|NATURAL ATTENUATION |
OF ORGANIC COMPOUNDS
The natural attenuation of organic compounds, particularly fuels and chlorinated solvents, has been subject to study for some time. The basic physical processes which, unaided by deliberate human intervention, reduce the concentration, toxicity, or mobility of such contaminants, are fairly well understood. However, techniques for predicting and monitoring natural attenuation are still in their infancy.
The mechanisms of natural attenuation can be classified as destructive and non-destructive. Destructive processes include biodegradation and hydrolysis. Biodegradation is by far the most prevalent destructive mechanism. Non-destructive attenuation mechanisms include sorption, dispersion, dilution, and volatilization. Dilution and dispersion are generally the most important non-destructive mechanisms.
Fuels and chlorinated solvents are organic compounds. That is, each molecule contains one or more carbon atom. Petroleum-based fuels not only contain molecules made up of hydrogen and carbon, but they generally contain benzene, toluene, ethyl benzene and xylene, known collectively as BTEX. These compounds, also made of carbon and hydrogen, are highly toxic. Chlorinated solvents, which are volatile organic compounds (VOCs), consist of molecules containing chlorine, carbon, and other elements, and they too are hazardous.
Biodegradation, also called bioremediation, is a process in which naturally occurring microorganisms, such as yeast, fungi, and bacteria, break down target substances, such as fuels and chlorinated solvents, into less toxic or non-toxic substances. Like larger living things, these microbes must eat organic substances to survive. Certain microorganisms digest fuels or chlorinated solvents found in the subsurface environment.
The ability of microorganisms to metabolize, or use nutrients depends on the chemical composition of the environment, and different microorganisms have evolved to take advantage of varying conditions. In most organisms, including bacteria, the metabolic process requires the exchange of oxygen and carbon. Biodegradation can occur in the presence of oxygen (aerobic conditions) or without oxygen (anaerobic conditions).
In general, there are three biodegradation processes: 1) when the contaminant is used by the microbe as the primary food source; 2) when the contaminant is used to transfer energy; and 3) (called cometabolism) when biodegradation occurs in response to a chance reaction between the contaminant and an enzyme produced during an unrelated reaction. For fuel hydrocarbons containing BTEX, the first process is dominant. The full degradation of chlorinated solvent plumes usually requires all three processes.
There are four principal non-destructive mechanisms that cause a decrease in measurable contaminant concentrations.
Predicting Natural Attenuation
By definition, natural attenuation processes occur without intervention. The key question, for responsible parties, regulators, and the public, is to what degree those processes are likely to contribute to the achievement of remedial action goals. In considering monitored natural attenuation as a remedy, it's necessary to evaluate the potential for biodegradation, chemical degradation, dispersion, dilution, sorption, and volatilization.
The evaluation of natural attenuation requires expertise in several technical areas including microbiology/bioremediation, hydrogeology, and geochemistry. The extent of contaminant degradation depends on a variety of parameters, such as contaminant types and concentrations, temperature, moisture, and the availability of nutrients and other compounds that influence the metabolism of microorganisms. Some studies suggest that high contaminant concentrations are lethal to microbes, and thus source removal of any free product is recommended. According to those studies, natural attenuation is not effective for fuel plumes when levels in soil exceed 20,000 parts per million. Information to be obtained during data review includes:
There are generally three lines of evidence that are used to support natural attenuation as a remedy. The first attempts to demonstrate attenuation through historical documentation of decreasing contaminant concentrations, in conjunction with hydrogeological information. The second uses chemical analysis to determine not only to what degree degradation is responsible for those decreased contaminant concentrations, but whether there is a sufficient supply of nutrients and other compounds to continue the process. This line of evidence can be used to show that conditions are sufficient for natural attenuation to occur. The third line of evidence requires the extraction of bacteria from the soil to demonstrate, in a laboratory, that the bacteria at the site do indeed destroy the contaminants.
Long-term monitoring is necessary to demonstrate that contaminant concentrations continue to decrease at a rate sufficient to ensure that they will not become a health threat or violate regulatory criteria. EPA recommends that monitoring be designed to accomplish the following:
EPA also recommends that monitoring continue as long as contamination remains above required cleanup levels, and for two to three years after attainment of cleanup levels to ensure that the plume is stable.
While the number of sampling points -- monitoring wells, for example -- required for any particular purpose is site-specific, greater certainty is generally achieved with the installation of more wells over larger areas, with samples taken at a greater variety of depths. More frequent sampling, as well as monitoring over longer time periods, also leads to more reliable conclusions. However, since additional monitoring costs money, natural attenuation becomes less attractive as a remedial option when regulators or others insist on more certain measurement of the results.
Hazardous Transformation Products
Sometimes compounds degrade into byproducts that are as toxic, or even more hazardous, then the original contaminants. For example, the reductive chlorination of PCE and TCE creates vinyl chloride, a confirmed carcinogen for which drinking water standards are typically ten times as stringent as the parent compounds. Though benzene, toluene, ethyl benzene and xylene are initially considered the most toxic compounds in petroleum-based fuels, a residue of heavier, potentially hazardous petroleum hydrocarbons may remain following BTEX degradation.
Though cleanup documents often focus on a "contaminant of concern," most subsurface contamination plumes contain a mix of chemicals resulting from a variety of operations and disposal activities. Even if BTEX or chlorinated solvents attenuate naturally, other contaminants may be resistant to natural processes.
For example, the gasoline additive methyl tertiary butyl ether (MTBE), which has been found in a large number of fuel-contamination plumes, is both resistant to biodegradation and more mobile than BTEX. A study of thirty MTBE sites indicated that on average the MTBE plume was 1.5 to 2.0 times further out than the leading edge of the benzene plume. MTBE is classified by U.S. EPA as suspected to cause cancer, and even at low levels it affects the taste and smell of drinking water. Similarly glycol ethers, a family of toxic industrial chemicals, are present in many plumes, but they are rarely named as chemicals of concern. These compounds are difficult to detect; they do not degrade well in anaerobic environments; and they are known to cause reproductive disorders.
Science now gives us a fundamental understanding of the mechanisms of natural attenuation, but we -- hopefully all stakeholders -- still must develop the policies and tools to evaluate when monitored natural attenuation is an acceptable remedy.
August 30, 1998
Principal author: Peter Strauss
Center for Public Environmental Oversight