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Rethinking the Limits of Oxygen-Based Biodegradation – More Oxygen Than We Think

USGS scientist holding a sample vile up to a dissolved oxygen color scale
USGS scientist holding a sample vile up to a color scale to determine the concentration of dissolved oxygen in groundwater. Photo credit: USGS
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At contamination sites where subsurface cleanup operations are taking place, oxygen may play a more important role in the biodegradation of contaminants than previously thought. Even at extremely low levels, oxygen may be critically important to the cleanup of subsurface contamination.

Environmental-cleanup professionals have been using a simple two-category model (with or without oxygen) to assess the role of oxygen in the biodegradation of contaminants in the subsurface. The cutoff between conditions where oxygen-based biodegradation was thought to be important (aerobic or oxic) or not important (anaerobic or anoxic) was largely determined by the limitations of the equipment used to measure dissolved oxygen at contamination sites. Scientists use oxygen probes or color-chemistry field kits that can typically detect oxygen in water down to concentrations of 0.1 to 0.5 milligrams per liter (mg/L). The tendency to assume that oxygen-based biodegradation is unimportant when oxygen concentrations are lower than the detection limit may lead to costly mistakes in the choice of environmental cleanup methods. In reality dissolved oxygen concentrations less than the detection limit are just that—less than the detection limit but otherwise unknown.

U.S. Geological Survey (USGS) scientists and others have recently shown that there is substantial oxygen-linked biodegradation of chlorinated solvents, such as dichloroethene (DCE) and the cancer-causing chemical vinyl chloride (VC), at dissolved oxygen concentrations well below the 0.1 to 0.5 mg/L field detection limit. Therefore, the assumption that “less than detection” oxygen concentrations are “unimportant” is not valid.

USGS scientists have developed a modified framework for assessing the importance of oxygen-based biodegradation of chlorinated solvents at groundwater contamination sites. The framework accounts for the possibility that there can be aerobic or anaerobic biodegradation taking place at locations in the subsurface where the dissolved oxygen is below the 0.1 to 0.5 mg/L measurement threshold. The framework has the potential to help environmental-cleanup professionals and regulators develop more cost-effective and environmentally responsible remediation plans.

This study was funded by the USGS Toxic Substances Hydrology Program and the Strategic Environmental Research and Development Program (SERDP).

References

Bradley, P.M., 2012, Microbial mineralization of cis-dichloroethene and vinyl chloride as a component of natural attenuation of chloroethene contaminants under conditions identified in the field as anoxic: U.S. Geological Survey Scientific Investigations Report 2012-5032, 30 p.

Bradley, P.M., 2012, Perils of categorical thinking—"Oxic/anoxic" conceptual model in environmental remediation: Remediation Journal, v. 22, no. 3, p. 9-18, doi:10.1002/rem.21317.

Bradley, P.M., 2011, Reinterpreting the importance of oxygen-based biodegradation in chloroethene-contaminated groundwater: Ground Water Monitoring and Remediation, v. 31, no. 4, p. 50-55, doi:10.1111/j.1745-6592.2011.01344.x.

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Created on August 1, 2013