Posted on July 26, 2016 by Charles Efflandt
The importance of a thorough technical evaluation of monitored natural attenuation (MNA) at chlorinated solvent and other groundwater-contamination sites cannot be overestimated. Regulatory acceptance of MNA as a preferred remedial alternative can save millions of dollars in response costs compared to common presumptive remedies. Because “active” remediation technologies rarely achieve complete contaminant treatment or removal, MNA is an implicit, if not specifically evaluated, component of most groundwater remedial actions. A proposal to use MNA as the primary cleanup mechanism, however, is often met with resistance from regulators, notwithstanding years of supportive data. Such resistance may be attributable to antiquated agency policies or, perhaps, an inadequate evaluation of evolving MNA science.
The use of MNA at groundwater sites has typically required a showing of a stable or shrinking plume, source control, sustainable natural attenuation conditions, and acceptable risk to health and the environment. Today, mathematical and modeling tools can systematically establish data trends demonstrating that remedial action objectives will be achieved through natural attenuation in a reasonable time frame.
Unfortunately, even if confronted with irrefutable data, many state regulators will reject meaningful consideration of MNA unless the attenuation mechanism can be pigeon-holed into policies that focus on the demonstration and scoring of anaerobic biodegradation conditions at a site. That is because after almost two decades, EPA’s 1998 Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water remains the framework for MNA evaluations and decision-making in many states. Because the 1998 Protocol presumed that the primary effective mechanism for natural attenuation was anaerobic biodegradation, the Protocol has unduly restricted state policies for screening and approval of MNA remedial action.
Numerous studies since the publication of the 1998 Protocol, however, have shown that a viable MNA remedial strategy can be supported by attenuation mechanisms other than anaerobic biodegradation These studies have documented other viable contaminant-destructive attenuation mechanisms and evaluation tools, such as aerobic cometabolism enzyme degradation, magnetic susceptibility, compound specific isotope analysis, and improved sampling and modeling techniques. Greater awareness of these scientific developments by regulators and environmental professionals will result in MNA being an increasingly important remedial tool at many groundwater sites.
We have learned the hard way that it’s much more difficult and expensive to clean up sites using default remedies than first thought. Fortunately, it is becoming increasingly apparent that nature has an ability to degrade various chemicals more quickly and effectively than previously believed. Regulatory acceptance should not, and need not, include unreasonable technical hurdles, such as imposing attenuation “causation” requirements that are neither feasible nor necessary to support what cannot be disputed. That a proposed MNA remedy does not neatly fit into the traditional anaerobic degradation box, and cannot with precision be attributed to one or more alternative degradation mechanisms potentially active at a site should not be determinative. At the end of the day, the data don’t lie. The MNA determination ought to begin with, and remain focused on, the empirical data and data trends.