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A Unique Approach to Evaluating Natural Attenuation is Applied Worldwide

A technician is pumping water from a multi-level well
A technician is pumping water from a multi-level well during an investigation of the natural attenuation of a chlorinated solvent plume under a fire training pit at the former Wurtsmith Air Force Base, Oscoda, Michigan. Tubing from the well leads to an in-line multi-parameter probe that records pH, dissolved oxygen, Eh, and temperature. Photo credit: Sheridan K. Haack, USGS.

The popularity of using natural processes to clean up contaminated sites (natural attenuation) is increasing dramatically; however, reliable methods have not been available for determining if natural attenuation is effective. As a result, regulators are trying to identify the facts needed for cleanup professionals to demonstrate that natural attenuation is successfully remediating a contaminated site. Toxics Program scientists and their partners have combined hydrological, chemical, and microbiological field methods to developed a unique approach for determining when subsurface microorganisms are effectively and efficiently degrading contaminants in the subsurface. This approach to evaluating natural attenuation involves the detailed mapping of overlapping biogeochemical zones in contamination plumes. The biogeochemical zones identify areas where microorganisms are flourishing and actively degrading contaminants. Such zone mapping is not possible when either microbiological or geochemical methods are used in isolation. Identification of these biogeochemical zones is key to evaluating the efficacy of natural attenuation because organic contaminants can degrade at different rates and to different extents within contaminant plumes.

This unique approach to evaluating the effectiveness of natural attenuation is starting to be used throughout the world; examples include:

  1. Wurtsmith Air Force Base, Michigan, where a former fire-training pit resulted in contamination of the subsurface with petroleum hydrocarbons and chlorinated solvents that are now naturally degrading;
  2. Six Mile Village near Fairbanks, Alaska, where a subarctic aquifer contaminated with chlorinated solvents had such low rates of biodegradation that dilution was determined to be the primary mechanism behind the observed natural attenuation;
  3. A shallow sandy aquifer in Denmark where the approach was applied to document the fate of herbicides in ground water; and
  4. A jet fuel spill in Norway where the approach was used to document the natural biodegradation of the contamination.

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References

Cozzarelli, I.M., Suflita, J.M., Ulrich, G.A., Harris, S.H., Scholl, M.A., Schlottmann, J.L., and Christenson, S., 2000, Geochemical and microbiological methods for evaluating anaerobic processes in an aquifer contaminated by landfill leachate: Environmental Science and Technology, v. 34, no. 18, p. 4025-4033, doi:10.1021/es991342b.

Arildskov, N.P., Pedersen, P.G., and Albrechtsen, H.-J., 2001, Fate of the herbicides 2,4,5-T, atrazine, and DNOC in a shallow, anaerobic aquifer investigated by in situ passive diffusive emitters and laboratory batch experiments: Ground Water, v. 39, no. 6. p. 819-830, doi:10.1111/j.1745-6584.2001.tb02470.x.

McGuire, J.T., Long, D.T., Klug, M.J., Haack, S.K., and Hyndman, D.W., 2002, Evaluating behavior of oxygen, nitrate, and sulfate during recharge and quantifying reduction rates in a contaminated aquifer: Environmental Science and Technology, v. 36, no. 12, p. 2693-2700, doi:10.1021/es015615q.

Richmond, S.A., Lindstrom, J.E., and Braddock, J.F., 2001, Assessment of natural attenuation of chlorinated aliphatics and BTEX in subarctic groundwater: Environmental Science and Technology, v. 35, no. 20, p. 4038-4045, doi:10.1021/es0108133.

Zheng, Z., Breedveld, G., and Aagaard, P., 2001, Biodegradation of soluble aromatic compounds of jet fuel under anaerobic conditions--Laboratory batch experiments: Applied Microbiology and Biotechnology, v. 57, p. 572-578, doi:10.1007/s002530100805.

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