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Natural Attenuation: Is Dilution the Solution?

Just as it was beginning to look like we were winning the remediation battle at groundwater contamination sites, the impact of MTBE releases reared its ugly head. In the midst of our remediation battle and our MTBE discoveries, we have seen the blossoming of a management strategy for contaminated groundwater known as monitored natural attenuation, or MNA. MNA refers to the reliance on natural attenuation (see definition below) processes within the context of a controlled and monitored site cleanup approach to achieve remedial objectives.

A close examination of the application of MNA, however, reveals some potential problem areas involving the misidentification of processes that govern contaminant plume behavior. These problems are often the result of the misapplication of simulation modeling techniques and/or consideration of unrepresentative data due to outdated or inappropriate monitoring well construction and sampling approaches. Dispersion on a grander scale has been advocated at municipal production wells as one approach to diluting the problem plume.

Such problems have become especially apparent at MTBE release sites, where the use of MNA could present the danger of a potential false sense of security. We may be encouraging “walk away” site closures when active remediation should really be implemented.

In this article we’ll discuss potential pitfalls associated with MNA and explore the limitations of monitoring networks. Ground- water sampling techniques can promote misidentification of plume biogeochemical parameters and in some instances excessive dilution. We’ll examine the location and construction of monitoring wells, seldom spelled out in state standards or guidelines. We’ll conclude by highlighting the potentially false sense of security we may have when we mistake concentration dilution for concentra- tion destruction at a LUST site or in a municipal production well.