Recycling and removal of radionuclides in forest soil resulting from nuclear accidents

As a result of the recent nuclear reactor accident at Chernobyl, a re-evaluation of the time required for removal of radionuclides from the upper soil layers has been initiated. Since the end of major atmospheric nuclear weapons testing occurred some 25 years ago, the earth surface has been labeled with the fallout tracers which are readily measurable and can serve as analogues for an actual nuclear accident. The principal objective of this study is to determine the time required for nature to ''clean up'' after a nuclear accident. A series of surface cores has been taken in the acid precipitation region of the forests of Western Pennsylvania and New York both at remote locations and nearby a potential source of radionuclides - the Shippingport and Beaver Valley, PA nuclear power reactors. The measurements for 137Cs show high deposition rates from nuclear weapons fallout at the surface (0-5 cm) section, with factors of 3 to 20 times lower at the deep (10-15 cm) section. The vegetation and topography influence the deposition amounts. Litter deposited from deciduous and coniferous forests, as well as, samples from nearby cleared land, show that the growing forest leaves and needles serve as collection surfaces for atmospheric fallout. This vegetation decomposes into the soil horizons through microbiological process over time. The radionuclide tracers become part of this ecological cycle. Such factors as the soil ion exchange capacity and the degree of complexation with humic substances determine the mobility of the radionuclides. The transport and removal of the radionuclides occurs through radioactive decay, surface runoff, infiltration, evapotranspiration and uptake in vegetation. Each of these factors is discussed with regard to movement of the man-produced radionuclides 137Cs, 90Sr, 239Pu, 240Pu, as well as, the natural radionuclides 210Pb, 238U, 232Th, 226Ra