Bioaugmentation and Natural Attenuation

An interesting paradigm exists with bioaugmentation and natural attenuation (or intrinsic bioremediation) as soil treatment methods for cold regions. Bioaugmentation, while controversial and expensive, is being used because proponents report achieving cleanup in short order. On the other hand, there is a strong desire to use less expensive natural attenuation, despite knowing little about its viability in cold regions. It appears that irrespective of long-term treatment and liability, responsible parties regard low-cost remediation as highly desirable.

19.2.4.1 Bioaugmentation

Allochthonous, designer, or genetically modified or engineered microorganisms amended with an emulsion or fertilizer and an enzyme catalyst are sold commercially as bioproducts. Bioproducts have been used to remediate petroleum-contaminated sites in Alaska, Canada, Greenland, and Norway. Usually applied with repetitive tilling, practitioners claim dramatic results within a single treatment season. However, laboratory trials of various bioproducts, comparison trials of bioproducts with garden variety and arctic-blend fertilizers, and field studies, all without tilling, found that bioproducts underperformed or fared no better than the fertilizers (Venosa et al. 1992; Margesin and Schinner 1997; Whyte et al. 1999; Braddock et al. 2000; Thomassin-Lacroix et al. 2002). Bioproducts are considerably more expensive than commercially available fertilizers. With climate change, ecologists are now documenting competition and proliferation of advancing flora in boreal forests and taiga and tundra ecosystems. It stands to reason that soil ecology might also be susceptible to potentially invasive microorganisms; a better understanding of the consequences of using bioproducts in the environment must be established before their further use.

19.2.4.2 Natural Attenuation

At the edge of the cost-time relationship (Fig. 19.1) is natural attenuation, a passive remediation strategy that relies largely on intrinsic biodegradation processes by indigenous microflora. In contrast to bioaugmentation, natural attenuation is based on the principle of ubiquity proposed by Baas Becking (1934) that "everything is everywhere, but the environment selects". Degradative potential clearly exists for petroleum in cold regions, as evidenced by laboratory studies demonstrating petroleum degradation at temperatures as low as 0-7°C by bacterial isolates and soil consortia from Arctic, Antarctic and alpine locations (Whyte et al. 1997; Mohn and Stewart 2000; Yu et al. 2000; Eriksson et al. 2001; Stallwood et al. 2005; Margesin 2007). Furthermore, in Antarctic soils impacted by a 36,000 L fuel spill resulting in soil concentrations of 10,000-20,000 mg kg-1 soil, fuel degradation from 40 L to 400 L per year were observed under ambient conditions (Snape et al. 2006). However, the majority of net natural attenuation was attributable to abiotic evaporative and dispersal processes (Snape et al. 2006). Other microcosm (Mohn and Stewart 2000) and field studies (Rayner et al. 2007) indicate that biodegradation rates in cold regions may be limited by oxygen and/or nutrient concentrations. By excluding environmental manipulations that biostimu-late degradation, natural attenuation is a long-term strategy, yet is appealing for several reasons. Low cost is appealing to responsible parties. From a regulatory perspective, natural attenuation coupled with long-term monitoring (i.e., monitored natural attenuation) is attractive when disturbances from assessment and remediation pose greater risk to sensitive ecosystems than the contamination. In Alaska, natural attenuation is considered for long-term treatment at experimental contaminated sites with site-specific cleanup criteria and institutional controls. The science of monitored natural attenuation is not yet fully developed, and more long-term field studies are needed to evaluate the methods' effectiveness and duration for petroleum-contaminated soils in cold regions.

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