In the Antarctic case study (see Section 1.3.3), three principal lines of evidence were presented as components that could be used for establishing trig- ger values, remediation guidelines, or site-specific risk assessment: physical and chemical; ecotoxicological; and community level ecological data (Figure 1.2). These lines of evidence are broadly compatible with the regulation frameworks used in Australia, Alaska, and Canada, and offer a simple conceptual model that allows for a mix of relevant quantitative data and professional judgement (Bat- ley et al. 2002; Chapman et al. 2002a). However, each of the lines of evidence requires further research both in the development of suitable methods and in the application of quantitative comparisons from a wide range of soils and envi- ronments.
1.5.2.1 Modeling oil and NAPL dispersal in freezing soils
Several experimental empirical studies have documented the behavior of spilled oil and fuel on frozen soil. Research by Chuvilin and coworkers (Chuvilin et al. 2001; Chuvilin and Miklyaeva 2003) has examined the factors that affect spreadability and transport of crude oil in frozen soil. They concluded that the amount of pore filled ice/water, oil composition, soil temperature, salinity, soil mineralogy and cryogenic structure all affect oil dispersion. Preliminary work has examined concentrations which indicate the presence of NAPL (DRO) in a range of Antarctic and sub-Antarctic soils through application of a partitioning model developed by Mariner et al. (1997). The model predicts that the hydro- carbon mass held in the soil in the dissolved, vapor, and adsorbed phases is predominantly controlled by soil organic carbon, and that NAPL is present at
concentrations in the range 50–1000 mg DRO kg−1(Rayner et al. 2007). However,
these models have yet to be extended to estimate at which point NAPL move- ment is possible. Charbeneau (1999) developed spreadsheet tools that assess the mobility of NAPL near the saturated capillary fringe. Qualitative observations indicate that soil type and the amount of pore filled ice/water have a significant influence on the spreadability (Barnes et al. 2004 and Chapter 3; Barnes and Adhikari 2006). However, there is virtually no quantitative information that can currently be used to predict what proportion of pore space needs to be filled by NAPL before movement is possible in soils that undergo freezing.
The most pressing future research need is to derive further experimental information to allow the development of a holistic dispersal model that can accommodate a range of freezing soil properties to enable calculation of disper- sal distances and rates for a range of oil types. This is needed for schemes such as the Canadian Tier 2 adjustment for the protection of groundwater for aquatic life so that dispersal in freezing ground can be accurately estimated. It is central to the ‘‘risk-based corrective action” proposed for Alaska. It is also needed where monitored natural attenuation is proposed, or where engineered controls such as permeable reactive barriers are to be implemented, such as in Antarctica.
1.5.2.2 Soil ecotoxicology
The dynamic relationships that exist between the biotic and abiotic com- ponents of the soil ecosystem are often changed by petroleum hydrocarbons, and soil ecosystem function is strongly linked to properties such as soil composition and structure. The current goal in polar ecotoxicology is to test and evaluate how sensitive organisms are to contaminants, and to relate toxic responses to ecosystem resilience (discussed below). The ideal way to undertake such sensi- tivity evaluations is to use organisms or criteria that can be quantified across a wide range of environment types (tropics to poles). In high polar environments,
the biotic components of the soil ecosystem are often relatively simple and of limited diversity (Wall and Virginia 1999), and the types and abundance of poten- tially suitable test organisms are low. For this reason soil microorganisms and invertebrates are likely to be the best indicators of petroleum hydrocarbon toxi- city in polar soils.
The terrestrial ecological toxicology of hydrocarbons in polar regions needs to take into account unique polar attributes that influence exposure. For example, the effect of temperature on toxicity of petroleum hydrocarbons is not known. It is commonly assumed that decreased temperature will result in decreased exposure due to lower biological activity (i.e. decreased contaminant uptake). However, some soil invertebrates (e.g. oribatid mites) remain active at tempera- tures well below that of other invertebrates (Hayward et al. 2003), and the effect of soil water freezing on the concentration of contaminants in the residual unfrozen water, or NAPL, and the second-order effects that this might have on toxicity is not known. In addition, hydrocarbons have a narcotic mode of toxic action (they disrupt cellular membranes). Regulation of cellular membrane com- position is critical to the survival of some polar organisms, and it is not clear if this disruption could yield lower toxicity thresholds. There are currently no data on terrestrial toxicological temperature dependence and further understanding of soil variability and its influence on toxicity is required to develop experimen- tal design parameters that will result in protection of polar environments. The priority is to identify native species that are suitable test organisms, and expose these organisms to petroleum hydrocarbon contamination for a range of soil types and environmental parameters, which will allow quantitative derivation of toxicity thresholds.
1.5.2.3 Ecosystem resilience to contamination
The toxicological effect of petroleum contaminants on one or more par- ticular species can have follow-on effects at community or ecosystem levels. In the cold regions, microorganisms and invertebrates drive ecosystem processes. They influence and are influenced by landform development, with features such as slope angle, aspect, cryoturbation, and snow/ice accumulation being critical to soil formation and the development of fluvial systems. The polar regions share a number of landscape similarities, however differences between the Arctic and Antarctic regions do exist, with one of the most important differences being that the Arctic has a much larger proportion of ice-free terrestrial habitat. Ter- restrial Antarctica comprises 99.5% monotonous ice. Much of the remaining 0.5% comprises barely weathered rock in the Transantarctic Mountains, with perhaps only 0.05–0.1% of the continent being the small and rare patches of ice-free land
where soils form (Poland et al. 2003). Such ice-free oases are important habitats for the vast majority of terrestrial Antarctic life.
Patch size and proximity to remnant populations are important factors influ- encing the abilities of disturbed populations to recover from disturbance (con- tamination) events. The physical environment (e.g. substratum type, aspect, snow accumulation, drainage) provides the background mosaic of habitat variabil- ity on which is superimposed the variability caused by biological processes (recruitment, competition, predation and grazing, biogeochemical processing, and atmosphere and hydrosphere interactions).
At small spatial scales, non-sorted landscapes are often linked in terms of chemical weathering products and the flow of biological ‘‘resources” that are moved down-slope by water migration. This contrasts with sorted landscapes where cryoturbation often leads to frost boils and associated sorting patterns. In such landforms there is tentative evidence that measures of biodiversity might
be highly disconnected and heterogeneous at small scales of∼1–10 m (Kade et al.
2005; Virginia and Wall 1999). In the Antarctic, these two different landform types typically occur in the landscape separated by continental ice, periodically frozen fjords, or ephemeral snow, and are potentially disconnected at scales of 100s of meters to 10s of kilometers. At the very largest scale, the individual ice- free areas of Antarctica are separated from each other by enormous distances (more than 1000 km in some places). Thus it is quite possible that there are a series of self-contained populations isolated from each other around the coast of Antarctica.
Temperate ecosystems have been shown to be robust against random per- turbations (habitat or species loss) but can be extremely fragile when critical elements in the system are selectively removed (Dunne et al. 2002b; Rhodes et al. 2006; Sol´e and Montoya 2001). It is not known whether high-latitude ecosys- tems show similar general characteristics. Examples of this are extinctions of species that cause secondary extinctions of other species (Dunne et al. 2002a), indirect effects that can be propagated through links of several inter-species
interactions or via non-trophic interactions such as competition ( Jord´an 2001),
and species that directly interact only weakly with other species but in fact have a strong role in maintaining overall ecosystem stability (Berlow 1999). All these system-based interactions are likely to be influenced by petroleum contamina- tion, and relating these changes back at a range of spatial scales that correspond to the landscape could be important. Integrating information on the predomi- nant scales of patchiness, variability, and connectedness of polar terrestrial soil ecosystems will help determine whether these properties of ecosystem struc- ture could potentially influence the susceptibility of Antarctic soil, lake, and nearshore marine ecosystems to the effects of petroleum contamination.