VOLATILIZATION OF A MULTICOMPONENT LIQUID THROUGH DRY SOILS - TESTING A MODEL

Spilled or leaked petroleum products often contaminate groundwater and subsequently evaporate through the soil to the atmosphere by gaseous diffusion. This study developed and tested a model that can predict the rate of diffusion through a dry soil column of the individual components in a liquid mixture when they are of low or intermediate volatility. We derived equations that relate the vapor pressures of the components to their sorption from the gaseous phase by the soil. We also provided for slow sorption. Three cases of increasing complexity were treated: evaporation of a mixture directly into the atmosphere; evaporation of a mixture through a soil column without significant sorption; and finally, evaporation through a highly sorbing soil column, for which a computer program was written. The model has been tested by comparing the evaporation of m-xylene and n-dodecane mixtures through sand, and air- and oven-dry Evesham clay soil (Aquic Eutrochrept or orthic grumaquert) in the laboratory. The equilibrium sorption isotherms and sorption kinetics for these soils and compounds were determined in separate experiments, and other parameters were obtained from the literature, so that the model contained no adjustable fitting parameters. There was fair agreement between the model predictions and the experimental results. Discrepancies were attributed to errors in chemical analysis and the boundary effects that arose from rather short columns, and to the assumptions that the compounds did not compete for sorption sites on the soil, that Raoult's Law was obeyed, and that there was no hysteresis in the sorption isotherms.