‘Time to crop’ for 137Cs in the surface soil and its long-term effects to population based on model assessment
Van Thang Nguyen, Ngoc Ba Vu, Nguyen Phong Thu Huynh, Cong Hao Le
Abstract:
Long-term behavior of artificial radionuclides in surface soils is very important to assess the radiological effects to population. Among artificial radionuclides, 137Cs is most important because of its long half-life and its biggest abundance in the environment. In this study, the fate of 137Cs in the surface soil layers was assessed by the Canadian Centre for Environmental Modelling and Chemistry (CEMC) soil model which is well known as a simple assessment of the relative potential for degrading reaction, evaporation, and leaching of a pesticide applied to a surface soil. The total decrease rate of 137Cs activity concentration in the surface soil (Te1/2) was 10.4 years found in the top 0–5 cm of the soil layer. The activity decrease of 137Cs and the corresponding Te1/2 values under the different depths of surface soil layer were investigated. The influences of soil organic material, soil water content and soil porosity to the losing rate of 137Cs were considerable. Long-term effects of 137Cs to population were assessed through activity concentrations of 137Cs in any parts of the food chain. Soil-to-plant transfer factor (TF), transfer coefficients Fm (transfer to animal milk) and Ff (transfer to animal meat) collected from many literatures were used for activity calculation. The effective doses to population due to ingestion of edible parts of plants, milk and meat was evaluated. The incorporation of four terms: radiological doses, soil-to-plant transfer factors (TF), plant-to-animal coefficients (Fm and Ff) and the total decrease rate of 137Cs is the new approach. The new concept ‘Time to crop’ (TC) based on the effective doses to population was explored and first used for agricultural proposals in the topsoils of the 137Cs exclusion zone. TC was calculated for many scenarios of radioactive exposures, many type of plants, animals and plant groups. TC values were found various in order fruits < leafy vegetables < tubers < cereals < grasses. The highest TC was found for grasses group as a result of the long-term accumulation of radionuclides in animals.
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