Assessment of Radionuclide Transfer in Terrestrial Ecosystem

Abstract

Radioactivity in the terrestrial environment is present since time immemorial and contributed by natural radionuclides of ²³⁸U and ²³²Th series and artificial radionuclides of ¹³⁷Cs, ⁹⁰Sr, etc. The major portion of food consumed by humans is grown in the terrestrial environment, and these radionuclides deliver radiation dose to humans and non-human biota through various exposure pathways. Soil is an important constituent of terrestrial environment comprising of mixture of sand, silt and clay in varying proportions, which give each soil type, characteristic texture and properties. The important physicochemical processes by which soils provide the nourishment for plants are controlled largely within the clay fraction of soil. The formation of soil in various agroclimatic zones is influenced by climatic factors of precipitation and temperature. Uptake of radionuclide depends on its availability in root zone of plant and its chemical form available for transport to root zone and translocation to edible portions of the plant. The major pathways and compartments considered for assessment of the impact to terrestrial environment are foliar interception, translocation and uptake from root zone by plants. Partitioning of radionuclide between soil and interstitial water is described by distribution coefficient K_d expressed in L kg⁻¹. There is a strong dependence of K_d for uranium, particularly within the pH range of 5–7, where K_d (U) is observed to be 10 times higher. The sorption behavior (K_d Iodine) of iodine is more complex. Data generated on soil-to-plant transfer factor (TF) or concentration ratio (CR) is an important parameter used in the Radiological Environment Impact Assessment (REIA) models. Important factors affecting TF are chemical and physical characteristics of soils, agricultural practices adopted, crop types, frequency of rain events and dietary consumption practices. Studies indicated that sorption of radiocesium in soils is influenced by ionic exchange processes at frayed edge sites (FES) regular exchange sites present in soil. Soils containing 14–50% organic matter, effectively complex Ra, more than its parent element Uranium. Lichens contain significantly higher concentrations of ¹³⁷Cs, ²¹⁰Po and ²¹⁰Pb as compared to vascular plants.