Process Overview:
Natural attenuation refers to the naturally occurring processes in soil and groundwater that degrade and dissipate contaminants. Natural attenuation occurs [in situ] and is used to reduce the mass, mobility, [toxicity], volume, or concentration of contaminants to acceptable levels. These in situ processes include [biodegradation], [dispersion], [dilution], [adsorption], [volatilization], and chemical or biological stabilization or destruction of contaminants (USEPA, 1992). Of these processes, the most important destructive attenuation mechanism is biodegradation, although some destruction via abiotic mechanisms may occur (Wiedemeier, et al, 1996). Where the circumstances are appropriate, natural attenuation used in conjuction with source control and performance monitoring can be an acceptable remediation approach (USEPA, 1997). In deed, natural attenuation has been applied over the past several years, and has become increasingly accepted as a remedial alternative (Wiedemeier, et al, 1996). A variety of other terms have also been used to describe natural attenuation including natural restoration, intrinsic remediation, intrinsic bioremediation, passive bioremediation, spontaneous bioremediation, and bioattenuation (National Research Council, 1993; Wiedemeier, et al, 1996).
Because the natural attenuation approach depends upon natural processes to reduce contaminant concentrations, there is no need for engineering intervention. For example, the addition of nutrients or oxygen is not needed to assist the process (On-Site Environmental, Inc., 1997). However, the process does require site characterization, source control, documentation of contaminant loss via field tests, and monitoring to establish its effectiveness (National Research Council, 1993).
There are several advantages to using naturally occurring process over conventional engineered remediation technologies. A main benefit is that natural attenuation is minimally disruptive and often less expensive than conventional remediation technologies. Because natural attenuation is nonintrusive, it allows for the continual use of infrastructure during remediation. In addition, engineered remediation technologies can cause limitations due to the use of mechanical remediation equipment and can pose a hazard to potential receptors when contaminants are transferred into the atmosphere during remediation activities. Natural attenuation does not generate wastes and the compounds that are the most toxic and most mobile are generally the most susceptible to biodegradation (Wiedemeier, et al, 1996). Another benefit is that there is minimal exposure to site workers through the use of natural attenuation. In addition, natural attenuation focuses on the destruction rather than transfer of contaminants (National Research Council, 1993).
Some problems have been encountered with natural attenuation. The primary problems include: the complexities of site characterization, the time required to achieve bioremediation, changes in site conditions, transformation products, public acceptance, and government regulations. Because natural attenuation requires a thorough site characterization, conducting site characterization may be complicated and costly. With natural attenuation, the time required for remediation can be a problem, because the process may require a longer time period than engineered remediation technologies to achieve acceptable remediation levels. In addition, site conditions may change over this long period of time and reduce the effectiveness of the process as well as disrupt previously stabilized contaminants (USEPA, 1997). Another potential problem associated with natural attenuation is the formation of transformation products. These products (e.g., vinyl chloride) may be more toxic than the contaminant itself. Natural attenuation could also cause an undesirable transfer of the contaminant from one type of media to another that may reduce the effectiveness of the process. Public acceptance may be a factor because natural attenuation is often thought of as a passive or “no-action” alternative. Therefore, outreach efforts may be required to gain public acceptance. In addition, government controls, such as permits and restrictions, may be imposed to prevent or reduce contact with hazardous substances (USEPA, 1997).
Process Implementation and Management:
Contaminant Subject to Treatment
The key factor in terms of a compound applicability to natural attenuation is that the biodegradation rate of the contaminant of concern must be fast enough to allow in situ biodegradation before contaminant migration. To date, natural attenuation has primarily been applied to the remediation of [petroleum hydrocarbons], such as [benzene], [toluene], [ethyl benzene], and [xylene]. However, the use of natural attenuation to remediate [chlorinated solvents] is a growing area (Renner, 1998). In addition, natural attenuation is potentially applicable for some [inorganic compounds], including metals and non-metals. The natural attenuation technique can also be applied to [pesticides], but less effectively (Federal Remediation Technologies Roundtable, 1997).
Site Characterization Requirements
Site characterization plays an important role in setting the constraints on the natural attenuation process because by definition natural attenuation occurs without adding anything to the site (National Research Council, 1993). In fact, the level of site characterization needed for natural attenuation is greater than that of other treatment technologies (USEPA,1997). In order to access any potential or current threat to human health or the environment, site characterization must be performed. Site characterization is also important in order to determine the fate and transport of the contaminant of concern over time (Wiedemeier, et al, 1996).
Site characteristics are seldom ideal and no one set of characteristics will lead to the bioremediation of all contaminants. However, there are certain site parameters and characteristics significant to natural attenuation. These include:
The ability to predict the groundwater flow is a critical site characteristic. The prediction of the flow of groundwater is required to determine the movement of contamination, whether the microorganism present will be able to naturally attenuate in all seasons and in all places on the contaminated site, as well as in a timely fashion to prevent the spread of contamination. The groundwater flows’ hydraulic gradient and trajectory should be consistent though out the year and from year to year. The seasonal variation in the water table should not change more than about 1 m. The seasonal fluctuation is the regional flow trajectory should be less than 25 degrees from the primary direction. The precise numbers are site specific (National Research Council, 1993).
The predictability of groundwater flow is dependent on the geological condition of the site. Sites with fractured bedrock aquifers or limestone are not successful candidates for natural attenuation because of the varying soil types present in these environments that result in unpredictable groundwater flow and movement of contamination (USEPA, 1996).
The nature, extent, and concentration of the contaminant of concern play a critical role in the effectiveness of natural attenuation. The contaminants must be effectively remediated by the naturally occurring microorganisms without forming toxic transformation by-products (USEPA, 1997). The fate and transport of the contaminant of concern is necessary in order to determine the future concentration and extent of the dissolved contaminant and to determine if there is potential for the contamination of downgradient receptors (Wiedemeier, et al., 1996). In addition, the concentration of contaminants affects the degradation rates of the natural attenuation process. Extremely [low concentrations] of a contaminant may reduce biodegradation rates because the contact between the contaminant and microorganism is limited. Very high contaminant concentrations can also reduce biodegradation rates because of the effects of [toxicity] (Leeson and Hinchee, 1997). The use of natural attenuation as a primary remediation option is not applicable where nondegradable contaminants are existing at levels that endanger human health or the environment (USEPA, 1997).
The natural attenuation process uses [microorganisms], such as bacteria, viruses, and fungi, occurring naturally in the soil or groundwater to transform contaminant molecules into nontoxic substances through a complex sequence of reactions. In this process certain strains of microbes are exploited for their ability to consume certain contaminates of concern (Zodrow, 1997). The appropriate microorganisms must be naturally present at the contaminated site in order for natural attenuation to be effective.
The presence of carbonate minerals buffer pH changes that occur from the biological production of acids and bases during bioremediation. Carbonates can result from limestone, dolomite, sand, or shell material in beach deposits (National Research Council, 1993).
Studies show that, nutrients generally do not need to be added to sustain microbial growth because they are recycled through the ecosystem. However, there are certain nutrients that are needed at minimum levels. These nutrients include calcium, cobalt, copper, iron, magnesium, manganese, molybdenum, nitrogen, phosphorus, potassium, sodium, sulfur, and zinc. Of these, nitrogen and phosphorus are required at the highest concentrations (Leeson and Hinchee, 1997). The amount of nutrients required is not as much as the amount of electron acceptors required (National Research Council, 1993).
Ambient concentrations of electron acceptors are needed in order for natural attenuation to be effective. Oxygen is needed for aerobic biodegradation. When there is insufficient oxygen, other electron acceptors such as nitrate, sulfate, and ferric iron are necessary for anaerobic bioremediation. The characteristics of the contaminants present and the natural circulation of the groundwater control the concentration of electron acceptors necessary for effective remediation. Large concentrations of electron acceptors are needed for large contaminant sources, very soluble contaminants, and sites where there is insufficient mixing between the contaminated and surrounding waters (National Research Council, 1993).
Electron donors may also be require for the natural attenuation process. Such is the case for natural attenuation to remediate chlorinated solvents. In this process, an adequate supply of both electron acceptors and electron donors are needed for the natural biodegradation of chlorinated solvents. Natural organic carbon and anthropogenic carbon (i.e. fuel hydrocarbons) are types of electron donors used in the natural attenuation process. If the concentration of electron donors is exhausted before the chlorinated solvents are removed, then the reductive dechlorination process will stop and the natural attenuation process will cease to be effective (Wiedemeir, et al, 1996). Certain sites such as former marshlands and swampy areas are good areas for natural attenuation because of the high levels of organic compounds present (USEPA, 1996).
System Design
Natural attenuation occurs at many sites, yet at different degrees of effectiveness. The degree to which natural attenuation is taking place depends on the characteristics of the soil and groundwater and the type and concentration of contaminant present. The processes involved in natural attenuation can be classified as either destructive or nondestructive. Destructive mechanisms, such as aerobic and anaerobic biodegradation, reduce the mass of the contaminant and ultimately destroys it. Non-destructive mechanisms, such as dispersion and sorption, reduce the concentration of the contaminant or prevent the spread of contaminants through adsorption (USEPA, 1996).
[Aerobic bioremediation] rely on the process whereby indigenous microorganisms use oxygen as an electron acceptor to degrade the contaminant. In [anaerobic bioremediation], alternative electron acceptors such as nitrates, sulfates, and iron are used. [Dispersion] is the mechanical and molecular mixing process that spreads and dilutes a contaminant as it moves throughout the groundwater. [Sorption] is the partitioning of the contaminant into a solid by physical or chemical attraction. [Volatilization] involves the transfer of a chemical from the liquid phase in groundwater to the gas phase of the [vadose zone] (Natural Research Council, 1993).
Natural attenuation is not a “no action” alternative. It is often thought of as a passive or “no action” alternative because it does not rely on human intervention. If an enhancer is used, the process is no longer “natural”. Although no human intervention takes place, natural attenuation does require source control measures, confirmation, and monitoring in order to be successful and accepted (USEPA, 1997).
Source Control
One of the first steps in remediation is to prevent the contaminant from spreading. The control of the contaminant source is the most effective way to reach remediation goals in a timely fashion. Some ways to obtain source control include removal, treatment, and containment (Natural Research Council, 1993).
Evaluation
When determining the effectiveness of natural attenuation, specific attention must be given to evaluation. In order to demonstrate in situ bioremediation is working, the National Research Council suggests that the evaluation strategy should include three lines of evidence: 1) a documented loss of contaminants, 2) laboratory assay showing the microorganisms at the site have the potential to transform the contaminants under the site conditions, and 3) laboratory and field data that show that the microorganisms present on site can actively degrade the contaminant (Natural Research Council, 1993).
There needs to be an observed reduction in contamination both at the source, as well as downgradient from the source in order to determine the effectiveness of the process. However, this simple observation is not enough. What is needed is proof that the contaminant is being destroyed. This proof can be obtained from either the second or third line of evidence. The first line of evidence comes from the chemical and geochemical analytical data that should be collected at every contaminated site. The second type of evidence can be obtained from conducted or documented laboratory studies. The third type of evidence is demonstrated through measurements of field samples, experiments run in the field, or mathematical modeling equipment (Natural Research Council, 1993). This “lines of evidence” approach has since been applied in published protocols.
Monitoring
Performance monitoring plays a critical role in the natural attenuation process. In fact, the importance of performance monitoring for natural attenuation is greater than that of other treatment technologies. Monitoring is critical because of the potential spread of contamination, the long time frames required for remediation, and various other concerns involved with natural attenuation. Monitoring programs should be able to demonstrate the expected occurrence of natural attenuation, verify that cleanup objectives are being obtained, detect environmental changes that may potentially reduce the effectiveness of the process, and determine if the source is expanding. In addition, monitoring should detect any impact to downgradient receptors, any new releases of contaminants, and any potentially toxic transformation products. Monitoring should be performed as long as there is contamination present above required cleanup levels (USEPA,1997).
A technical protocol to implement natural attenuation has been established by the Air Force Center for Environmental Excellence. The key steps involved in natural attenuation are outlined in this protocol. They include:
Integration with Other Technologies
At some sites, natural attenuation can effectively remediate contaminants without the use of other remediation techniques. However, natural attenuation is typically incorporated with other remediation technologies. In fact, the United States Environmental Protection Agency recommends the use of natural attenuation “in conjunction with, or as a follow-up to, other (active) remedial measures” (USEPA, 1997). An example of integration of technologies is natural attenuation for groundwater remediation in conjuction with in situ vapor recovery for remediation of areas bove the water table. This technique is known as [bioventing]. Natural attenuation can also follow engineered bioremediation. In this case, natural attenuation is used as a finishing step after the majority of residual contaminants have been removed (Natural Research Council, 1993).
Process Performance:
Historical data show that in most cases, natural attenuation will reduce dissolved contaminant concentrations to below regulatory standards. According to the Air Force Center for Environmental Excellence, technical protocol for implementing natural attenuation of fuel contamination had been fully or partially implemented at 40 Air Force sites by September 1995. Contaminant concentrations and mass had declined at the 20 sites where historical data are available. Natural attenuation was expected to reduce contaminant concentration levels to below regulatory standards in 28 out of 30 Air Force sites that have been fully evaluated (Wiedemeir, et al, 1995). The technical protocol for evaluating natural attenuation of chlorinated solvents had been fully or partially implemented at 10 Air Force sites by November 1996 (Wiedemeir, et al, 1996).
Costs
The basic costs associated with natural attenuation include site characterization, chemical analysis, numerical modeling, report preparation, and regulatory negotiations. An unnecessary cleanup activity is not needed and therefore reduces cost. The total cost to fully implement the Air Force Center for Environmental Excellence protocol ranges from $100,000 to $200,000. This cost is only applicable for sites contaminated with fuel hydrocarbons or chlorinated solvents (Wiedemeir, et al, 1996).
Unsolved Issues
Natural attenuation has been used at several sites, however there are many areas where knowledge is limited. Further research is needed in order to determine what types of contaminants can be successfully treated by natural attenuation and the biochemistry of the reaction. In addition, more appropriate procedures are needed to evaluate sites. Further attention needs to be given to innovative site characterization techniques. Finally, further research is needed to improve mathematical fate and transport models to include all of the potential natural attenuation processes (Nation Research Council, 1993).
Case Studies
Bemidji, MN
Natural Gas Manufacturing Plant, MI
References:
Federal Remediation Technology Roundtable. “Natural Attenuation: In Situ Soil Remediation Technology.” Remediation Technologies Screening Matrix and Reference Guide Version 3.0 (1997): 5 pg. Online. Internet. 28 Jan. 1998. Available: http://www.frtr.gov/matrix2/section4/4_4.html.
Federal Remediation Technology Roundtable. “Natural Attenuation: In Situ Ground Water Remediation Technology.” Remediation Technologies Screening Matrix and Reference Guide Version 3.0 (1997): 5 pg. Online. Internet. 28 Jan. 1998. Available: http://www.frtr.gov/matrix2/section4/4_35.html.
Leeson, A. and R.E. Hinchee. Soil Bioventing: Principles and Practices. CRC Lewis Publishers, New York, 1997.
McAllister, P.M. and C.Y. Chiang. “A Practical Approach to Evaluating Natural Attenuation of Contaminants in Ground Water.” Ground Water Monitoring Review. Spring 1984: 161-173.
National Research Council. In Situ Bioremediation: When Does It Work? Washington: National Academy Press, 1993.
On-Site Environmental, Inc. “Terms and Definitions.” (1997): 4 pp. Online. Internet. 30 May 1998. Available: http://netdial.caribe.net/~onsite/osedef.html.
Renner, Rebecca. “Intrinsic Remediation Under the Microscope.” Environmental Science and Technology. April 1, 1998: pgs. 180A-182A.
Terra Vac Corporation. Air Sparging (Sparge VAC). (1996): 1 p. Online. Internet. 30 March 1998. Available: http://www.terravac.com/toolas/index.html.
U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response. A Citizens Guide to Natural Attenuation. (1996): 4 pg. Online. Internet. 17 June 1998. Available: http://clu-in.com/citguide/natural.htm.
U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response. National Directive 9200.4-17: Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites. (1997): 30 pg. Online. Internet. 17 June 1998. Available: http://www.epa.gov/swerust1/directiv/9200_417.htm.
Wiedemeier, T.H. et al. Technical Protocol for Implementing the Intrinsic Remediation with Long-Term Monitoring Option for Natural Attenuation of Fuel Contamination Dissolved in Groundwater. San Antonio: U.S. Air Force Center for Environmental Excellence, Brooks Air Force Base, 1995.
Wiedemeier, T.H. et al. Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Groundwater. San Antonio: U.S. Air Force Center for Environmental Excellence, Brooks Air Force Base, 1996.
Zodrow, John J. “Second Generation, In Situ Bioremediation: Enzymes, Super-bugs and Geobotany.” Environmental Solutions Feb. 1997: 20-25.