Diffusive transport of volatile pollutants in nonaqueous-phase liquid contaminated soil: A fractal model

Volatile organic chemicals from nonaqueous phase liquids trapped in soil spread quickly both into the atmosphere and the groundwater system, resulting in long-lasting pollution of the environment. The extent and speed of spread is strongly influenced by the heterogeneities present in the soil, which interact with the various mechanisms of transport in a complex manner. A semianalytical model is presented to investigate the effect of soil-heterogeneities on the spreading of pollutants from a nonaqueous phase liquid trapped in a soil. Based on field evidence, soil heterogeneities are modeled as a self-similar fractal process, and diffusive transport of organic chemicals in such a substrate is studied. Various mechanisms of, and resistance to, transport of these contaminants are considered, and it is illustrated how each of these are altered by the presence of heterogeneities. More specifically, it is shown that, while the transport processes at early time are governed by volatilization and dissolution and are quite unaffected by the heterogeneities, the transport processes at late time are strongly dependent on the heterogeneities of the soil and its sorption characteristics. In addition, how failure to recognize these heterogeneities in the soil may result in unsuccessful design of appropriate remediation techniques is discussed.