Remediation of Petroleum-Contaminated Soil--Summary

Millions of underground storage tanks (USTs) have been installed over the years at various locations and facilities. A significant percentage of these UST systems have leaked and released their stored contents, usually petroleum products, into the environment. To minimize the threat of groundwater contamination, strict federal, state, and local regulations require the owners and operators of these petroleum storage tanks to clean up the environmental damage that is caused by past leaks and spills.

A state transportation agency that has underground storage tanks containing petroleum products (e.g., gasoline, diesel fuel, heating oil) at its vehicle fueling sites or maintenance facilities may be affected by these requirements. Many transportation agencies have been found responsible for cleaning up the petroleum-contaminated soil and/or groundwater from past facility operations and practices that resulted in petroleum leaks or spillage. Many agencies have also become responsible for cleaning up the petroleum contamination from leaking storage tanks they encounter when acquiring the land for, or during the construction of, new transportation facilities.

This synthesis describes the technologies that may be available to transportation agencies faced with the regulatory responsibility to clean up or remediate petroleum- contaminated soils at a site. Some of these remediation technologies remove and destroy i.e., treat the petroleum contaminants that exist in the soil, while others are designed only to contain the movement or leaching of the contaminants. The technologies differ from one another by the type of process that is employed e.g., biological, physical, chemical, thermal, or containment to accomplish cleanup objectives. In addition, some remediation technologies can be applied in-situ, or while the soil is still in the ground; others are ex-situ technologies that require soil excavation before deployment. A total of 18 different soil remediation technologies are described: passive biodegradation, bioventing, in-situ biodegradation, biopiles, land treatment or landfarming, slurry biodegradation, soil venting, soil flushing, soil washing, coal tar agglomeration, solvent extraction, radio frequency heating, vitrification, thermal desorption, solidification/stabilization, capping, land disposal (landfilling), and incorporation in cold-mix asphalt.

A survey found that approximately 90 percent of the petroleum-contaminated sites being cleaned up by state transportation agencies involve gasoline and/or diesel fuel- contaminated soils. Transportation agencies have also been responsible for remediating soils containing kerosene and heavier weight petroleum products, but at far fewer sites. It was also found that most of the petroleum-contaminated sites undergoing cleanup by transportation agencies contain relatively small volumes of contaminated soil. Approximately 62 percent of all the sites remediated by transportation agencies during the 2 years prior to a fall 1994 survey conducted for this synthesis have involved less than 380 cubic meters (m3) (500 cubic yards, yd3) of contaminated soil; 75 percent of the sites have involved less than 765 m3 (1,000 yd3) of soil.

Because most sites contain readily volatile petroleum contaminants in relatively small amounts, state transportation agencies have been able to select from and employ a broad range of alternative soil remediation technologies. The following conditions and practices are found to exist:

• Many state transportation agencies place petroleum-contaminated soils in landfills, particularly in states where such practice is allowed by the environmental regulatory agency, and the cost of landfilling is not too high. The practice of landfilling occurs despite the long-term liability that may accompany this activity.
• Where landfilling is not allowed or is cost prohibitive, and air quality conditions are satisfactory, landfarming (including use of biopiles) often becomes the most cost-effective soil remediation technology choice to a transportation agency. This appears to be particularly true in the South and West where temperatures are suitable for biodegradation and sufficient land area is available.
• The thermal treatment of contaminated soil in modified hot-mix asphalt plants or low temperature strippers is also performed by many state transportation agencies. Thermal treatment technologies appear to be used most frequently by highway agencies in northern states with climates that are not conducive to landfarming. Although the cost of thermal treatment is typically greater than the cost of landfarming, environmental conditions in many states render landfarming either impractical or cost prohibitive because of the need to construct and maintain temperature-controlled enclosures.
• When allowed by state regulators, some transportation agencies use a number of biological treatment technologies other than landfarming to remove petroleum contaminates from the soil. Natural biodegradation is allowed by some state environmental regulatory agencies. In general, however, biological treatment technologies are not used extensively by transportation agencies because: (a) low temperature and other conditions make biodegradation impractical in many geographic areas, (b) biotreatment is not yet widely accepted by many environmental regulatory agencies (although this is changing rapidly), (c) biological treatment is often costly due to the need for extensive site characterization studies and treatability tests, and (d) in-situ biotreatment is simply not very practical when there are small amounts of contaminated soil involved and insufficient land or time available because of project scheduling demands.
• Transportation agencies rely more on ex-situ technologies than they do on in-situ soil remediation techniques. Faced with project deadlines and the unavailability of suitable space for treatment within the right-of-way, there often remains no alternative but to excavate the soil and perform off-site treatment. The high rate of ex-situ treatment may also be a reflection of the fact that transportation agencies continue to discover some petroleum-contaminated sites late in the project development process. In addition, even with the cost of soil excavation and transport, ex-situ treatment is often the lowest cost option available to an agency when dealing with relatively small volumes of contaminated soil.

The cleanup practices of state transportation agencies at petroleum-contaminated sites were found to mirror closely those used by other responsible parties to remediate UST sites nationally. However, the cost and regulatory feasibility of soil remediation technologies cannot be considered static. In terms of cost, for example, it is unclear how much longer petroleum-contaminated soils will be welcome at many landfills. While relatively inexpensive now in many geographic areas, the cost of landfilling petroleum-contaminated soils will inevitably rise as landfill space becomes more and more scarce and landfill operators become more selective in what they accept. As the cost escalates, many transportation agencies may find that land disposal no longer emerges as the most cost-effective technology at even small petroleum-contaminated sites.

The regulatory environment surrounding petroleum-contaminated sites is also changing. Biological treatment technologies are becoming more widely accepted by many environmental agencies as the risks of petroleum contamination are better understood. State environmental regulations and policies that have hindered or prohibited the use of certain soil remediation techniques are being re-evaluated. In addition, many states are now adopting approaches to the establishment of site cleanup levels that are site-specific, risk-based, and do not use total petroleum hydrocarbons (TPH) as the indicator parameter. As more state environmental agencies embrace such chemical-specific risk-based cleanup guidelines, the use of biological treatment methods (including natural attenuation) by transportation agencies at many of their sites may be viewed more favorably. This change in regulatory attitude may provide transportation agencies with increased opportunity to argue for the controlled "re-use" of lightly contaminated soils as suitable subbase or other construction material within the limits of their projects.

The regulatory and technology related information that is needed to select the most cost-effective soil remediation technology for a site is constantly changing. The relative cost-effectiveness of alternative technologies differs not only from site to site, but also over time as the cost of existing treatment and disposal options change and new options (e.g., permitting of local asphalt plants to accept petroleum-contaminated soils) become commercially available in a particular geographic area. Transportation agencies and their consultants are encouraged to assemble and use the most up-to- date cost and other information available as they undertake the technology evaluation and selection process at each petroleum-contaminated site they encounter.

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