Relative importance of atmospheric and root uptake pathways for 14CO2 transfer from contaminated soil to plants

Plants growing on soil contaminated with 14C can receive the radionuclide from both direct uptake through the roots or indirect uptake from the atmosphere. The contribution from the atmosphere depends on the rate of volatilization from the soil, the size of the source area, and the meteorological dispersion conditions. An atmospheric dispersion relationship was used to calculate the air concentration for a given area source term. The air concentration was then used to estimate the flux to foliage, which was compared with direct plant uptake through the roots. The ratio of atmospheric to root uptake pathways depended on plant height and source area, exceeding unity when the fetch was greater than a few meters for bean plants, but much greater for maize. A very large fetch, greater than 1000 m, is required before a uniform specific activity throughout the local biosphere can be assumed for a soil source. This quantitative analysis describes the limitations for calculations and experiments used to define simplified transfer coefficients for 14C from soil to vegetation.