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GeoInsighter Spring 1999 Newsletter

Natural Attenuation of Hydrocarbon Plumes: How Long? How Far?

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Since 1995, researchers and consultants have demonstrated that anaerobic processes in oxygen- limited environments dominate the biodegradation of dissolved petroleum hydrocarbons, especially benzene, toluene, ethylbenzene and xylenes (BTEX), during natural attenuation (intrinsic remediation) of contaminated aquifers. In addition, it has been demonstrated that dissolved chlorinated hydrocarbons, such as perchloroethylene (PCE) and trichloroethylene (TCE), naturally biodegrade in similar environments; however, the degradation process of these compounds are more complex and may produce compounds more toxic than the parent compounds. It has been estimated that, at approximately 80 percent of fuel hydrocarbon spills sites, natural attenuation alone can be protective of human health and the environment. However, for chlorinated hydrocarbon sites, it has been estimated that natural attenuation alone can be protective at only 20 percent of sites studied. The significance of these findings is that oxygen (or other electron acceptors) and bacteria injection are not necessary at many sites to promote biodegradation, making monitored natural attenuation a more attractive remedial alternative to the standard "pump and treat" or air sparging/vapor extraction technologies. Monitored natural attenuation is generally more cost-effective, non-disruptive, and easy to implement in comparison to other physical remedial alternatives. The U.S. Environmental Protection Agency (EPA), the American Society of Testing and Materials (ASTM), and other professional organizations have published guidance on implementing the natural attenuation alternative, including the use of natural attenuation computer models.

When applying the natural attenuation approach in place of active remediation, clients and regulators typically ask two primary questions: 1) how long will the plume persist in the environment, and 2) how far will the plume extend if no engineered controls, such as pumping/ containment, are employed to stop its migration. The answers to these questions can be quite complex due to the number of processes affecting dissolved-phase hydrocarbon compounds in ground water, including ground water velocity and flow patterns, retardation of plume movement through interaction with soil, plume dispersion (mixing), and, most importantly, biodegradation mechanisms and rates. For chlorinated hydrocarbons, the answers are even more complex due to the sensitivity of the multi-step reductive dechlorination process that produces intermediate compounds during degradation.

To answer these primary questions, a number of predictive computer models have recently been developed that simulate the migration and fate of dissolved petroleum and chlorinated hydrocarbons in the subsurface. These models have been expanded from earlier versions to more accurately simulate the complex aerobic and anaerobic biodegradation of hydrocarbon releases. These models are typically employed after sufficient hydrogeologic information has been collected at the spill site to adequately characterize current plume magnitude and extent, to demonstrate that natural attenuation is occurring, and to identify potential human and ecological exposure points and pathways.

After these models are calibrated, they can effectively simulate natural attenuation of dissolved hydrocarbons in ground water, producing concentration distributions of hydrocarbon plumes over extended periods of time. The simulation results can then be used to demonstrate to regulators or other potentially effected parties:

  • that natural attenuation of the hydrocarbon plume is occurring, resulting in the stabilization or contraction of plume size and magnitude;

  • the appropriateness of a ground water management zone or activity and use limitation (AUL) area;

  • that downgradient receptors will not be impacted in the future by plume migration; and

  • that the length of time before concentrations attenuate to applicable standards are reasonable in comparison to other remedial alternatives.

Model results are also useful in evaluating the feasibility of remedial alternatives. They can be used to assess the feasibility of natural attenuation or evaluate the effectiveness of other remedial alternatives such as air sparging, ground water extraction, or biological nutrient-enhancement systems. In addition, if natural attenuation is the favored remedial approach, modeling results are instrumental in establishing where and when to monitor ground water quality over time. 

In summary, the application of these models, along with adequate hydrogeologic data, may provide strong evidence that natural attenuation, independently or in combination with other remedial alternatives, is an effective approach to cleaning up dissolved hydrocarbon contamination in ground water. Due to the relatively lower costs of monitored natural attenuation, as compared to other engineered remedial alternatives, you may want to consider using natural attenuation models in evaluating the effectiveness of monitored natural attenuation if you are responsible for the cleanup of hydrocarbon releases or if other remedial approaches seem to be costing you money with little effect toward achieving remedial goals. 

Richard J. Wozmak
 info@geoinc.com

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