Transport and Biological Fate of Toluene in Low‐Permeability Soils

Abstract

The effect of simultaneous sorption, diffusion, and biodegradation on the fate and transport of toluene in low‐permeability soil formations was examined. A transport model accounting for vapor and liquid sorption, vapor diffusions, and first‐order biodegradation was developed to describe the movement of volatile solute in unsaturated soils. Modeling studies were followed with laboratory batch and column studies on fine‐grained soil samples obtained from a gasoline‐contaminated site. Batch experiments yielded the sorption and diffusion coefficients for generating theoretical solute transport profiles. Column studies were conducted to examine toluene sorption, diffusion, and biodegradation under aerobic and denitrifying conditions. Results from the column studies indicated that vapor sorption onto the soil was minimal due to the high moisture content of the soil. Comparison of model predictions with experimental results indicated that the SASK model, which is based on the resistivity theory, provided a more accurate prediction of the vapor phase tortuosity than the frequently used Millington‐Quirk equation. Laboratory results of toluene concentration profiles matched well with the model predictions and yielded degradation rates comparable to those obtained in the field. Column studies, examining toluene biodegradation under aerobic and denitrifying conditions in low‐permeability soils, indicated that the presence of excess nitrate in aerobic environments yielded higher solute degradation rates than those observed under exclusively aerobic systems.

The behavior of clomazone in the soil environment

“…The batch equilibration sorption isotherm method is used widely to predict the potential movement of organic compounds into and from soils 18–20. Some literature reports incorporating this method use commercially available synthetic ‘model’ organic compounds to explain sorbate and sorbent behavior.…”

Transport of BTEX Vapours Through Granular Soils with Different Moisture Contents in the Vadose Zone

“…Weerts et al (2001) illustrated the distribution of water among the soil particles at different water contents. At low moisture content, the pore water covers the surface of the soil particles (Shoemaker et al 1990;Moldrup et al 2001) and occupies the small pores (Venkatraman et al 1998). Venkatraman et al (1998) studied the pore-filling behaviour of water molecules in fine grained soil (sandy silt) and found that the fine pores are filled with water first.…”

“…At low moisture content, the pore water covers the surface of the soil particles (Shoemaker et al 1990;Moldrup et al 2001) and occupies the small pores (Venkatraman et al 1998). Venkatraman et al (1998) studied the pore-filling behaviour of water molecules in fine grained soil (sandy silt) and found that the fine pores are filled with water first. At a moisture content of 5%, pores with a diameter of 0.1 lm or lower are filled while the larger pores are unfilled.…”

Experimental Study of Gas Transport Parameters in Unsaturated Silica Flour

“…They used a simulated landfill apparatus to experimentally determine the effective diffusion coefficient of the unsaturated clay at an air‐filled porosity >0.2 and found this coefficient to be on the order of 10 −6 m 2 s −1 Schaefer et al (1997) assessed gas‐phase diffusion in a soil with >60% silt and clay content and obtained a diffusibility value lower than 10 −5 for the soil at a water saturation >80%. Venkatraman et al (1998) applied the diffusibility model presented by Schaefer et al (1997) to assess the tortuosity effect on gas‐phase diffusion in a low‐permeability soil with 64% silt and clay contents. Johnson et al (1998) conducted field experiments and obtained some effective diffusion coefficient values on the order of 10 −8 m 2 s −1 Moldrup et al (2000b) developed a water‐induced linear reduction model modified from the Marshall model (Marshall, 1959) for predicting the effective diffusion coefficient in unsaturated repacked soils with clay content <54%.…”