FIELD VALIDATION OF A MODEL OF THE UPTAKE OF GASEOUS SOC IN LOLIUM-MULTIFLORUM (RYE GRASS)

The concentrations of a variety of semivolatile organic compounds (SOC) including polychlorinated dibenzo-p-dioxins, dibenzofurans, and biphenyls measured in rye grass under field conditions are compared with the concentrations predicted by a mathematical model based on laboratory studies with a fugacity meter. The agreement is excellent, with a maximum difference of 30% between the predicted and the measured concentrations for those compounds where dry gaseous deposition is the main uptake pathway. It was found that compounds with a log octanol/air partition coefficient > 8 did not approach equilibrium in the field study. The uptake of these compounds was independent of the physical-chemical properties of the substance and was postulated to be governed by the rate at which air is exchanged between the grass canopy and the free atmosphere. For more volatile compounds that approached equilibrium, the bioconcentration factor is determined primarily by the octanol/air partition coefficient of the substance and the temperature. This study represents the first validation of a model of plant uptake of gaseous SOC that we are aware of and demonstrates that the results of fugacity meter studies can be extrapolated to environmental conditions.