Transfer analysis of ²¹⁰Po and ²¹⁰Pb in the terrestrial environment

The transfer of ²¹⁰Po and ²¹⁰Pb between various compartments in the terrestrial environment has been analysed by using published data. The average activity concentration of ²¹⁰Po in dry soil is 61 ± 14 Bq.kg^-1 (median 44 Bq.kg^-1). Ground water concentrations of ²¹⁰Po in drilled wells might be as high as 6.5 Bq/l. But in regular drinking water it is just about 3-5 mBq/l. The uptake of radionuclides from soil to plant is usually given as the ratio of dry matter radionuclide-activity concentrations of plant (AC_plant) and soil (AC_soil) respectively. This ratio is called the soil transfer factor: STF = AC_plant/AC_soil. The soil transfer factor varies widely between various types of crops with an average of about 0.056±0.003. The activity concentrations in leafy plants are, however highly affected by the atmospheric deposition of ²¹⁰Pb and ²¹⁰Po. By comparing the activity concentrations in plants grown on an open field with those grown on a field sheltered by a polyethylene tent, it has been possible to estimate a deposition transfer factor: DTF = Difference of the dry matter activity concentration (Bq.kg^-1) of plant grown in open field and plants grown in tent shelter, divided by the atmospheric deposition during the vegetation period (Bq.m^-2). The deposition transfer factor for ²¹⁰Pb thus estimated is in the order of 0.5-1.0 (m².kg^-1 dry matter) for leafy plants like grass and 0.1-0.5 for less leafy plant and straw. For various grains it is < 0.2 and for root fruits it is < 0.003 (m².kg^-1 dry matter). Corresponding values for ²¹⁰Po are about a factor 3 times higher. The world average activity concentration of ²¹⁰Po in fresh leafy vegetable is estimated to 320±190 mBq.kg^-1, and in cereals and grain products 240±80 mBq.kg^-1. The average activity concentration in milk products is 59 ± 13 mBq.kg^-1 and in meat products about 70 ± 39 mBq.kg^-1. The dietary intake of milk and meat products is 170 kg.a^- which is the highest of all food items of terrestrial origin. The overall world average activity concentration in terrestrial food products is estimated to 70±40 mBq.kg^-1 for ²¹⁰Pb and 150±120 mBq.kg^-1 for ²¹⁰Po. A few studies have been performed to quantitatively study the transfer of the natural radionuclides ²¹⁰Pb and ²¹⁰Po from fodder to milk. The transfer coefficient F_m that describes the fraction of the daily intake of radio-nuclide (Bq.day^-1) secreted per litre of fresh milk (Bq.litre^-1) has been estimated from such studies by assuming a daily food intake of 16 kg dry matter per day. The transfer coefficient F_m for ²¹⁰Pb thus obtained is 0.01 day.Iitre^-1 and for ²¹⁰Po 0.003 day.litre^-1. These values are about 17 and 8 times higher than those recommended by IAEA for these elements respectively. A steady state model of the food chain lichen-reindeer and man has been applied to estimate the fractional residence time for different steps in the food chain to man. (author)