SIMULATING THE VOLATILIZATION OF SOLVENTS IN UNSATURATED SOILS DURINGLABORATORY AND FIELD INFILTRATION EXPERIMENTS

This paper describes laboratory and field experiments which were condu cted to study the dynamics of trichloroethylene (TCE) as it volatilize d from contaminated groundwater and diffused in the presence of infilt rating water through the unsaturated soil zone to the land surface. Th e field experiments were conducted at the Picatinny Arsenal, which is part of the United States Geological Survey Toxic Substances Hydrology Program. In both laboratory and field settings the gas and water phas e concentrations of TCE were not in equilibrium during infiltration. G as-water mass transfer rate constants were calibrated to the experimen tal data using a model in which the water phase was treated as two pha ses: a mobile water phase and an immobile water phase. The mass transf er limitations of a volatile organic compound between the gas and liqu id phases were described explicitly in the model. In the laboratory ex periment the porous medium was nonsorbing, and water infiltration rate s ranged from 0.076 to 0.28 cm h-1. In the field experiment the water infiltration rate was 0.34 cm h-1, and sorption onto the soil matrix w as significant. The laboratory-calibrated gas-water mass transfer rate constant is 3.3 x 10(-4) h-1 for an infiltration rate of 0.076 cm h-1 and 1.4 x 10(-3) h-1 for an infiltration rate of 0.28 cm h-1. The ove rall mass transfer rate coefficients, incorporating the contribution o f mass transfer between mobile and immobile water phases and the varia tion of interfacial area with moisture content, range from 3 x 10(-4) h-1 to 1 x 10(-2) h-1. A power law model relates the gas-water mass tr ansfer rate constant to the infiltration rate and the fraction of the water phase which is mobile. It was found that the results from the la boratory experiments could not be extrapolated to the field. In order to simulate the field experiment the very slow desorption of TCE from the soil matrix was incorporated into the mathematical model. When des orption from the soil matrix was added to the model, the calibrated ga s-water mass transfer rate constant is 2 orders of magnitude lower tha n that predicted using the power law model developed for the nonsorbin g laboratory soil.