New Report Presents a Framework for Assessing the Sustainability of Monitored Natural Attenuation

(Click on image for larger version)

Relying on natural processes to reduce, or even destroy, contaminants at toxic waste sites is known as monitored natural attenuation. These natural processes can be physical, chemical, or biological and are important to those in the waste management regulatory world. But when can monitored natural attenuation be relied on as a long-term method to clean up toxic waste sites? What makes monitored natural attenuation sustainable? The U.S. Geological Survey (USGS), in cooperation with the Strategic Environmental Research and Development Program (SERDP) and Virginia Tech University, has developed a framework to answer these questions. The framework presents methods to assess the sustainability of natural attenuation at toxic waste cleanup sites.  This methodology, recently published as U.S. Geological Survey Circular 1303, provides environmental planners and managers with a quantitative plan for using monitored natural attenuation as the solution for cleaning up toxic waste sites.

Natural Attenuation and Environmental Sustainability

Humans, and all other living creatures as well, produce wastes.  If these potentially toxic wastes accumulated endlessly in the environment, life on Earth would be impossible.  Or, put another way, life would be unsustainable.

The reason life on earth is sustainable is due to what are called waste-substrate cycles.  In waste-substrate cycles, the waste product of one group of organisms becomes a necessary food (substrate) for other organisms.  Thus, the waste product of photosynthesis (oxygen) is consumed as a necessary substrate (food) for organisms that breathe air (such as humans).  The key to waste-substrate cycles is the presence of processes that continuously consume the waste products of living organisms.

The same reasoning applies to chemical wastes generated by humans.  If chemical waste-consuming processes (called natural attenuation processes) are available and balanced with chemical wastes, then wastes will not accumulate to toxic levels in the environment.  In other words, natural attenuation is sustainable.  Conversely, if waste-consuming processes are not available, or if waste-production overwhelms waste consumption, wastes will accumulate to toxic levels and natural attenuation is not sustainable.

USGS scientist examining an outcrop of organic-rich sediment that drives sustainable natural attenuation of a chlorinated-solvents plume in the ground-water system underlying Kings Bay, GA (Click on image for larger version)

Assessing the Sustainability of Natural Attenuation

While all of this seems simple and logical, methods for assessing the sustainability of natural attenuation of human wastes, like petroleum hydrocarbons or chlorinated solvents, have only recently been developed.  The USGS, in cooperation with SERDP and Virginia Tech, has developed methods for assessing the balance between the delivery of contaminants to the environment and their natural attenuation. The assessment consists of three parts.

1. Using a computer model to estimate the time required for contaminants to dissolve/disperse/degrade in the subsurface (time of remediation).
2. Estimating the energy (organic carbon) needed for the biodegradation reactions to completely degrade the contaminants to harmless byproducts. The energy needed is estimated by determining the amount of organic carbon needed to drive the biodegradation reactions to completion (energy balance).
3. Comparing the required amount of organic carbon needed to degrade the contaminants (part 2) to the pool of available organic carbon in the subsurface (see accompanying photograph) to see if enough organic carbon is available for natural-attenuation processes to clean up the contaminants within the estimated time of remediation (part 1). If enough organic carbon is at the site (along with other factors), then natural attenuation will most likely be sustainable (long-term sustainability).

Environmental professionals can use these general principles to assess the sustainability of monitored natural attenuation in hydrologic systems.

References

Chapelle, F.H., Novak, J.T., Parker, J.C., Campbell, B.G., and Widdowson, M.A., 2007, A framework for assessing the sustainability of monitored natural attenuation: U.S. Geological Survey Circular 1303, 46 p.

More Information

• Chlorinated Solvents Contamination Research Projects
• Microbial Degradation of Chloroethenes in Ground Water Systems
• Solvent Plume Source Removal Test (Fenton's Reagent), Naval Submarine Base Kings Bay, Georgia

USGS scientists collecting data to assess the sustainability of the natural attenuation of a chlorinated-solvents plume at an old dry-cleaning facility near Soldonta, AK. (Click on image for larger version)

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