VOLATILE ORGANIC VAPOR DIFFUSION AND ADSORPTION IN SOILS

Knowledge of the relationship between D(P)/D0 (diffusion coefficient i n soil divided by diffusion coefficient in free air) and the volumetri c soil-air content, epsilon, is important when modeling gaseous moveme nt of volatile organic compounds (VOCs) in soils. The effective diffus ion (i.e., diffusion and retardation) of trichloroethylene (TCE), tolu ene and freon in Yolo silt loam (fine-silty, mixed, nonacid, thermic T ypic Xerorthent) were measured in a two-chamber diffusion apparatus. T he experiments were conducted on packed soil cores over a range of wat er contents. Vapor retardation factors were calculated from soil param eters and equilibrium partition coefficients. Partition coefficients w ere measured in batch experiments. It was found that for water content s higher than four molecular layers of water surface coverage, solid/v apor partition coefficients, K(D)', were consistent with values predic ted by Henry's Law constants (K(H)), and squeous/solid partition coeff icients, K(D). For less than four molecular layers of water, sorption increased by orders of magnitude. The vapor retardation factors, along with the measured effective diffusion, allowed a calculation of diffu sion coefficients (D(P)) for the investigated species by using the ana lytical solution to diffusion in a two-chamber apparatus. Values of th e ratio D(P)/D0 were generally higher than the values predicted by the Millington-Quirk equation, and lower than the values predicted by the Penman equation. Compared with the nonreactive tracer freon, D(P)/D0 values for TCE and toluene agreed very well for higher water contents. Values obtained for air-dry soil, however, were under-predicted. The experimental work for determination of the effective diffusion of reac tive tracers can, therefore, for sufficiently high water contents be l imited to the determination of D(P)/D0-epsilon relations for a nonreac tive tracer and measurement of K(D), K(D)', and K(H) values for the re active tracers.