Relative Importance of Atmospheric and Root Uptake Pathways for 14CO2 Transfer from Contaminated Soil to Plants

Abstract

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.

Depth profiles of radiocarbon and carbon isotopic compositions of organic matter and CO2 in a forest soil

“…Radiocarbon is of concern in health physics because of its potential inventory, rapid mobility, and long half-life (5730 years) (Amiro et al, 1991). It is produced not only naturally in the upper atmosphere by the reaction of neutrons originating from cosmic rays with nitrogen (Libby, 1946) and, to a lesser extent, with oxygen and carbon, but also by anthropogenic activities such as nuclear weapons testing and operations of both nuclear power plants and reprocessing facilities (Kunz, 1985;Levin et al, 1988;Uchrin et al, 1992).…”

Impacts of 14C discharges from a nuclear fuel reprocessing plant on surrounding vegetation: Comparison between grass field measurements and TOCATTA-χ and SSPAM14C model computations

“…The sink compartment is used to represent losses of 14 C from the system, including both loss of plant material as a result of harvesting and also 14 C in the turbulent atmosphere carried away from the area of interest by diffusion and air movement. A specific activity approach has been adopted for the uptake of plant 14 C, which includes 1% uptake from roots (Amiro et al, 1991). Although the atmosphere is separated into two compartments, it is assumed that the aboveground portion of the plant grows inside the diffuse part of the atmosphere only.…”

Comparison of modelled uptake to cereal crops of 14C from gaseous or groundwater mediated pathways

“…It was assumed that plants obtain 2% of carbon from soil and 98% from atmosphere, via photosynthesis or foliar uptake (Amiro et al, 1991). The stable carbon content (g C kg À1 dry weight) of the plants and basis of derivation (e.g.…”

“…Literature values indicate that Z d varies from onetenth to two-thirds of Z c (e.g. Amiro et al, 1991;Avila and Pröhl, 2008) for a range of vegetation types. In this study, for a wind speed of 5 m s À1 at a height of 10 m, a crop height of 1 m, and Z d of 0.17 m a windspeed at 1 m height of 2.8 m s À1 was adopted; for a Z d of 0.67 m a windspeed at 1 m height of 0.75 m s À1 was adopted.…”