The RBCA Success Story

It has been five years since the American Society of Testing and Materials (ASTM) issued their Risk-Based Corrective Action (RBCA or "Rebecca") standard for remediating contaminated sites. Since then, the cleanup landscape for underground storage tanks (USTs) has been significantly altered. Most states have changed their approach to UST management and have implemented a risk-based decision making (RBDM) process into their corrective action programs for USTs. Additionally, most states have developed guidance for evaluating natural attenuation as a potential remedial technology that can be used to achieve the risk-based site cleanup goals.

Natural attenuation refers to the physical, chemical and biological capacity within the aquifer system for assimilating or reducing hazardous chemical concentrations. The paradigm shift to RBCA and the evaluation of natural attenuation in the remedial technology selection process has allowed many states to reduce their backlog of "active" UST sites and to allocate resources more efficiently among sites that represent a higher risk to human health and the environment.

Risk-based decision making (RDBM) and risk-based corrective action (RBCA)

The number of UST sites requiring corrective action has rapidly increased in recent years. As of March 31, 2000, more than 742,805 USTs requiring corrective action had been reported. Cleanups had been initiated at 357,268 sites and completed at 241,858 of them , which leaves more than 110,000 cleanups still underway. Forty-six states have established state financial assurance UST funds and even though these funds represent more than $1.3 billion a year, many are facing solvency problems due to the rapid increase in reimbursement requests.

To help state and local agencies make cleanups faster, less expensive and more effective, EPA is encouraging agencies to incorporate risk-based decision-making (RBDM) into their corrective action programs. When applied to UST corrective action, RBDM is also referred to as Risk-Based Corrective Action (RBCA). An RBDM program typically includes three main activities:

  • prioritization of sites based on the timing or magnitude of potential impacts;
  • determination of site-specific, risk-based remediation goals; and
  • remedy selection based on exposure control and technical feasibility.

RBDM is "a science-based process that offers clearly defined and consistent basis for site evaluation and remediation. As a result, implementation of RBDM corrective action programs is expected to result in increased program efficiency and improved risk reduction."

Risk-Based Corrective Action (RBCA) refers to a systematic, tiered method for managing environmental contamination (Figure 1). In the RBCA approach, decisions related to source allocation, urgency of response, cleanup levels and remediation alternatives are based on the evaluation of the potential risk to human health and the environment. The goals of a RBCA process, according to EPA, are as follows

  • Protection of human health and environment;
  • Practical, cost-effective application of risk-based decision-making; and
  • Consistent and technically defensible administrative processes.
Figure 1.
Overview of 3-tiered RBCA evaluation process, explained below




Tier 1

Look-up Tables

Scope: Generic risk-based screening levels for exposure at source zone.

Data needs: Requires max source zone concentrations and receptor identification

Tier 2

Analytical Models

Scope: Site specific target levels to protect human health due to exposure at source zone or at separate point(s) of exposure

Data needs: Requires affected media delineation to POE site specific transport & receptor data

Tier 3

Numerical Models

Scope: Site specific target levels to protect human health due to ecological resources at complex points of exposure

Data needs: Requires affected media delineation to POE detailed site-specific data on transport mechanisms and receptors

RBCA integrates EPA risk assessment practices with traditional site assessment and remedy selection in order to find a sufficiently protective, cost-effective alternative to site closure. A detailed framework for taking risk factors into account in making corrective action decisions has been developed by ASTM . EPA considers the ASTM standard as a good starting point for the development of a risk-based process tailored to applicable state and local laws and regulatory practices. One limitation that UST implementing agencies must take into account when using ES-38-94 standard is that it deals exclusively with human health risks (there will, of course, be cases in which ecological risks have to be considered in establishing cleanup goals).

Status of RBCA implementation

With support from EPA and other organizations, the ASTM has developed tools (e.g., worksheets and software) and a training program to help UST implementing agencies understand the concepts of risk-based decision-making and the ASTM standard. To date, 49 states/territories have adopted RBCA, with 41 states in the implementation design phase (U.S. Environmental Protection Agency, 1998c). Table 1 Click to view includes the status of the RBCA program for some fo the states. Seven states/territories (N.D., Nev, Vt., Md., Guam, the Commonwealth of the Northern Mariana Islands and American Samoa) have abstained from entering the RBCA program. Additionally, several states have incorporated elements of the RBCA program to areas other than USTs (16 states apply RBCA to Voluntary Corrective Action VCA, seven to Brownfields reclamation, six to Superf und pro grams an d seven to RCRA programs).

In the RBCA approach, decisions related to source allocation, urgency of response, clean up levels and remediation alternatives are based on the evaluation of the potential risk to human health and the environment.

More recently, the EPA and ASTM conducted an RBDM performance assessment study to evaluate the impact of risk management strategies on corrective action programs of five pilot states (Texas, Utah, North Carolina, Iowa, and Illinois). The EPA study detemined that in the majority of the pilot states, implementation of a RBCA program resulted in an increase in case closures and a decrease or stabilization in case backlog. Additionally, the study showed that these RBCA programs successfully targeted low-risk sites for closure while retaining higher-risk sited for further action.

Figure 2.
Impact of RBCA implementation on Site Cleanups

In addition to the five states evaluated in the above mentioned study, Michigan and Alabama have independently evaluated the performance of their LUST programs after the implementation of RBCA. The Alabama Department of Environmental Management (DEM) tracked a reduction of 106 active cases classified as low-risk sites and a corresponding increase of 115 closed sites (4 percent of active tanks) in the first year of the program (RBCA was implemented in April 1998). On the other hand, in 1996, the Michigan Department of Environmental Quality (DEQ) reported a 61 percent increase in LUST case closures compared to the average case closure rate for 1990 to 1995. In addition, a 30 percent decrease in case backlog from 1995 to 1998 was achieved in Michigan. Furthermore, the Michigan DEQ reported a 24 percent average reduction in remediation/closure costs for UST due to the implementation of the RBCA program .

Impact of RBCA on UST cleanup

The use of RBCA has shifted the remediation goals from concentration-based target levels to performance-based target levels. Instead of trying to achieve a given concentration in soil and groundwater at every point of a site, cleanup goals are determined based on plume containment or achieving a given risk-based concentration at a point of compliance downgradient of the site . This approach is aimed at answering how much contamination we should clean up, rather than how much contamination we can remediate. The implementation of RBCA programs has sparked an increase in cleanups completed between 1995 and March 2000 as compared with the number of cleanups completed between 1990 and 1995 (Figure 2) for most EPA regions. Overall, the number of remediation cases completed in the period 1995-2000 is approximately 139,000 cleanups compared with 104,000 cleanups in the preceding 5 years).

Natural attenuation in the context of RBCA

Natural attenuation, also referred to as intrinsic remediation, has been recently accepted as a feasible cleanup alternative for sites contaminated with petroleum hydrocarbons and other chemicals. A number of organizations have dedicated significant resources to develop guidance on implementing natural attenuation.

Remediation by natural attenuation relies on the physical, chemical and biological processes that contribute to the decrease of both the contaminant concentration and plume size. Natural attenuation involves the use of the indigenous microorganisms at polluted sites to detoxify and degrade environmental contaminants through their physiological and metabolic capabilities. Extensive analysis of these intrinsically occurring processes is required to assess the time required for cleanup and the extent of future migration of the contaminant plume. Natural attenuation processes include biodegradation, dilution, dispersion, sorption and abiotic (related to nonliving elements such as climate, soil and water) reactions. The large number of contaminated sites across the United States and great costs of most other remediation procedures as well as the lack of efficient remediation technologies for some of these sites make intrinsic remediation an attractive option when its use can be justified.

EPA believes that natural attenuation is a feasible alternative for remediating leaking UST sites and that it is compatible with the risk-based corrective action approach. While the majority of states allow natural attenuation (29 according to GSI, 1998), there are at least seven states that do not allow it as a stand-alone remedial option. Natural attenuation, as any other remediation option, requires an assessment of its appropriateness based on the risks, site characteristics and the potential to achieve remediation goals in a case-by-case basis.

The use of RBCA has shifted the remediation goals from concentration-based target levels to performance-based target levels.

It is important to point out that natural attenuation should always be accompanied by an analysis of the risk the contaminant left in place poses to human health and the environment. Sites with high risk of negative impacts are usually not susceptible to remediation by natural attenuation mainly because the time frame for remediation is usually much longer than that for an engineered remediation.

MTBE and the RBCA-natural attenuation paradigm

The Clean Air Act (CAA) requires that reformulated gasoline (RFG) contain two- percent oxygen by weight in order to reduce the emission of numerous air pollutants. A widely used gasoline oxygenate is methyl tertiary butyl ether (MTBE)-more than 85 percent of RFG contains this oxygenate. While beneficial to air quality, the use of MTBE in the program has resulted in growing incidences of MTBE in drinking water, with about five to ten percent of drinking water supplies located in high oxygenate use areas showing at least detectable amounts of MTBE. The major source of groundwater contamination seems to be releases from leaking USTs containing gasoline.

Several states have incorporated elements of the RBCA program to areas other than USTs, such as corrective actions, brownfield reclamation, superfund and RCRA programs.

MTBE presents a challenging issue for the RBCA approach since MTBE, due to its persistence and mobility in water, is more likely to contaminate ground and surface water than the other components of gasoline. At this time, MTBE does not appear to be readily biodegradable. Therefore, EPA, does not consider natural attenuation as an appropriate remediation option for this compound . EPA, however, encourages states to incorporate MTBE as a chemical of concern into their RBCA programs. To date, at least 10 states do not allow natural attenuation if MTBE is a chemical of concern. Nevertheless, the presence of MTBE at a site should not exclude natural attenuation as a remedial alternative for other contaminants present at a site.

The need for developing MTBE plume management strategies has never been greater. MTBE production is a key economic component for many states and concerns associated with MTBE contamination would have many implications for the RBCA programs. Industry and regulatory and research communities are actively seeking low-cost remediation technologies that would be appropriate for managing MTBE plumes.


RBCA is a program based on an evaluation of potential impacts of a given plume on human health and the environment. This approach allows environmental agencies to classify contaminated sites according to their level of risk in order to prioritize the allocation of limited resources as well as to expedite the process of cleanup while being protective of the environment. Forty-nine states/territories have implemented (or are implementing) RBCA at a local level.

The use of a risk-based approach has accelerated site cleanup and resulted in an increase in case closures and a decrease in case backlog. In addition to adoption risk-based decision making approaches for cleanup, many states have accepted the use of natural attenuation as a viable technology for site-cleanup, thus significantly reducing cleanup costs. The emergence of MTBE as a pollutant of national concern in ground water may threaten the implementation of RBCA and natural attenuation cleanup approaches at many USTs nationwide.


Description of RBDM in UST Corrective Action Programs-

Frequently Asked Questions:

Issues Associated with Natural Attenuation-

Status of ASTM RBCA Training In the UST

Database of Corrective Action

RBDM Performance Assessment

MTBE Information from EPA and Non-EPA

Information on Monitored Natural

RBCA State Policy Issues

Hanadi S. Rifai, PhD, PE, is an assistant professor and Monica Suarez, MS, is a research assistant at the department of Civil and Environmental Engineering at the University of Houston, Houston, Texas. Dr. Raifai can be reached via email at

This article originally appeared in the 12/01/2000 issue of Environmental Protection.

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