Modelling the influence of terrestrial vegetation on the environmental fate of xenobiotics

A terrestrial vegetation sub-model for use in multimedia Mackay-type models is developed. The purpose is to evaluate the influence of terrestrial vegetation on the fate of xenobiotics in the environment on a regional spatial scale. The vegetation sub-model treats the above-ground plant parts as one well-mixed box. Fate processes included for this above-ground plant compartment are uptake from soil with transpiration stream, diffusive exchange with air, stomatal uptake from air of micropollutant sorbed to fine particles, metabolism, plant death and harvest. Advective transport to leaves by wet and dry deposition is ignored, which may not be valid for pesticides administered by spraying and near emission sources of xenobiotics sorbed to particles. The equations used for diffusive exchange with the gas-phase of air and uptake from soilwater are only valid for non-ionized organic micropollutants. Roots are treated as an integral part of the soil, taking into account harvest of root crops. The vegetation sub-model is implemented as a new module in SimpleBox, which is a multi-media model for modelling the fate of xenobiotics on a regional to global spatial scale. Parameters representing vegetation on natural and agricultural soils in the Netherlands are selected. Model calculations were performed for hypothetical compounds to show the potential influence of intermedia transfer processes introduced by adding terrestrial vegetation to the model. These calculations indicated that the added processes can be equally, and sometimes even more, important for the regional fate as the air-soil intermedia transfer processes included in recently published Mackay-type models. The only exception was stomatal uptake of fine particles. Although influencing concentration levels of xenobiotics in plants, this transport process was not significantly influencing the concentrations in air, water, natural soil and agricultural soil. For specific chemical properties, the influence on air and soil concentrations can be up to a factor of 50. Case studies with dimethoat, hexachorobenzene, bromacil and benzo[a]pyrene showed that the concentrations in soil are considerably influenced by including vegetation in the model. While metabolism in above-ground plant parts and harvest of those parts were identified as significant fate processes, calculations also showed that it is unlikely that harvest of root crops is significant.