Contaminated Site? Let Mother Nature Help
"Using natural attenuation is a less costly cleanup option in certain cases."
The owner of a contaminated property frequently is faced with the uneviable responsibility of conducting a cleanup action. Any fellow property owner with cleanup action experience will most assuredly attest: 1) it was costly; 2) it affected the ability to use or sell the property; and 3) the contamination is never as easy to get out as it was to get in.
Consultants have an arsenal of cleanup technologies that were engineered with the best intentions. However, application of these technologies often may conflict with the property owner's business needs, may exceed the funds available for cleanup, or simply cannot effectively eliminate all of the contamination. Fortunately, regulators have come to grips with the reality that sometimes the most cost-effective and efficient solution to cleaning up a site is to let Mother Nature run her course, applying her own cleanup technology dubbed "natural attenuation."
And just what is natural attenuation? According to the Washington State Department of Ecology (Ecology), natural attenuation, “…includes a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of hazardous substances in the environment.” In layman’s terms, this means that contaminants may go away on their own without a need for any active cleanup. The benefits of a natural attenuation approach translate to no invasive or destructive excavations, no expensive or noisy cleanup equipment, no skyrocketing operation and maintenance costs, and no trench and pipe mazes across the site.
Various hazardous substances such as chlorinated solvents (including the notorious dry-cleaning solvent PCE), petroleum hydrocarbons, and some metals can be successfully remediated by various biologic, chemical, and/or physical natural attenuation processes. The U.S. Environmental Protection Agency and many states have acknowledged that naturally occurring subsurface bacteria can successfully reduce these toxic chemicals to harmless metabolic by-products such as carbon dioxide and water within a time frame that is acceptable to the regulatory community. These microorganisms have proven incredibly convenient for those hard-to-reach areas of contamination beneath buildings, railroads, and other types of critical infrastructure that are better left undisturbed.
The key to using a natural attenuation approach in a cleanup strategy is identifying when “favorable conditions” exist. The fundamental criteria for affirming favorable conditions are demonstrating that:
• There is a chemical or biological component to the attenuation process;
• The rate of attenuation will result in cleanup within a reasonable time frame; and
• Human health and the environment are protected.
Meeting the first two criteria can be accomplished within a relatively short time frame and at a very reasonable cost, often using existing knowledge of the site history, a limited number of strategically placed monitoring wells, and a little focused geochemical analysis. Meeting the third criterion involves determining whether potential receptors such as a surface water body or a groundwater aquifer will be affected during the time frame predicted for achieving cleanup goals.
This low-cost, practical cleanup approach may sound too good to be true: a cleanup remedy that allows continued use of the affected property while cleanup goals are achieved unattended? Unfortunately, yes, it may be too good to be true. Following are a few limitations to keep in mind when considering natural attenuation as a cleanup remedy.
One limitation is the regulatory requirement to make certain Mother Nature is indeed doing her job. A natural attenuation remedy must include a provision for periodic monitoring to ensure that degradation of contaminants is occurring as predicted, and that human health and the environment remain protected throughout the cleanup action. The cost of the long-term monitoring required may increase total cleanup costs to the point that natural attenuation is not the most cost-effective solution, at least as a stand-alone remedy. Reducing the time frame for cleanup may be economically desirable, and focused treatment or removal of areas of highest contaminant concentrations may be necessary to make natural attenuation a feasible component of a cleanup strategy.
If a quick cleanup time frame is a priority, a natural attenuation remedy may not be appropriate because it can exceed 5 to 10 years when no source treatment or removal is performed. However, the scientific community has come to the rescue again in devising a variety of means to “enhance” the natural attenuation process. Assorted media have been engineered to provide extra food and energy to beneficial bacteria, boosting the rate of biodegradation of contaminants.
In some cases, a process known as bioaugmentation is employed, in which specific bacteria known to successfully biodegrade a suite of contaminants are injected into the contaminated area. These enhancement processes have the potential to dramatically reduce the cleanup time frame. (But this “enhanced natural attenuation” process is a subject for another article.)
Farallon’s Senior Project Manager Jeffrey Kaspar has been applying natural attenuation remedies for sites with releases of petroleum hydrocarbons and chlorinated solvents since the 1990s. He provided technical support to Ecology in the development of the guidance and supporting spreadsheet-based models used as an essential component of the natural attenuation evaluation process. Mr. Kaspar was recruited by Ecology to participate in educational workshops designed to provide the public with comprehensive training on correctly understanding, evaluating, selecting, and implementing natural attenuation as a cleanup action remedy. Following the successful completion of workshops in Spokane and Seattle, Mr. Kaspar was invited to conduct the internal training workshop for Ecology Project Managers conducted at Ecology headquarters in Olympia, Washington. He also created the natural attenuation training seminar for the Northwest Environmental Training Center, and has served as an instructor for this course.
Farallon strives to actively participate in the development of the guidance documents and regulations that impact the environmental consulting industry and the services we provide our clients. If you have questions regarding evaluation of natural attenuation as a component of a remedial action, Farallon has the expertise to assist you.