The movement of water in freezing soil is caused by disequilibrium in the soil-water-ice system that results from a complex combination of differ- ences in temperature, pressure, concentration, and other internal factors such as humidity, and electrical and magnetic potentials. Considerable effort has been made to understand freezing ground phenomena such that we can predict the parameters that influence ice segregation, frost heave, and thaw settlement.
2.2.4.1 Frost heave and thaw settlement
In a freezing fine-grained soil, such as a sandy-silt, silt, and clay, water is drawn to the freezing front from the surrounding soils. This water movement (wicking) occurs because of the temperature and moisture gradients in the soil. A soil can wick moisture if water is able to flow under negative pressures in the thin capillaries between the soil particles. A sandy-gravel for instance, is a non-frost susceptible soil, as it cannot wick moisture through its relatively large pore spaces. Therefore, the three necessary conditions for frost heave are sub- freezing temperatures, the availability of moisture, and the ability of water to wick through the soil. Wicked water freezes in a segregated fashion, forming ice lenses. Ice lenses push soil upward differentially, causing damage to the overlying engineered structure. Similarly, during the thawing period, heaved soils now settle differentially, also causing damage. Telephone poles, foundation posts, and fences are jacked out of the ground by a similar mechanism.
2.2.4.2 Thermokarst
The thawing of ground ice and consolidation of thawing soil results in the deformation of soil and the formation of specific forms of relief called thermokarst. As thawing ice-rich soils consolidate by squeezing out the available moisture, the resulting depression in the ground will either drain away water or water will accumulate in the depression and create a thermokarst lake.
2.2.4.3 Taliks
It is usual that permafrost limits the possibility of sub-permafrost water recharge. According to van Everdingen (1974), in permafrost regions, rates of groundwater recharge are orders of magnitude lower than in non-permafrost regions. Taliks are layers or bodies of unfrozen soil in permafrost. In relation to permafrost integrity, taliks are divided into two groups: open and closed. An open talik penetrates the entire thickness of permafrost, while a closed talik is a thaw bulb bordered by permafrost and the soil surface. Taliks occur both in continuous and discontinuous permafrost regions, and many areas of unfrozen soil in discontinuous permafrost were developed as taliks. In areas of continu- ous permafrost, taliks beneath lakes are usually closed and prevent interaction between suprapermafrost and sub-permafrost waters. Rare open taliks beneath big rivers and deep lakes can provide connection between surface water and sub-permafrost water. Open taliks also occur below mountainous rivers where the channel deposits are presented by coarse material. High hydraulic conduc- tivity provides extensive water flow in channel deposits and thermal conditions favorable for talik existence.
Numerous shallow lakes are abundant in low-lying continuous Arctic per- mafrost regions, but not every lake has a talik. Taliks occur only beneath lakes whose depths are greater than a critical depth (Kudriavtsev 1978). In the Arc- tic Coastal Plain of Alaska, the critical depth of lakes is about 1.5–1.8 m. Only beneath lakes with a greater depth is soil perennially unfrozen to some depth. As previously mentioned, open taliks are very rare. For example, to develop an open talik in continuous permafrost, where mean annual temperature of water
in lakes is about 1◦C and the permafrost temperature is about−8◦C, the lateral
extent of a lake has to be an order of magnitude greater than the permafrost thickness (Grechishchev et al. 1980). And in the northern parts of Alaska, the permafrost thickness is greater than 500 m!
In a discontinuous permafrost region, size of a lake in the lateral direction is less critical. Here, the age of a lake is an important factor in formation of the talik. For example, if a section of ice-saturated permafrost is about 50 m thick, a young thermokarst lake (with an age to hundreds of years) could have a closed talik. Lakes which are thousands of years old could be underlain with open taliks. Soil stratigraphy is also a very important factor as a thinner layer of ice-saturated soil will result in the faster formation of an open talik. Also the permeability of lake sediment is usually very low. Some authors (Kane and Slaughter 1974; Tishin 1983) acknowledged the possibility of interaction of lake water and sub-permafrost groundwater.
2.3 Guidelines and recommendations
An evaluation of a contaminated site in cold regions depends not only on the physical properties of the site but also on the time of the year. An oil spill on tundra in winter will exhibit different impact behavior than a spill that occurs in summer. Two test sites near Fairbanks were contaminated with crude oil, one during the winter of 1976 and the other during the summer of 1976 (Collins et al. 1994). The purpose of these trials was to determine the fate of the environment after a crude oil spill on forest underlain by permafrost. Fifteen years later, the sites were re-examined to determine the long-term effects of the spill. It was found that the winter spill had affected a larger area as the oil traveled over the frozen ground. There were also indications that some of the volatile compounds still existed in the subsurface and that little biodegradation took place during the 15 years (see Chapter 3, Section 3.1 for more discussion). The population of hydrocarbon-degrading bacteria may increase at such a spill, while overall microbial diversity may decline (Aislabie et al. 2004).
The upper permafrost is almost impermeable (Shur 1988a; Olovin 1993). But seasonal freezing of soils and their thermal contraction form cracks of different
sizes in the upper permafrost. Fractures and cracks in frozen soil allow con- tamination to penetrate into the upper permafrost and impact the root systems of vegetation (Filler and Barnes 2003). Removal of trees and compaction and other disturbances to the surface organic layer can result in an increase in the active layer depth and melting of the highly impermeable intermediate layer of the upper permafrost. In discontinuous permafrost regions, contaminants may follow a lowering permafrost table as it degrades.
Lessons learned through mitigation of numerous petroleum spills and con- taminated sites has resulted in a Tundra Treatment Manual (Athey et al. 2001) in Alaska. This guidance document considers season and physicochemical proper- ties of petroleum contaminants relative to environmental media in the develop- ment of early response and recovery tactics. Management of permafrost, or per- mafrost control, will be integral to remediation planning at Arctic and Antarc- tic contaminated sites. Where engineered bioremediation is used with heating schemes, particular attention must be given to heat transfer to the subsurface to prevent settlement and further intrusion of contamination with depth.
2.4 Future research
At present we have little understanding of the effects of repeated freez- ing and thawing of soils on contamination in the ground. A key area of future research concerns the impact that petroleum spills have on unfrozen water content, which in turn can have an effect on properties such as frost heave. Furthermore, since climate change is anticipated to have significant impact on permafrost and the active layer, it stands to reason that it will also influence contamination within these soil regimes.