Hydraulics of the Aquifer and Barrier Media

The permeability of the soil to both dissolved (e.g., metal ion) and free-phase (e.g., hydrocarbon) contaminants depends on the frozen and unfrozen moisture content of the media (Wiggert et al. 1997; Wolfe et al. 2003). Importantly for contaminant migration, the moisture content in both the soil and the PRB varies with depth from the surface, according to the history of wetting and the direction and characteristics of freezing. In particular, in areas (such as the high Arctic and Antarctic) where the soil is underlain by permafrost, the soil freezes both from the top down and bottom up, leading to greater water content and potentially ice saturation and the development of impermeable layers, at least seasonally, in the upper and lower parts of the active layer (Wolfe et al. 2003). There is an inverse relationship between permeability and ice content (Wiggert et al. 1997; McCauley et al. 2002). Frozen, water-saturated materials approach impermeability, a characteristic which has been exploited for the creation of frozen soil barriers to contain hazardous materials (Andersland et al. 1996). However, Chuvilin et al. (2001) found some migration of oil, even into saturated, frozen soil, possibly as a result of movement along cracks that develop in the soil during freezing (Chamberlain and Gow 1979; Biggar et al. 1998). Cracking enhances the infiltration of water (Benson and Othman 1993) and other fluids, and so can act as an important control on the direction and flux of contaminants in soil and PRB media. Lateral patterns of contaminant movement can also be controlled by capillary suction (Barnes and Filler 2003).

The redistribution of fine-grained material by washing can create significant changes to the hydraulics of barrier media, depending on the ratio of the size of the immobile:mobile grains. If the ratio is <10, where immobile grains are <10 times larger than the mobile grains, an impermeable layer can develop in the barrier media or the aquifer dowstream. If the ratio is 10-20, hydraulic conductivity can be impaired, and if the ratio is >20 only slight changes to hydraulic conductivity (K) might occur (Abadzic and Ryan 2001). These ratios may change depending on the abundance of the mobile fraction, and the size and shape of the media.

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