Consideration of canopy structure in modelling ¹⁴C-labelled gas behaviour in the biosphere for human dose assessments

¹ Limer Scientific Consulting Limited, Unit 1702, Block 3, Central Residence, No. 167, 1038 Hua Shan Lu, Shanghai, 20050 Shanghai, China
² Mike Thorne and Associates Limited, Hamsterley, County Durham, UK
³ Low Level Waste Repository Limited, Cumbria, UK

Abstract

The LLW Repository Limited has recognised the potential importance of the processes being considered in the BIOPROTA ¹⁴C working group and funded the development a new ¹⁴C model that addresses the exchange of gas in a soil-plant-atmosphere system. This model considers two regions in the above-ground atmosphere and utilises concepts from the field of micrometeorology to describe the exchange of air between these regions and losses from the area of interest. The lower layer only experiences molecular diffusion processes in relation to the movement of molecules of CO₂, whereas the upper layer experiences some degree of turbulent mixing as a result of winds which flow over the area of interest. The thicknesses of these layers depend upon the canopy density, which will affect the light intensity and thus the rate of photosynthetic uptake of carbon in the canopy profile. Model results demonstrate the impacts of ¹⁴C-labelled gas from the soil upon the calculated ¹⁴C concentration in plants for a variety of plant species (pasture and garden crops) and subsequent doses to human exposure groups. The technical modelling work described has been funded by the LLW Repository Ltd in support of its 2011 Environmental Safety Case.