 10‐ × 12.5‐cm NaI (thallium) scintillation crystal detector. The concentration of 137Cs measured ranged from 0.01 to 3.28 pci/g in soils and from 0.01 to 10.05 pci/g in sediments. Uranium, thorium, and potassium were determined simultaneously with 137Cs. Total potassium ranged from a trace to 5.4%, uranium from 0.5 to 9.4 ppm, and thorium from 1.9 to 42.0 ppm. The concentration of 137Cs in reservoir sediments varied with depth. In some reservoirs the years of greatest 137Cs fallout were correlated with layers containing peak concentrations of 137Cs in the sediment profile. The occurrence of 137Cs in sediments was significantly correlated with the amount of clay and of organic matter present. The amount of 137Cs was greater per unit area in sampled sediments than in the contributing watersheds, and the relative concentration factor was as large as 24. This accumulation of 137Cs in reservoir sediments is indicative of the pollution possible in reservoirs as a result of fallout within the watershed but not necessarily within the reservoir.