By balancing monitored natural attenuation with engineered remediation solutions, cleanup goals can be met at the lowest possible cost while still protecting human health and the environment.
While evaluating remediation options at a creosote-contaminated site in Texas, scientists discovered that dissolved hydrocarbons, some of which were hazardous, were being dissipated in the groundwater at rates similar to those provided by engineered remediation solutions, such as soil vapor extraction and groundwater sparging (1). A naturally occurring bioactive zone of microorganisms was transforming the contaminants into carbon dioxide and water.
This phenomenon is an example of natural attenuation - the reduction in mass, mobility, or toxicity of contaminants in soils, sediments, or groundwater by naturally-occurring physical, chemical, or biological processes. Nature becomes the cleanser.
Natural attenuation is also referred to as intrinsic bioremediation, intrinsic remediation, intrinsic biodegradation, passive remediation, and passive bioremediation. It may include any or all of the following processes: biodegradation, dilution, dispersion, adsorption, volatilization, and chemical and biochemical stabilization. Biodegradation alters or destroys the contamination by transforming contaminants into carbon dioxide, water, and other nontoxic compounds. The other processes reduce the concentration or the mobility of contaminants without destroying the contaminant.
According to the Preamble of the National Oil and Hazardous Substances Pollution Contingency Plan, the regulatory framework for the Superfund Program (2), natural attenuation can be effective at reducing contaminants in the groundwater to concentrations that are protective of human health and sensitive ecological environments in a reasonable time frame.
The intentional use of these natural processes for achieving remediation objectives in corrective action programs has been termed "monitored natural attenuation" by the U.S. Environmental Protection Agency (EPA) Office of Solid Waste and Emergency Response (OSWER) (3). This new term acts as a reminder that its use in no way is to be regarded as a "do nothing, walk away, default" approach. Rather, it is an alternative method to achieve objectives that are fully protective of human health and the environment.
According to the OSWER directive (3), the use of monitored natural attenuation must include sufficient evidence that corrective action levels will be achieved. Proponents are expected to perform sufficient site investigation and characterization to set a cleanup objective, estimate a defensible time frame, provide evidence that natural attenuation is occurring, demonstate that it will be as effective as other solutions, and set up monitoring procedures to compare results against expectations. Contingency strategies must be provided to ensure that human and ecological resources are protected if natural attenuation is not working. Monitored natural attenuation will typically be used in conjunction with other corrective action measures, and rarely as a stand-alone approach.
Increased knowledge about the limitations of remediation technologies, the emergence of risk-based approaches, and the need to allocate money and other resources where they are most needed have provided impetus for the increased use of monitored natural attenuation. Many state and federal remediation-implementation guidelines now include a discussion of site circumstances that favor natural attenuation strategies and list the steps to determine whether or not it is occurring and at what rates.
Working in partnership with nature is not a new idea, since many of its principles are currently being used in engineered remediation processes. These technologies basically accelerate rates of natural attenuation by aeration, vapor extraction, mixing, and the addition of beneficial nutrients, oxygen, and/or microbes.
Advantages and disadvantages
There are numerous advantages of monitored natural attenuation. Since by definition it is an in situ process, less volume of remediation wastes is generated, resulting in a reduced risk of human exposure to contaminated media.
In many cases, monitored natural attenuation is nonintrusive. The site can be used with minimal disruption while remediation is occurring, and few surface structures are required. Property does not need to be purchased for easements or the installation of equipment.
More importantly, the implementation of monitored natural attenuation enables managers, remediators, and regulators to differentiate between sites that are cleaning themselves and those that are not, so that engineering resources can be allocated to sites where they will provide the greatest benefit. Its use in conjunction with other remedial measures lowers the overall remediation costs.
As many as 20% of the sites contaminated with chlorinated organics and 80%-85% of fuel-contaminated sites may be amenable to using just natural attenuation. Translated into dollars, use of monitored natural attenuation could help save hundreds of millions of dollars in unnecessary engineered solutions.
One disadvantage of monitored natural attenuation is that remediation time frames are long and not easily predicted, even with historical data. During the next decade, as it is used and its results are monitored, a clearer understanding of attenuation rates and results will emerge.
In some cases, site characterization may be more costly and complex and institutional controls may be necessary to ensure long-term protection of human health and the environment. Because hydrologic and geochemical conditions may change over time and could result in renewed mobility of previously stabilized compounds, long-term monitoring will be necessary to show the effect of the new conditions on the remedial effectiveness of monitored natural attenuation.
Limitations of available technologies
Experience with site investigations and remediation over the last decade has provided a much clearer understanding of the mechanics of cleanup: how fast and far contaminants spread, under what circumstances, and what restoration goals can and cannot be achieved. As cleanup programs progressed, the regulatory community and industry acknowledged that achieving restoration goals with the best available technology was an impossible task at many sites. Current technology can restore portions of the nation's contaminated groundwater sites to meet drinking-water standards, but total cleanup is not feasible at many sites because of the diversity of contamination and the technical complexity of groundwater cleanup (4).
Excavation is expensive and presents significant economic risk. Once the source of contamination has been contained, the site must be restored. At many sites, restoration goals are met by excavating and treating large amounts of soil to remove unwanted chemicals. Costs range from $180 to $300 per cubic yard, most of which go for excavation, backfill, debris handling, analytical testing, and oversight. Only $40-$80 per cubic yard is spent for actual treatment and disposal. This is a particularly wasteful strategy when the chemicals do not jeopardize human health, drinking water, or other ecological resources.
And, although pump-and-treat systems are an important remediation tool in groundwater cleanup, they have several limitations. According to one study, cleanup goals have not been achieved at 69 of 77 sites where pump-and-treat systems were used and, more importantly, no time frame for achieving cleanup goals could reasonably be estimated (5).
If an engineered solution is not sufficiently contributing to achieving cleanup goals, why continue to pour money into a strategy that isn't working - particularly if alternatives such as monitored natural attenuation can be demonstrated to be equally as effective or even more so?
A risk-based approach
Today, many states are rewriting rules and passing legislation that will make it easier to allocate remediation resources where they are most needed and speed up remediation and project closures. To accomplish these goals, policy makers are shifting toward risk-based approaches. Site characteristics are measured against the potential of risk to human health and the environment and site management strategies are prioritized so that they are commensurate with that risk.
Risk Based Corrective Action (RBCA), developed by the American Society for Testing and Materials (ASTM), was the first formalization of how this approach could be used to develop remediation strategies at leaking underground fuel-tank (LUFT) sites. Under this new approach, four components must be evaluated:
contaminant sources;
exposure pathways;
affected environmental media; and
existing and potential human and environmental receptors.
By focusing on actual and potential risks, the goal of protecting human health and the environment is achieved more quickly and effectively.
Monitored natural attenuation is a corollary to a risk-based approach. The key to whether its use will be accepted by regulatory agencies as an alternative or in conjunction with other remediation techniques depends on whether it can be demonstrated to be as effective as other corrective actions in protecting human health and the environment. Due to the rapidly changing federal and state regulatory climate, regulators should be consulted first before proceeding with natural attenuation or another risk-based strategy to determine what data are needed to support such an approach.
Evaluating
natural attenuation
The decision tree in Figure 1 illustrates the use of information provided by site investigations and risk assessments as the scientific framework within which to evaluate the potential of monitored natural attenuation at a particular site. Figure 2 shows the sequence of steps usually taken to determine whether monitored natural attenuation will be effective and whether accelerating the rate of natural attenuation through an engineered remediation process may be warranted.
Figure 1 addresses sites that must submit a new remedial plan prior to commencing remediation.
Figure 3 is similar, but it shows how to reevaluate a site where engineered remediation is already reducing contamination (as discussed later).
In a risk-based approach to preparing a new remedial plan, the first question to be addressed is whether residuals pose any immediate risks to human health and the environment. The answer is arrived at by examining the data from site and risk assessments, which provide information about the source of the problem and its transport through the soil or groundwater. When the residuals are transported through the soil, a person may assimilate them through ingestion, inhalation of vapors, skin contact, etc. Chemicals that are present in the groundwater may adversely affect drinking water sources and may subsequently transform to vapors that are considered a potential risk to human and ecological receptors.
Immediate risks must be rapidly mitigated by taking corrective measures with active strategies, such as source removal, plume containment, or vapor extraction. Where active treatment is impractical or contaminants pose a relatively low long-term threat, engineering controls such as containment should be considered.
Once any immediate risks have been mitigated, long-range risks must be examined. Will migration of contaminants along exposure pathways or predetermined points of compliance jeopardize human health and ecological resources? If the answer is yes, process modeling must be used to explore the various remediation options available, including monitored natural attenuation, and a cost/benefit analysis conducted to evaluate their potential to achieve restoration goals.
If a site is not posing any immediate or future threat to human health and the environment, and can be shown to be naturally attenuating over reasonable time frames, the use of engineered solutions may be an unnecessary waste of human and financial resources.
However, if regulators are reluctant to consider a risk-based approach or monitored natural attenuation, managers should consider the technical impracticability provisions allowed under many regulations. The inherent limitations of engineered remediation may mean that cleanup levels and objectives cannot practically be attained within a reasonable time frame using any remediation technologies. In such a case, alternative strategies must be identified that are protective of human health and the environment. Such strategies may include a combination of containment, engineered remediation, and monitored natural attenuation.
In many instances, a technical impracticability analysis will only be possible after an engineered solution has been implemented and operated for several years. Site managers should consider designing the performance monitoring program to observe any natural attenuation that is occurring in order to better determine whether remediation is occurring due to the natural or engineered processes. Monitoring should include observing concentrations of dissolved constituents within the impacted area over time, as well as monitoring the dissolved oxygen and other indicators of natural attenuation. Such data may be valuable in arguments for considering monitored natural attenuation during a project review after data have been collected over several years.
Demonstrating effectiveness Figure 2 outlines the steps in evaluating the effectiveness of monitored natural attenuation.
After risks have been assessed, circumstances favoring and limiting the use of a monitored natural-attenuation strategy must be assessed. Generally, site managers must demonstrate that:
the original source of the contamination has been stopped;
the contaminant plume has stabilized or contracted;
no additional contamination will occur;
migration of contaminants does not cause present or potential risk to human and ecological receptors; and
future use of contaminated groundwater as a water resource is unlikely.
If circumstances favor the use of monitored natural attenuation, then site managers must show that natural attenuation is occurring. Generally, the same techniques developed to document, analyze, and demonstrate the potential effectiveness of engineered remediation are used:
evaluate historical data to show that contaminant concentrations have been reduced over time;
provide evidence that microbial or other attenuation mechanisms are occurring; and
evaluate factors that could affect the mobility of the contaminants.
Historical data are obtained through monitoring soil and groundwater conditions. These data show the reduction of contaminant concentrations downgradient from the groundwater flow path or in the soil.
Modeling the data helps project a time frame for achieving restoration objectives. Monitoring measurements taken over time will produce evidence that will help refine conceptual models to predict what will occur at a site.
If the data show contaminant reductions, more elaborate documentation may not be required.
If the historical data are insufficient to support a monitored naturalattenuation approach, data such as the nature and rates of natural attenuation processes or other studies may be required. Such data will likely be required at sites "with contaminants that do not readily degrade through biological processes (e.g., most nonpetroleum compounds, inorganics), at sites with contaminants that transform into more toxic and/or mobile forms than the parent contaminant, or at sites where monitoring has been performed for a relatively short period of time (3)."
More elaborate evidence can include supportive studies that show decreases in terminal electron acceptors (such as dissolved oxygen) and increases in biodegradation products (such as increased carbon dioxide) at numerous locations in the contaminant plume. Treatability studies and counts of microbial populations at representative locations in the contaminant plume can establish rates of dilution, adsorption, and dispersion and provide evidence that constituents are biodegrading at acceptable rates. Other studies can establish the biodegradability of the contaminants and evaluate the presence of other compounds that enhance or impede biodegradation.
To assess the factors that affect the mobility of the contaminants, hydrogeological conditions are studied to determine their effect on natural attenuation. These include aquifer permeability, aquifer thickness, depth to aquifer, homogeneity of geologic strata, and hydraulic conductivity.
The soil chemistry is studied to determine whether site conditions are conducive to natural attenuation and to evaluate what chemical and physical factors will affect the mobility of the plume. This includes an examination of soil moisture, the organic carbon content of the soil, porosity, and pH.
The development of a rating system to assess the value of the data collected is still being explored. One of the first such rating systems examined the feasibility of in situ bioremediation by assigning rating points on a scale of -4 to +2 to such parameters as contaminant characteristics, hydrogeology, and soil and groundwater chemistry (6). Its goal was to provide a defensible method for interpreting concentration levels of various chemical components by showing how their interactions affected degradation rates. The system helps evaluate the relative value and consistency of data, the relationship of various site parameters, and where speculation about how to interpret the data might be occurring.
The biodegradation protocols recently developed by the Air Force Center for Environmental Excellence include a more elaborate rating system (7). The protocols focus primarily on biological degradation of chlorinated solvents and were developed for use at large sites that are impacted by many different chemical compounds, and do not take into account all the other types of natural attenuation that may be occurring at a site.
A new approach, bioavailability, examines whether chemical compounds are available to human and ecological receptors (see sidebar at right).
Fate and transport modeling evaluates the effect of hydrogeologic and chemical parameters on the rate of natural attenuation over time and helps predict whether monitored natural attenuation can attain protective remediation objectives over a reasonable time frame.
Biodegradation rate constants accurately simulate the fate and transport of contaminants dissolved in groundwater. Various methods for calculating these rates have been proposed in the Air Force protocols (7). If natural attenuation is not occurring at acceptable rates, site-specific variables that are inhibiting it should be identified to ascertain what techniques might be used to augment that rate, such as adding oxygen or hydrogen peroxide or installing a passive containment system.
As with any remedial options, the goals of restoration must be identified. Cleanup concentrations at specified points of compliance must be established, including predictions of when they will pose no risk to human health and the environment. These goals will depend on whether any plans exist for developing the site for commercial or residential use.
Even when evidence supports the use of monitored natural attenuation, evaluations of other remedial options should be conducted to determine whether it will be equally as cost-effective as other strategies (or more so), that remedial objectives will be achieved in a reasonable time frame, and that it will be protective of human health and the environment. Sometimes, monitored natural attenuation is not always the most cost-effective option because it can be very slow.
Finally, it is necessary to specify monitoring requirements, such as the locations of monitoring wells, the types of samples and analyses that will be conducted, and monitoring intervals, so that the performance of remedial solutions can be evaluated over time and adjustments made for changing conditions. When possible, monitoring should be designed to differentiate the remediation that is being achieved by natural processes and engineered processes.
Institutional controls should be specified to prevent exposure from contamination to sensitive receptors. These can include fencing off the property, placing real estate covenants on future residential or commercial usage, ensuring that the groundwater will not be used as a water supply, or providing an alternative groundwater supply.
Contingency strategies should be provided to ensure that human and ecological resources are protected if monitoring results show evidence that monitored natural attenuation is not working as well as predicted.
Reevaluating engineered-solution sites
Figure 3 illustrates the process for reevaluating sites where remediation is ongoing. It can provide a defensible justification for changing a previously issued record of decision (ROD) at sites where engineered solutions are being used to achieve specified corrective action goals. These sites are candidates for risk-based approaches where monitored natural attenuation can be implemented in conjunction with other corrective measures. Even if monitored natural attenuation is not appropriate, it is useful to periodically review and consider alternative remedial strategies and compare their cost and performance to existing ones.
According to EPA guidelines for reviewing proposals to amend previously issued RODs (8), amendments could be accepted if the following conditions are met:
1. A different technology would result in more cost-effective cleanup;
2. Achieving cleanup levels is not technically practical from an engineering perspective, or a remediation system has reduced contaminant levels but contaminant recovery efficiency is so low that a concentration plateau has effectively been reached, or remediation has become demonstrably inefficient; and
3. Reduced monitoring will significantly reduce costs but not harm the effectiveness of the remedy. The EPA guidelines specifically recommend the use of monitored natural attenuation.
In order to determine whether a ROD should be reviewed, new site assessments and process modeling must be performed to evaluate how much remediation has occurred and whether the conditions specified above can be met. The resulting data on contaminant plume, concentrations levels, and migration can be compared to previous site assessments so that the progress of the remedial options currently in use can be properly evaluated.
Such an evaluation is similar to what would occur for a site proposed for remediation, with this difference: its purpose is to replace existing solutions with new ones that will be equally or more protective of human health and the environment, and, hopefully, with lower remediation costs. I
Acknowledgments
The authors wish to thank John T. Wilson, one of the authors of the U.S. Air Force's technical protocols on natural attenuation, for his personal insights.
[Sidebar]
Contaminant Availability: Redefining "How Clean Is Clean?" There is increasing evidence that not all chemicals detected in soil are available to leach to groundwater or for uptake by human or ecological receptors, and that this reduced availability results in reduced toxic effects. If research continues to support this concept, it will provide a new basis for evaluating risk that will change the way soil cleanup concentration goals are set.
In a project initiated by the Gas Research Institute (9), scientists are examining the effects of aging in the environment on the availability of soil-bound hydrocarbons. The results of these studies indicate that hydrocarbon availability declines with time, even though the total concentrations of the hydrocarbons remain constant. This observation suggests that soil/chemical interactions are occurring over time that bind the hydrocarbons and prevent their release to the environment or living organisms. The GRI researchers have also discovered that soil treatment technologies can effectively treat the available fraction of the hydrocarbon contaminant while leaving behind the fraction that is tightly bound to, or sequestered within, the soil. This residual concentration (which is the plateau concentration often observed during soil treatment) is not readily susceptible to leaching by water or uptake by ecological or human receptors. It, therefore, is significantly less toxic and presents significantly less risk than the untreated soil. We can infer from these observations that the effects of aging (or weathering) and treatment are similar and yield significant reductions in leachability and toxicity. The aged or treated materials can often be left or placed in the environment because the rates of release to groundwater or organisms are at or below de minimus levels. De minimus levels are concentrations at which the rates of release and uptake are balanced by natural attenuation processes within the environment or the organism. Since it is the readily available fraction of the hydrocarbon that is largely responsible for the risk, it is now being proposed that risk-based, tiered testing schemes use this fraction as the basis for management decisions at hydrocarbon-contaminated sites. One result of this important new research is the formation of a new coalition of industry groups at both the state and federal levels. At the federal level, the Bioavailability Policy Project is working to establish an awareness among the public and key policymakers of the need to incorporate the theory of contaminant availability into federal and state decision-making processes through guidance, regulation, and legislation. On the state level, contaminant availability is being examined by several states and has already been recognized as part of the ASTM risk-based corrective action approach and other tiered riskbased management processes.
Bioavailability is discussed further in (9).
[Reference]
Literature Cited
1. Borden, R.C., et aL, "Transport of Dissolved Hydrocarbons Influenced by Oxygen Limited Biodegradation; 2. Field Application," Water Resources Research, 22 (13), pp. 1983-1990 (Dec.1986). 2. National Oil and Hazardous Substances Pollution Contingency Plan. Federal Register, SS, pp. 8706, 8733-8734 (Mar. 8, 1990).
3. US. Environmental Protection Agency, "Draft Interim Final OSWER Monitored Natural Attenuation Policy," Directive 9200.4-17, U.S. EPA, Office of Solid Waste and Emergency Response, Washington, DC (Nov. 17,1997). 4. National Research Council, "Alternatives for Ground Water Cleanup," news release issued by National Research Council, National Academy of Sciences, Washington, DC (June 23, 1994).
S. Rice, D. W., et al., "Recommendations to Improve the Cleanup Process for California Leaking Underground Fuel Tanks (LUFTs)," Lawrence Livermore National Laboratory, Livermore, CA, p. 1 (Nov. 1995).
[Reference]
6. Brubaker, G. R., "Screening Criteria for In Situ Bioremediation of Contaminated Aquifers," in Proceedings of 2nd Annual Hazardous Materials Management Conference, Central Tower Conference Management Co., Glen Ellyn, IL (Mar.1416, 1989).
7. Wilson, J. T., et aL, "Overview of the Technical Protocol for Natural Attenuation of Chlorinated Aliphatic Hydrocarbons in Ground Water Under Development for the U.S. Air Force Center for Environmental Excellence," presented at the U.S. EPA Office of Research and Development's Symposium on Natural Attenuation of Chlorinated Organics in Groundwater, Dallas, TX (Sept.11-13, 1996). 8. U.S. Environmental Protection Agency, "Superfund Reforms: Updating Remedy Decisions," memorandum to EPA Regional Directors by Stephen Luftig, Director of the Office of Emergency and Remedial Response, and Barry Breen, Director of the Office of Site Remediation Enforcement, U.S. EPA, Washington, DC (Sept 27, 1996).
9. Linz, D. Gv and D. Nakles, "Environmentally Acceptable Endpoints in Soil: RiskBased Approach to Contaminated Site Management Based on Availability of Chemicals in Soils," American Academy of Environmental Engineers, Washington, DC (1997).
[Author Affiliation]
G. H. SWETT is director of the environmental management systems group at RETEC, a management, information technology, and environmental consulting, engineering, and construction firm in Tucson, AZ (E-mail: gswetttretecinc.com). He specializes in the implementation of environmental management systems for the refining, petrochemical, steel finishing, and natural gas processing industries. He received his BA in chemistry and mathematics from the Univ. of Denver and an MBA in finance from Golden Gate Univ. He serves on a number of environmental management committees of industry associations, including the National Association of Manufacturers, American Petroleum Institute, and National Petroleum Refiners Association,
D. RAPAPORT is founder of Jerome Headlands Press, a public relations firm in Jerome, AZ IE-mail: jhpress(sedona.net). She is a freelance writer specializing in environmental technology issues.
