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.

Target risk levels dictate the degree of human health protection to be achieved by risk-based soil and groundwater cleanup standards. The degree of conservatism incorporated in the selection of applicable target risk levels significantly affects cleanup standard calculations and associated remediation action cost. This paper reviews the significance of the carcinogenic target risk level and provides background information regarding target risk values incorporated in various regulatory programs. This paper does not discuss target risk levels for non-carcinogenic chemicals.

Significance of Target Risk Levels

The target risk level serves as a starting point in the development of soil and groundwater cleanup standards. The adverse human health effect is quantified by the individual excess lifetime cancer risk (IELCR). The IELCR represents the incremental (over background) probability of an exposed individual's getting cancer (i.e., a risk occurring in excess of or above and beyond other risks for cancer such as diet, smoking, heredity). Cleanup standards calculated on the basis of excess risk limits correspond to allowable levels in excess of the background concentrations of the chemicals of concern normally present in the source media. In the "forward mode" of risk assessment, the end result of a baseline risk assessment is an estimated IELCR at the point of exposure. If the IELCR exceeds the regulatory specified target risk level, remediation measures are necessary. In the "backward mode" of risk assessment, the acceptable level of risk at the point of exposure, as specified by the regulatory authority, is used to back-calculate soil or groundwater target cleanup concentrations for the source area. If the target concentrations exceed the actual concentrations in the source area, no remediation measures are necessary.

Back-calculated cleanup standards are directly proportional to the applicable target risk level. For example, if the acceptable risk level for benzene in residential drinking water is increased from 10^-6 to 10^-4, the target concentration for benzene changes from 2.94 µg/1 to 294 µg/1 (ASTM, 1995), based on the application of standard reasonable maximum exposure (RME) factors. In practical terms, this means that persons ingesting water containing 2.94 µg/l of benzene on a daily basis for 30 years will incur an additional 1 in 1,000,000 risk of cancer, while persons ingesting water containing 294 µg/l of benzene, incur an additional 1 in 10,000 risk of cancer. Persons who do not ingest this water incur no additional risk. Soil and groundwater cleanup standards for all exposure pathways exhibit a similar sensitivity to the acceptable risk levels.

Examples Of Acceptable Carcinogenic Risk Levels In Existing Regulatory Programs

There is a general perception that a carcinogenic risk limit of 10^-6 represents a standard risk protection factor embodied in State and Federal regulations. However, careful review of historical standards reveals use of a broad range of effective risk limits, wherein 10^-6 represents a lower bound zero risk value (FSC, 1980). Also, there is an apparent historical trend for the use of greater acceptable risk levels as the conservative nature of risk assessment calculations has been recognized. Examples of existing regulatory standards are listed below:

• U.S. Food and Drug Administration (FDA): A risk-based recommended standard of 10^-8 was first proposed by Mantel and Bryan in 1961. In 1977, the FDA instead used 10^-6 as a risk-based standard for food residues of animal drugs after originally proposing the 10^-8 risk level in 1973. The 10^-6 target risk level used by the FDA for meat products represents essentially zero risk.
• Mean Regulatory Trigger: Travis et al (1987) reviewed over 132 Federal regulatory decisions and concluded that every situation involving a risk due to chemical exposure of 4x10^-3 triggered a regulation. Further, action was never taken to reduce upper-bound cancer risks below 1x10^-6. The decisions to regulate at levels between 4x10^-3 and 1x10^-6 were based on size of exposed population, technical feasibility, and costs.
• Superfund NCP: In its final National Contingency Plan for Superfund site remediation (USEPA, 1990a), EPA codified a range of acceptable risks (i.e., 1x10^-4 to 1x10^-6) as a basis for remediation of Superfund sites. The decision to use a range of acceptable risk levels and not specify 1x10^-6 as the ultimate cleanup goal was reaffirmed in subsequent Agency guidance (USEPA, 1991).
• Safe Drinking Water Act: The starting point for most MCLs is a 10^-6 carcinogenic target risk level, but many MCLs exceed the 10^-6 risk level for practical and economic reasons. For example, the Federal MCL for beryllium is based on a carcinogenic target risk level of 2x10^-4 for these reasons.
• State RBCA Target Risk Policies: A recent survey of States participating in the EPA/ASTM RBCA Training Initiative shows that roughly one-third of them have selected carcinogenic target risk limits in the range of 10^-5 to 10^-4 for development of risk-based soil and groundwater cleanup standards. Of the 14 States responding to the 1995 RBCA Policy Issue Survey, nine had tentatively selected a 10^-6 carcinogenic target risk level or a drinking water MCL as the basis for derivation of cleanup limits, while five intend to employ either a 10^-5 or 10^-4 target risk limit depending on applicable land-use conditions (Conner et al., 1997).
• EPA Hazardous Waste Management System Toxicity Characteristics Revision: In its Hazardous Waste Management System Toxicity Characteristics Revisions (USEPA, 1990a), USEPA has selected a single risk level of 1x10^-5. EPA believes that "due to the conservative nature of the exposure scenario and the underlying health criteria," a 10^-5 target risk level is the highest risk level that is likely to be experienced by an exposed population.
• Clean Air Act Amendments (1990b): The Clean Air Act , as amended, 42 USC 7412(f), contains a provision which specifically references and endorses the basis on which EPA determined acceptable risk and margin of safety in the benzene NESHAP regulations. Under this policy, EPA considers all relevant risk factors, including uncertainty in the risk estimates, with a presumptive risk of approximately 1 in 10,000 (10^-4) in making acceptable risk decisions under Section 112 (hazardous air pollutants) of the Clean Air Act (USEPA, 1989).
• National Council on Radon Protection: While evaluating radon exposures, the National Council on Radiation Protection and Management adopted a remediation action level corresponding to a lifetime risk of 2.8x10^-2. This was adopted to balance risk and feasibility. A lower level would have resulted in "a great societal cost."

Thus, although the common belief is that Federal regulations are written with an acceptable risk of 10^-6, this is not true as is clear from the above examples.

Options For Defining An Acceptable Carcinogenic Level Of Risk

Considering that 10^-6 is an arbitrary number as is any other number (e.g., 10^-5 or 10^-4), the following are a few policy options available to the decision makers:

• A single specified acceptable risk level applicable for all carcinogens (i.e., the Bright-line standard such as 10^-6, 10^-5).
• Different specified risk levels depending on the weight-of-evidence classification of a chemical (i.e., 10^-6 for A carcinogens, and 10^-5 for B and C carcinogens, or 10^-5 for A carcinogens and 10^-4for B or C carcinogens).

A range of acceptable risk levels (e.g., 10^-4 to 10^-6) with accompanying discussion as to how the range is to be used (e.g., land use).

Lower acceptable risk level for actual exposures, higher acceptable risk level for potential future exposures. This may be justified on the basis that most of the petroleum-derived chemicals are known to naturally attenuate.

References

American Society for Testing and Materials (ASTM). 1995. Standard Guide for Risk-Based Corrective Action Applied at Petroleum Release Sites. ASTM E 1739.

Conner, J.A., R.K. McLeod, and J.P. Nevin. 1997. State RBCA Policy Issue Database, American Petroleum Institute.

Food and Drug Administration (FDA). 1973. Compounds used in food-producing animals. Procedures for determining acceptability of assay methods used for assuring the absence of residues in edible products of such animals--proposed rule. Federal Register 38:19226-19230.

Food and Drug Administration (FDA). 1977. Compounds used in food-producing animals. Criteria and procedures for evaluating assays for carcinogenic residues in edible products of animals. Federal Register 42:10412-10437.

Food Safety Council (FSC). 1980. Fd. Cosmet. Toxicol. 18:711-734.

Mantel, N. and W. Bryan. 1961. Safety testing of carcinogenic agents. J. Natl. Cancer Inst. 27:455-470.

Travis, C.C., S.A. Richter, E.A. Crouch, R. Wilson, and E. Wilson. 1987. Cancer risk management: A review of 132 Federal regulatory decisions. Environmental Science and Technology 21(5):415-420.

US Environmental Protection Agency (USEPA). 1989. National emission standards for hazardous air pollutants: Benzene emissions for maleic anhydride plants, ethylbenzene/styrene plants, benzene storage vessels, benzene equipment leaks and coke by-product recovery plants--final rule. Federal Register 54:38044.

US Environmental Protection Agency (USEPA). 1990. Hazardous waste management system toxicity characteristics revisions. Federal Register 55:11798-11863.

US Environmental Protection Agency (USEPA). 1990a. National oil and hazardous substances pollution contingency plan--final rule. Federal Register 55:CFR Part 300:666.

US Environmental Protection Agency (USEPA). 1990b. Clean Air Act. 42 USC 7412(f).

US Environmental Protection Agency (USEPA). 1991. Role of the Baseline Risk Assessment in Superfund Remedy Selection Decisions, OSWER Directive 9355.0-30.

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