Estimating the soil erosion on hill slopes in Korea using radionuclide 137Cs

This study presents the intensity of soil erosion inferred from spatial distribution of radionuclide 137Cs in undisturbed soils on different bedrock hill slopes, underlain by granite, gneiss and sedimentary rocks in Korea. For estimating the intensity of the soil erosion, both bulk and core samples were taken from various geomorphic parts along the hills including reference sites which represents the expected inventory of a site experiencing neither erosion nor deposition, i.e., where 137Cs was initially absorbed in the 1950s. The initial distributions of the radionuclide 137Cs in top soils are indicated at approximately exponential with soil depths with each geological feature. The primary factors influencing the post-depositional redistribution of the radionuclide in hill slope soils have been represented as erosion by overland flow along the hill slopes and diffusion processes by throughflow within the soil profiles. The soil erosion on hill slopes is shown to depend on the bedrocks, soil properties and different runoff processes. The radionuclide 137Cs provides the average areal activities of the baseline 137Cs inventory for the uneroded soils with 2100 Bq/m2, 2840 Bq/m2 and 3366 Bq/m2 on granite, gneiss and sedimentary hill slopes, respectively. Estimation of the rate of soil erosion inferred from spatial distribution of the radionuclide 137Cs confirms that the soil loss from hill slopes is positively relative to loss of 137Cs in soil particles. At most sampling points along the hill slopes, soil loss is accurately related to 137Cs loss within soil particles. The 137Cs inventory within soil profiles at the reference sites shows an exponential curve with soil depths regardless of the bedrock types on these hills. However, as compared with that at reference sites, the loss of the 137Cs inventory on surface soil tends to increase on the upper slopes due to erosion, whereas, redistributions of the 137Cs in soil particles take place on down slopes by deposition of sediments stripped from upper slopes. The average rates of soil erosion are detected with 40.71 t/ha/yr, 26.56 t/ha/yr and 22.27 t/ha/yr on the granite, gneiss and sedimentary hills, respectively. The variations in rates of the 137Cs loss-induced soil erosion indicate that surface soil is more intensively prone to erosion by overland flow on the granite hill than that on the gneiss and sedimentary hills.