June 28, 2007

Contaminated site? Let Mother Nature help

  • Using natural attenuation is a less costly cleanup option in certain cases.
    Farallon Consulting


    Frequently, owners of a contaminated property are faced with the unfortunate responsibility of conducting a cleanup action. Ask any fellow property owner who has had some experience with a cleanup action and they will most assuredly inform you that it was: 1) costly; 2) 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 engineered with the best intentions but application of these technologies may often conflict with your business needs, exceed the funds available for cleanup, or just cannot effectively eliminate all of the contamination. Luckily, regulators have recently 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.

    Image courtesy of Washington State Department of Ecology
    Using natural attenuation requires monitoring to ensure the contamination is degrading as predicted. [enlarge]

    Just what is natural attenuation? According to the state Department of 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 and destructive excavations, no expensive and noisy cleanup equipment, no skyrocketing operations and maintenance costs, and no trenching and piping 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 Environmental Protection Agency and many states have recently acknowledged that naturally occurring subsurface bacteria can successfully reduce these toxic chemicals to harmless metabolic by-products such as carbon dioxide and water within an acceptable time frame 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 biologic component to the attenuation process.

    • Demonstrating that the rate of attenuation will result in cleanup within a reasonable time frame.

    • Demonstrating protection of human health and the environment.

    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 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 can be walked away from until cleanup goals are achieved and allows continued use of affected properties? Unfortunately, yes it is, and there are some limitations to consider.

    One limitation when considering a natural attenuation approach is that there is a 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 the degradation of the 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. Reduction of 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 the cleanup strategy.

    Lastly, if a quick cleanup time frame is a priority, a natural attenuation remedy may not be appropriate since it can exceed five to 10 years when no source treatment or removal is performed. However, the scientific community has come to the rescue again; it has devised 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.

    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. However, this “enhanced natural attenuation” process is a subject for another article.

    Jeff Kaspar has been involved in the evaluation and application of natural attenuation remedies for more than 10 years, and contributed to the 2005 Washington State Department of Ecology’s natural attenuation guidance. Kaspar has served as an instructor for natural attenuation courses with Ecology and the Northwest Environmental Training Center.

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