Dependence of hydraulic and pneumatic characteristics of soils on a dissolved organic compound

The physical properties of the interfaces between solid, liquid, and gaseous phases determine the fate of organic pollutants in soils. Choosing butanol as a model substance, we investigated the effects of a dissolved organic compound on the relationships between capillary pressure, phase saturation, and permeabilities of a quartz sand mixture. Steady-state column experiments were performed at various degrees of phase saturation using air as the non-wetting phase and aqueous solutions with 0%, 2% or 6% (by weight) butanol as the wetting phase. The capillary pressure–saturation curves were found to scale according to the surface tension ratio. While the saturation curves showed considerable hysteresis between wetting and drying with respect to capillary pressure, no hysteresis was found in the hydraulic permeability curves with respect to saturation and all curves described a unique function of saturation. Contrary to hydraulic permeability, pneumatic permeability however showed significant hysteresis with respect to saturation. The drainage and imbibition branches of the air permeability functions were not found to depend on the butanol concentration. These results indicate that for a dissolved organic compound the effect of concentration on the hydraulic and pneumatic characteristics of a soil can be predicted well from a single set of capillary pressure–saturation and permeability–saturation curves if the corresponding surface tensions are known and if changes of contact angles remain small.