Further studies of the air–pasture transfer of polychlorinated biphenyls

Field survey and field experimental data are presented and interpreted in the context of the factors controlling the supply of polychlorinated biphenyls (PCBs) to the pasture sward. The field experimental data show that an air (gas phase)–pasture equilibrium was approached rapidly, within a few days, with a similar equilibrium attained between different grass species studied (Lolium perenne, Holcus lanatus, Festuca ovina). A mass balance model for a typical UK air–pasture–soil system is presented and used to estimate the distribution and rates of air–surface exchange and sequestration of PCBs into freshly growing plant biomass. A soil–air–vegetation model developed by Trapp and Matthies (1997, Environmental Science and Technology 31, 71–74) and other information is used to consider the possible influence of soil outgassing and sequestration versus the supply of PCBs from the bulk air to the pasture. It is hypothesised that a rapid approach to air–pasture equilibrium conditions could occur because PCBs are supplied from both the bulk air and by volatilisation from the underlying pastureland soil.