In contrast to petroleum released to unfrozen soils, the flow of petroleum released to frozen active layer soil will be influenced by the pres- ence of ice at different scales. At the millimeter scale, ice present as pore ice will act as a solid, changing the pore geometry and thus, the capillarity and permeability of the soil. In the extreme, the ground surface will be nearly imper- meable and downward migration will be minimal for the most part. Under these conditions surface flow will dominate upon release, though the higher viscosity at cold temperatures will inhibit lateral movement. In contrast to a release of petroleum to an unfrozen active layer, the increased exposure of the petroleum to the surface elements leads to greater losses of petroleum hydrocarbons by physical weathering (evaporation and photochemical oxidation (Snape et al. 2006a).
Winter releases of petroleum were studied in two separate field tests (Mackay et al. 1975; Johnson et al. 1980). Mackay et al. (1975) applied known volumes of crude oil to frozen ground at two separate field test sites. The ecosystem of each site was characterized as mature black spruce forests. Crude oil released in each of these controlled tests was at different temperatures (heated and ambient). Areas sampled soon after the release showed minimal penetration below the
top moss layer. Penetration below the moss layer did not occur until spring thaw (Mackay et al. 1975). Downslope migration of oil was limited in both of these controlled releases owing to increases in viscosity as the crude oil cooled shortly after the releases. Downslope migration recommenced during spring thaw. Similar results were found in a controlled release of 7600 l of Prudhoe Bay crude oil to frozen ground reported in Johnson et al. (1980). As expected, the summer release resulted in greater downslope migration in comparison to the winter release (Johnson et al. 1980).
Due to high pore-ice contents commonly found in the upper few meters of permafrost, the migration of petroleum into permafrost should be minimal in most cases. Nevertheless, petroleum hydrocarbons and liquid petroleum have been measured at depths of meters in permafrost (Biggar et al. 1998; McCarthy et al. 2004). In both these instances, movement was attributed to free phase petroleum movement through interconnected air voids in the frozen soil. These air voids may result from unsaturated compacted soil, fissures resulting from thermal contraction, or naturally occurring air voids in granular material (such as beach deposits) due to natural processes.
Biggar et al. (1998) measured significant petroleum hydrocarbon concentra-
tions (1200–17 000 mg TPH kg−1 in soils at depths ranging from 0.5 to 1.5 m
below the permafrost surface at old spill sites at Canadian Forces Station Alert and Isachsen High Arctic weather station in Canada’s high Arctic. They attributed the contaminant migration in the permafrost to be free phase NAPL movement through air voids in compacted fill or fissures in the native permafrost, depend- ing on the site. The air voids in the fill at Alert would have been a consequence of fill placement. The air voids in the native silty clay (Isachsen) and weather rock (Alert) would likely have been the result of contraction induced fissures during coldest soil temperatures; a similar mechanism to that attributed to ice wedge growth.
Migration of uncontrolled releases of refined petroleum occurring over sev- eral years near Barrow, Alaska was reported by Braddock and McCarthy (1996) and discussed further in McCarthy et al. (2004). The site was a sandy gravel beach deposit adjacent to the ocean and a nearby lake. These researchers found migration to be strongly influenced by the non-uniform nature of thaw depths brought about by the heterogeneous nature of snow depths and ground cover. They also found isolated ‘‘reservoirs” of petroleum at depths greater than 3.0 m, where the active layer was 0.5 to 2.0 m thick and highly variable. Air voids and hydrocarbon seeps were observed in the frozen soil with a down-hole cam- era. The existence of petroleum hydrocarbons below the permafrost table was attributed to both the presence of air voids and unfrozen brine at concentrations approximately three times that of seawater.
Frozen fine soils can contain unfrozen water at the soil surface boundary (Chapter 2). Lacking sufficient displacement pressures for petroleum to flow into ice rich permafrost, a possible transport mechanism is diffusion of petroleum hydrocarbons through the unfrozen water content. Aqueous phase diffusion is a relatively slow transport process in comparison to advection. The contribution of diffusion to the overall movement of contaminants into permafrost soils is minimal.
In summary, results from studies conducted by Biggar et al. (1998) and McCarthy et al. (2004) illustrated that migration of petroleum into permafrost is possible most likely through air voids in the frozen soil. However, from a health risk perspective, a potential risk might be associated with the contaminant actu- ally altering the active layer and/or permafrost, and leading to greater lateral migration.
While the case histories are very informative providing a relative ‘‘big pic- ture” understanding of petroleum releases to frozen ground, they do not provide detailed insight into the mechanisms controlling the subsurface movement of petroleum in freezing and frozen soils. It is instructive therefore to examine theoretical concepts and available laboratory results that better define these mechanisms.
3.3.3 Theoretical concepts of petroleum movement in freezing