Release of Radon-222 by Vascular Plants: Effect of Transpiration and Leaf Area
Lewis, B. G. | MacDonell, M.M.
At disposal sites for wastes containing radium (Ra), vegetative stabilization may allow escape of radon (Rn) via uptake and mass flow of the gas in the plant transpiration stream. The main objectives of this work were to determine the magnitude and factors of such transport. Corn (Zea mays L.), sunflower (Helianthus annuus L.), and tall fescue (Festuca arundinacea Schreb.) were grown in uranium mill railings solids containing ²²⁶Ra (4.4 × 103 Bq kg⁻¹). The quantities of ²²²Rn released by the plants in a controlled environment chamber were measured in relation to water transpired and leaf areas. The ²²⁶Ra and ²²²Rn in corn xylem liquid, and diffusion of ²²²Rn across paraffin wax barriers, were also measured. The ²²²Rn release rates by the plants ranged from about 8 to 28 mBq s⁻¹ m⁻² of leaf area, and were unrelated to the quantity of water transpired. Generally, the plants tended to release twice as much radon at 15°C than at 30°C, a phenomenon explained in part by the increased solubility of Rn in water at the lower temperature. We suggest that the bulk of ²²²Rn released by the plants is taken up by mass flow in water, but at the leaf mesophyll Rn diffuses independently of water, through the entire leaf cross-section, unimpeded by the cuticle and epicuticular wax. The species of plant does not appear to be a major factor in the quantity of Rn released to the air; rather, the amount of Rn released into the atmosphere from a given area of vegetated land is expected to be in direct proportion to the leaf area index.
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