Effect of dynamic competitive sorption on the transport of volatile organic chemicals through dry porous media

Dynamic (time-varying) competitive sorption, modulated through fluctuating relative humidity, is shown to significantly affect the diffusive flux of 1 ,2,4-trichlorobenzene through soil columns. The flux varied by a factor of up to 2 during a 24 hour cycle in the experimental system when subjected to humidity fluctuations of +/-40%. This effect was observed despite conditio ns in which sufficient water coverage would have been present to significan tly depress volatile organic chemical (VOC sorption). Many vadose zone chem ical transport models do not incorporate vapor-solid sorption, which is kno wn to be important in low-moisture soils. The models that do incorporate va por sorption do not account for dynamic variations of the partitioning beha vior associated with variable moisture content. We present and validate a m athematical model for VOC transport through soils that accounts for dynamic nonlinear competitive sorption on volatile species transport. Simulations are presented which show that the effect of a thin dry zone at the soil-air interface can result in large variations in the instantaneous chemical rel ease rate. This implies that for some conditions in the held, VOC sampling should be conducted over longer periods to avoid bias due to short-term spi kes in the chemical release rate.