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Superfund Probabilistic Risk Assessment to Characterize Uncertainty and Variability

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Environmental Protection Agency.


Notice of availability with request for comment.


The Environmental Protection Agency (EPA) has developed and is requesting public comment on a draft guidance entitled “Risk Assessment Guidance for Superfund Volume 3 Part A: Process for Conducting Probabilistic Risk Assessment (RAGS 3A).” It is available electronically on the Internet at​superfund/​pubs.htm#r. RAGS 3A addresses the technical and policy issues associated with the use of probabilistic risk assessment (PRA) in the EPA hazardous waste sites cleanup program, commonly known as Superfund. PRA, if applied appropriately, can better characterize uncertainty and variability in the risk estimates than the traditional point estimate approach. The guidance presents a recommended tiered process for conducting both human health and ecological PRA using Monte Carlo analysis, with emphasis on applying sensitivity analysis to identify important sources of variability and uncertainty in risk estimates, applying frequency distributions to characterize variability in exposure, and quantifying uncertainty in the mean contaminant concentration. The draft RAGS 3A should not be used or cited until it is finalized. RAGS 3A provides guidance to EPA staff and also to the public and to the regulated community on how EPA generally intends that the PRA be implemented to evaluate risk at more complex Superfund sites addressed under the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA). The guidance is designed to describe EPA's national policy on the use of PRA. PRA is not expected to be relevant at every site. The document does not substitute for EPA's statutes or regulations, nor is it a regulation itself. Thus, it cannot impose legally-binding requirements on EPA, States, or the regulated community. EPA may change this guidance in the future, as appropriate.

Further, Executive Order 13132, entitled “Federalism” (64 FR 43255, August 10, 1999), requires EPA to develop an accountable process to ensure “meaningful and timely input by State and local officials in the development of regulatory policies that have federalism implications.” “Policies that have federalism implications” is defined in the Executive Order to include regulations and regulatory policies that have “substantial direct effects on the States, on the relationship between the national government and the States, or on the distribution of power and responsibilities among the various levels of government.”

RAGS 3A does not have federalism implications. It will not have substantial direct effects on the States, on the relationship between the national government and the States, or on the distribution of power and responsibilities among the various levels of government, as specified in Executive Order 13132. As explained above, RAGS 3A does not impose legally-binding requirements on the States. It is a technical risk assessment guidance which discusses a statistical risk assessment approach that may be used at more complex Superfund sites. Thus, the requirements of section 6 of the Executive Order do not apply to RAGS 3A.


You may submit comments until April 21, 2000. Comments received after Start Printed Page 7551that date will be considered to the extent feasible; however, EPA will not delay finalizing the guidance in order to accommodate late comments.


You are invited to submit written comments to: EPA, Superfund Docket RAGS 3A-2, Mail Code 5202G, Ariel Rios Building, 1200 Pennsylvania Avenue, N.W., Washington, DC 20460. For cost savings the draft RAGS 3A document is available electronically on the Internet and EPA plans to print the document only after it is finalized. The Superfund Docket containing the RAGS 3A document and public comments is physically located at 1235 Jefferson Davis Highway, Crystal Gateway I Building street level, Arlington, Virginia. The docket is available for inspection between 9:00 a.m. and 4:00 p.m., Monday through Friday, excluding Federal holidays. Appointments to review the docket can be made by calling (703) 603-9232. The public may copy a maximum of 266 pages from the docket free of charge, however a charge of 15 cents will be incurred for each additional page, plus a $25.00 administrative fee.

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S. Steven Chang, Office of Emergency and Remedial Response (5204G), U.S. Environmental Protection Agency, Ariel Rios Building, 1200 Pennsylvania Avenue, N.W., Washington, DC 20460, at (703) 603-9017, by E-Mail at, or the RCRA/Superfund Hotline at (800) 424-9346 (in the Washington, DC metropolitan area, (703) 412-9810). The Telecommunications Device for the Deaf (TDD) Hotline number is (800) 553-7672 (in the Washington, DC metropolitan area, (703) 412-3323).

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The U.S. Environmental Protection Agency (EPA) responds to releases and threatened releases of hazardous substances under the authority of CERCLA. Regulations governing such responses are found in the National Oil and Hazardous Substances Pollution Contingency Plan or NCP. The process for remedy selection in the NCP generally involves performance of a remedial investigation to identify the nature and extent of contamination at National Priorities List sites. In general, sampling results and site observations obtained in the field are used in the baseline risk assessment to identify specific contaminants and exposure pathways of concern and to determine whether remedial action is warranted.

Today's Federal Register notice introduces a draft guidance on use of a tool which could evaluate the uncertainty and variability associated with risk estimates developed as part of the baseline risk assessment for hazardous waste sites. The RAGS 3A document builds upon basic concepts of risk assessment outlined in the RAGS Volume 1 (U.S. EPA, 1989),[1] the “Guiding Principles for Monte Carlo Analysis” and the “Policy for Use of Probabilistic Analysis in Risk Assessment” (U.S. EPA, 1997).[2] PRA is not a requirement, and will not be appropriate at many sites. The guidance focuses on Monte Carlo analysis as a method of quantifying uncertainty and variability in risk. Primarily targeted toward the risk assessors, it is intended to be most accessible to those readers who are familiar with risk assessment and basic statistic concepts. The development of a PRA could involve significant investment of time by the risk assessor and risk manager to determine the extent and scope of the assessment. A tiered approach to PRA is advocated, beginning with evaluating the results of a point estimate approach. Important considerations include the time required to perform the PRA, the additional resources involved in developing the PRA, the available data on exposure that will be used in the assessment, and the value added by conducting the PRA.


Probabilistic risk analysis, as exemplified by Monte Carlo analysis, has been in use since 1946. However, the application of PRA to human health and ecological risk assessment is a more recent application. As a result, the Agency believes that those using PRA analysis would benefit from development of additional guidance.

In 1997, the EPA announced the “Policy for Use of Probabilistic Analysis in Risk Assessment at the U.S. EPA” (U.S. EPA, 1997), indicating the Agency's interest in probabilistic analysis in human health and ecological risk assessment. This 1997 policy states that “It is the policy of the U.S. Environmental Protection Agency that such probabilistic analysis techniques as Monte Carlo analysis, given adequate supporting data and credible assumptions, can be viable statistical tools for analyzing variability and uncertainty in risk assessments. As such, and provided that the conditions described below are met, risk assessments using Monte Carlo analysis or other probabilistic techniques will be evaluated and utilized in a manner that is consistent with other risk assessments submitted to the Agency for review or consideration. It is not the intent of this policy to recommend that probabilistic analysis be conducted for all risk assessments supporting risk management decisions. Such analysis should be a part of a tiered approach to risk assessment that progresses from simpler (e.g., deterministic) to more complex (e.g., probabilistic) analyses as the risk management situation requires. Use of Monte Carlo or other such techniques in risk assessments shall not be cause, per se, for rejection of the risk assessment by the Agency. For human health risk assessments, the application of Monte Carlo and other probabilistic techniques has been limited to exposure assessments in the majority of cases. The current (1997) policy, Conditions for Acceptance and associated guiding principles are not intended to apply to dose response evaluations for human health risk assessment until this application of probabilistic analysis has been studied further. In the case of ecological risk assessment, however, this policy applies to all aspects including stressor and dose-response assessment.”

Based on this (1997) Policy the Superfund program is developing guidance for implementation of PRA to better characterize variability and uncertainty in fate and transport, and exposure assessment for human health and ecological risk assessments, and dose-response assessment for ecological risk assessments.


EPA welcomes feedback on today's draft RAGS 3A document. EPA will review public comments received on the draft RAGS 3A document and, where appropriate, incorporate changes responsive to those comments.

EPA is seeking public comment at this time in order to ensure hearing the widest range of views and obtaining all information relevant to the development of policy, not because doing so is a legal requirement. EPA does, however, expect to respond to the principal comments received on the draft RAGS 3A document as a matter of public information.

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Dated: February 9, 2000.

Stephen Luftig,

Office Director, Office of Emergency and Remedial Response.

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1.  U.S. EPA, 1989, Risk Assessment Guidance for Superfund: Volume 1: Human Health Evaluation Manual, Part A, Interim Final. EPA/540/1-89/002. Office of Emergency and Remedial Response, Washington, DC. NTIS PB90-155581.

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2.  U.S. EPA, 1997, “Guiding Principles for Monte Carlo Analysis.” EPA/630/R-97/001. Office of Research and Development Risk Assessment Forum, Washington, DC.

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[FR Doc. 00-3492 Filed 2-14-00; 8:45 am]