Underground Storage Tanks

PIRI's (Partnership in RBCA Implementation) official members include representatives from the major oil companies (i.e., Amoco, BP, Chevron, Exxon, Mobil, and Shell), ASTM, EPA's OUST, and States. The paper presented here represents the points of view of the various PIRI authors and not the companies or states with which they are affiliated. This document is not an EPA document. This document will be updated as changes in technologies and policies require. It is critical that readers check with the implementing agency in their States before acting upon the policies discussed in these papers.

There are a multitude of technical and policy issues associated with using natural attenuation to remediate leaking USTs containing petroleum. The U.S. Environmental Protection Agency (EPA) believes that natural attenuation is a viable option for remediating leaking UST sites and that it is compatible with the risk-based corrective action approach. The majority of States allow natural attenuation, and most State Funds reimburse for associated expenditures. Some State implementing agencies do not, however, allow the use of natural attenuation.

Background

The use of natural attenuation at UST sites with petroleum releases has increased significantly over the past few years. As of 1995, natural attenuation is the second most popular option for soil sites; it is being used at roughly 29,000 sites or at 28 percent of the active contaminated soil universe. Natural attenuation is the most common treatment option at groundwater sites; it is being used at 17,000 sites or about 47 percent of the active contaminated groundwater universe.

The first issue for natural attenuation is whether or not it is accepted as a remediation option by a State. Although most States allow the use of natural attenuation, there are at least seven States that do not allow it as the sole remediation option. About 43 States have established or are planning to establish guidance, regulations, or statutes on natural attenuation. At this time, MTBE does not appear to readily biodegrade. EPA, therefore, does not currently believe that natural attenuation is an appropriate remediation option for MTBE. EPA encourages States to incorporate MTBE as a chemical of concern into their RBCA programs. About 10 States do not allow natural attenuation if MTBE is a chemical of concern; this number may begin to increase as more States start testing for the presence of MTBE. However, the presence of MTBE at a site should not preclude natural attenuation as a remedy for other contaminants (e.g., BTEX) at a site.

Natural attenuation is one of several remediation options possible at a site; it is not a "default" option. As with any remediation option, natural attenuation should be evaluated for its appropriateness based on the risks, the site characteristics, and the potential to achieve remediation objectives at each individual site.

The next issue is "how to implement" the use of natural attenuation. EPA recently released an EPA/OSWER Policy Directive entitled Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites to promote consistency in how monitored natural attenuation remedies are proposed, evaluated, and approved. The Directive provides guidance to EPA and state staff, to the public, and to the regulated community on how EPA intends to exercise its discretion in implementing national policy on the use of Monitored Natural Attenuation. ASTM is developing a Standard Guide for Remediation by Natural Attenuation at Petroleum Sites and plans to release it soon. These documents will assist States in developing processes and criteria for implementing natural attenuation as a remedy.

Natural Attenuation Versus No Further Action

If natural attenuation is chosen as a remediation option, then, by definition, the site is not "protective [of human health and the environment]." The site would be designated as a No Further Action (NFA) site if it were "protective [of human health and the environment]"; hence, work is still needed at the site. Natural attenuation is an active process that includes site characterization, risk assessment, and monitoring of remediation progress. EPA believes that free product removal is essential when remediating by natural attenuation and that the cleanup is not complete until the site has reached the cleanup levels of the State or local implementing agency. EPA also believes that it is important for the implementing agency to include contingency measures in its planning, in case natural attenuation is shown through monitoring to be unable to meet the cleanup levels.

Of course, States can and do incorporate flexibility in how they implement and oversee free product removal. One must remember that the Federal UST regulations pertain only to Federally regulated USTs that contain petroleum (and hazardous substances). Petroleum from other sources is not subject to the Federal UST regulations, although EPA's Oil Pollution Act, Resource Conservation Recovery Act (RCRA), or Safe Drinking Water Act (SDWA) may apply. In addition, the Federal UST regulations state that "...owners and operators must remove free product to the maximum extent practicable as determined by the implementing agency." For example, site-specific (e.g., geological) conditions may render free product removal impractical. In this case or at other sites as determined by the implementing agency, free product removal would probably be assessed within a risk-based framework base that evaluates proximity to potential receptors, land use, groundwater classification, and other factors as determined by the State or local implementing agency. EPA recommends that the responsible party work closely with the implementing agency to ensure that it complies with State requirements.

Site Characterizations

As with any remediation alternative, an adequate site characterization is essential. Three types of site-specific information or "lines of evidence" may be used: Primary, secondary, and tertiary. Each is described below.

• Primary lines of evidence are data from historical groundwater and/or soil chemistry samples that demonstrate a clear and meaningful trend of declining contaminant mass and/or concentrations at appropriate monitoring or sampling points. Primary lines of evidence are used to determine whether plumes are shrinking or stable.
• Secondary lines of evidence include data from the site characterization that indirectly demonstrate the type of natural attenuation processes active at the site and determine the rate at which such processes will reduce contaminant concentrations to required levels. For example, the rate of biodegradation can be indirectly determined by measuring the levels of dissolved oxygen and nitrate, iron (II), sulfate, methane, carbon dioxide, and other parameters.
• Tertiary lines of evidence include data from field or microcosm studies (conducted in or with actual contaminated site media) that directly demonstrate microbial activity in the soil or aquifer material and its ability to degrade the contaminants of concern.

EPA recommends collecting both primary and secondary lines of evidence unless there are sufficient historical data (as determined by the State or local implementing agency) to adequately characterize the site. The amount and type of information necessary will depend upon site-specific factors including the hydrogeological settings, the size and nature of the problem, the existence and proximity of any receptors, and the potential risk to receptors. For petroleum sites, EPA does not expect that tertiary lines of evidence will be necessary. ASTM is developing similar recommendations. Nineteen States currently require secondary lines of evidence (i.e., geochemical indicators) for natural attenuation.

Remediation Objectives

Every remediation action plan must have clearly defined objectives. These objectives include identifying cleanup levels or performance requirements, determining points of compliance, and establishing acceptable timeframes for cleanup. Once the remediation objectives are established, natural attenuation can be evaluated for suitability as a remediation alternative. Natural attenuation may be the sole remediation option, a component of the remediation action plan, or unsuitable--depending on the site.

Performance Monitoring

EPA emphasizes that performance monitoring is essential when relying on natural attenuation as a remediation option. Performance monitoring is needed to demonstrate that natural attenuation is occurring according to expectations, to detect contaminant migration or any new releases from the site, and to verify the attainment of cleanup objectives. EPA stresses that performance monitoring is required as long as contamination levels remain above required cleanup levels at points of compliance as determined by the State implementing agency. The majority of States require quarterly monitoring generally for a period of 1 to 3 years or until definite trends are established or standards are attained.

Practical and Economic Issues

The potential advantages of natural attenuation include: The generation of less remediation wastes; less disruption of the environment; ease of use in conjunction with other remediation technologies; no equipment down time; and likelihood of lower overall costs than from active remediation. There are, however, a number of disadvantages including: Longer timeframes may be required to achieve remediation goals; the public may not perceive the process correctly; site characterizations can be more costly and complex; responsibility must be assumed for performance monitoring; the potential exists for continued migration; and implementing a contingency plan that uses a more active remediation alternative might be necessary if natural attenuation fails.

The disadvantages can have economic implications: Property transfers may be delayed or stopped; longer timeframes may not be compatible with future land uses; active remediation may be more economical than performance monitoring at certain sites; and liability could be an issue for present and future land users. In addition, the owner and operator may find it more difficult to obtain insurance. EPA and the States are working closely with the private sector to overcome some of these issues, but many of them will need to be resolved on a site-specific basis.

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