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.

The term "off-site" refers to properties or areas that are not within the boundaries of the property on which the release occurred or where the source area(s) is located. The primary driver (from a pure risk-based corrective action perspective) for considering the off-site movement of a chemical of concern is the actual or potential exposure of a receptor to a chemical of concern at a concentration that presents an unacceptable risk. There are, however, a number of non-risk-related issues that impact (and many times drive) the decisions associated with off-site movement. These issues include:

• "Trespass" or the concept that the chemical of concern was caused to exist off-site without the permission or consent of the property owner;
• "Property devaluation" or the concept that the property value has been lessened by the stigma associated with the presence of the chemicals of concern on a property even though the presence of that chemical does not present a risk;
• "Diminished use" or the concept that the property can not be fully utilized or that its use is partially restricted by the presence of chemical of concern; and
• "Nondegradation" or the concept that the environment has been degraded and should be restored to its original quality even though the chemicals of concern do not present a risk. (This is particularly true for ground water.)

Off-site movement is a difficult issue for a State program to address because it is driven not only by protection of human health and the environment but also by subjective criteria, emotional responses, and non-health-based considerations. From a RBCA perspective, the ideal approach to this issue is to address it based purely on the actual or potential risk that is posed by the chemical of concern, leaving the other issues to be resolved among the stakeholders under common law principles. This approach addresses the acceptable risk associated with a source area based on known or reasonably anticipated point(s) of exposure. Although this approach provides considerable flexibility, implementing it may be difficult because of current State policies (e.g., nondegradation) and a general reluctance to accept the concept of site-specific acceptable concentrations of chemicals of concern in the groundwater or soil.

Different Approaches For State Programs

As described in the following paragraphs, there are several alternate approaches being considered by State programs. The most restrictive approach is to establish groundwater standards (i.e., apply drinking water standards or background levels) and require that all groundwater, on or off the site, meet these standards. This approach does not take into account the use of the groundwater, the potential for exposure to the chemicals of concern, or the technological limitations of remediation methods. It requires the implementation of a complex and sometimes extensive remediation action system to reduce concentrations of chemicals of concern in the soil and groundwater simply to achieve the standard. It assumes that all soil and groundwater can be restored to pre-release condition.

A State could allow a demonstration showing that existing levels of chemicals of concern do not present a current or future unacceptable risk but requiring that concentrations of chemicals of concern in the groundwater ultimately reach established groundwater standards within an identified timeframe through natural attenuation. This approach recognizes that natural attenuation processes will reduce the concentrations of chemicals of concern over distance and time. This approach does, however, require that an established groundwater standard be achieved; typically, it will require the implementation of a remediation action plan if the standard cannot be achieved in the timeframe identified.

A State could assume that the property line is the point of exposure for the purpose of calculating site-specific target levels at the source area. This approach recognizes the use of a point of exposure (e.g., a drinking water well, surface water) but artificially fixes the location of this point rather than specifically establishing it at the actual or reasonably potential future point where a receptor may be exposed to the chemicals of concern. For smaller sites or sites where the source is close to the property line and there is no likely future receptor in close proximity, this approach is not significantly different from the one above.

A State could assume an arbitrary point of exposure set at a defined (limited) distance (often set as groundwater travel time) down-gradient of the source, regardless of property size, for the purpose of calculating site-specific target levels at the source area. This approach recognizes both the difficulty of establishing reasonably potential future points of exposure and the problems associated with using the property line as a point of exposure. It still does not address actual points of exposure unless they are within the defined distance; it could, therefore, be conservative in some cases.

Policy Considerations

There are three considerations or policy decisions that must be evaluated in order to define an approach to the off-site movement of chemicals of concern: Land use, groundwater use, and point(s) of exposure. Land use (current and reasonably potential future) includes the land use of the site and the surrounding property. Determination of land use should be based on the following factors:

• The current land use of the property;
• The current land use of properties immediately adjacent and across any streets from the property;
• The current zoning or planning designation for the property;
• The current zoning or planning designation for the surrounding properties;
• Reasonably potential future land use of the property using the duration of exposure as the maximum time into the future to evaluate; and
• Reasonably potential future land use of the surrounding properties using the durations of exposure as the maximum time into the future to evaluate.

Groundwater use (current and reasonably potential future) includes the quality and potential use of the groundwater (groundwater classification) as a drinking water source and State nondegradation policies. While this determination can be linked to the use of the property, it is typically addressed as a separate issue. Determination and documentation of groundwater use could include the development of a groundwater classification system based on characteristics such as urban setting, yield rates, existing quality (both ambient chemicals resulting from human activities and natural quality as measured against the primary and secondary drinking water standards).

Determination of the location of the point(s) of exposure, specifically when a known or reasonably potential future receptor is not identified, could include:

• An actual or known location of a current or future receptor;
• The determination of an assumed location for a receptor in cases where future use is likely, but the exact circumstances for that use cannot be determined; or
• A calculated position based on a time/distance relationship (e.g., groundwater travel time).

It is important that a RBCA program identify these three policy decisions to ensure a consistent approach that clearly defines unacceptable risk.

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