A Multitracer Study of Radionuclides on the Surface Soil Erosion in the Yunnan-Guizhou Plateau, China. I. Distribution and Geochemical Speciation of [sup(7)]Be, [sup(137)]Cs, [sup(226)]Ra, [sup(228)]Ra in the Soils

Soil erosion is one of the major environmental problems in the world. The YunnanGuizhou Plateau is located in southwestern China on the eastern slope of the Himalayan Mountains. There are large areas of carbonate rocks with violent karstifîcation in the Yunnan-Guizhou. Surface soils in this area are seriously eroded with an increasing trend of desertification of landscape because of long­ term geological action and the negative influence of humankind. It is of note that the erosion modulus does not really indicate the degree of soil erosion and the seasonal and partial transport of soil particles owing to the karst landforms and erosion mechanism. In order to control soil erosion, it is necessary to reveal its erosive mechanism and particle transportation. Studies of soil erosion using fallout radionuclide cesium-137 have been made in the last thirty years (Brown et al. 1981; Ritchie and McHenry 1990; Wan and Santschi 1987; Wan et al. 1991; Zhang et al. 1989; Murray et al. 1992, 1993; Wallbrink et al. 1993). 137Cs (half-life = 30.3 years), whose flux to the earth’s surface shows a dependence on bomb testing density, region, and latitude, has appeared to be satisfactory for tracing the accumulation rate of soil erosion. Transportation rate of soil particles, however, has a seasonal dependence due to precipitation, and could not be traced by 137Cs.