MODEL OF THE LONG-TERM EXCHANGE OF PCBS BETWEEN SOIL AND THE ATMOSPHERE IN THE SOUTHERN UK

Measurements of concentrations of four PCB congeners (PCB-28, -52, -138, and -153) in air, herbage, and soil of the southern U.K. over the period 1942-1992 are compiled to demonstrate that emissions of PCB to the atmosphere increased from low levels in 1940 to a peak in the late 1960s and then fell to low current levels. This emission pulse resulted in a corresponding peak in air concentrations. Soil concentrations have responded to this pulse but with some delay, reaching a maximum in the early 1970s. At that time PCB fugacities in soil apparently exceeded atmospheric fugacities by about a factor of 10 because of nondiffusive deposition processes. Since then, fugacities in air and soil have fallen with a half-life of 10-20 yr probably as a result of volatilization or ''degassing'' of PCB, although it is possible (but not proved) that the degradation processes are also occurring with a half-life of approximately 15 yr. A simple two-compartment (air-soil) mass balance model is developed and applied to these data which gives a reasonable simulation of the longterm trends in concentration. It is concluded that there is a need to develop and improve models of this type to explain and predict the extent to which soils and vegetation act as a buffer of atmospheric concentrations of persistent chemicals.