AIR-PASTURE TRANSFER OF PCBS

A field experiment was conducted to study the air to pasture transfer of PCBs at a rural site in northwest England. Strong positive linear c orrelations were obtained between the log plant-air partition coeffici ents (m(3) of air g(-1) of plant dry weight-defined here as the scaven ging coefficient) and log octanol-air (K-oa) partition coefficients. P asture typically retained amounts of PCB per g dry weight equivalent t o that in similar to 7 m(3) of air for congener 18 and ranging up to s imilar to 64 m(3) for congener 170, regardless of whether the pasture growth (exposure) time had been 2, 6, or 12 weeks. This indicates that airborne PCBs partition onto freshly grown pasture and approach plant surface-air gas-phase equilibrium rather rapidly at this site, i.e., within 2 weeks of exposure. In late April-June, when grassland product ion is at a maximum, sequestering rates could approach 1.2 ng of PCB-1 8, 0.17 ng of PCB-170, and 12 ng of Sigma PCB m(-2) day(-1). With 7 mi llion ha of managed and rough grassland in the U.K., fresh pasture pro duction in the spring and summer is estimated to remove an average of Sigma 0.8 kg of Sigma PCB day(-1) from the air during these times (sim ilar to 80 kg of Sigma PCB per growing season). Some buffering influen ce may he exerted on surface air concentrations during the most active periods of plant biomass production, while the incorporation of PCBs into pasture following air-pasture transfer processes controls the sup ply of PCBs to grazing animals and the human food chain.