Large areas of surface soil from remote, cold-climate regions have been contaminated with petroleum and are difficult to remediate. There are many such Department of Defense sites in Alaska. These sites are generally not easily accessible, increasing the costs of mobilization and demobilization. They also have limited infrastructure and are subject to harsh winters that can cause equipment failures. Conventional cleanup strategies are sufficiently costly to limit their use at these sites, yet there are few remedial alternatives.
The rhizosphere-enhanced remediation of petroleum in surface soils was evaluated as a possible effective solution to this problem. Rhizosphere-enhanced remediation is a subset of phytoremediation, which includes techniques for many contaminants and many remediation mechanisms. The mechanisms active in rhizosphere-enhanced remediation are based on enhanced microbial activity at the root-soil interface. For petroleum remediation, plant uptake is not thought to be significant. Rather, the process is believed to be driven by at least three factors: (1) "pseudo-mixing" of soil due to root exploration, (2) analogue enrichment from carbon compounds released by the root or during periods of plant senescence, and (3) greater contaminant solubility due to biosurfactants or pH changes near the root surface.
The results from this project are some of the first cold-regions field data to become available that use scientifically defensible techniques to confirm that plants have a positive effect on petroleum depletion relative to either nutrients alone or control treatments. The data shows that plant-associated effects do occur but not uniformly for all petroleum fractions, and the effects are greatest for more recalcitrant petroleum fractions. Because rhizosphere-enhanced treatment of surface soils is a long-term strategy, using standard analysis techniques to monitor sites may lead users to conclude that rhizosphere treatment is not working. This field data supports both theory and laboratory data, and it demonstrates that the treatment is working.
The benefits of rhizosphere-enhanced remediation include low costs, applicability to large areas, minimal or no infrastructure needs, the self-repairing nature of plant growth, generally high public acceptance, and reduced dust and runoff. It may be an option for surface-soil treatment of petroleum in situations where no other options exist, such as remote sites and live-fire training ranges, or at other sites where costs limit options such as Brownfields. (Project Completed - 2004)