Multiyear Temporal Changes in Chlorinated Solvent Concentrations at 23 Monitored Natural Attenuation Sites

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There is an increasing interest in predicting how long it will take natural attenuation processes to remediate groundwater plumes. The U.S Environmental Protection Agency (USEPA) Monitored Natural Attenuation (MNA) Directive (USEPA 1999) specifies that MNA relies on “ . . . a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or groundwater.”

MNA involves data collection to estimate both the rate of attenuation processes and the “anticipated time required to achieve remediation objectives.” The USEPA adds that “ . . . determination of the most appropriate time frame is achieved through a comparison of estimates of remediation time frame for all appropriate remedy alternatives.”

Some state regulatory programs also require a relative or absolute estimate of remedial time frame. For example, the Texas Commission on Environmental Quality (TCEQ) mandates all MNA demonstrations address the requirement that remedial goals be achieved in a reasonable time frame by either: (1) comparing times required using other remedial alternatives at the same affected property under similar conditions; and/or (2) developing estimates of how long MNA will take to reach the cleanup goals (CEQ 2001). The TCEQ program states that the simplest method to estimate a remediation time frame for MNA is to calculate a concentration versus time rate constant and then use the rate constant to determine the time when concentrations reach the required response goal.

Temporal Trend Studies

The potential lifetime of chlorinated solvent plumes and source zones has been evaluated using laboratory studies, mathematical models database studies, and heuristic judgement. Using laboratory data from Schwille (998) and an analysis developed by Johnson and Pankow (1992), a hypothetical 50 L pool of dense nonaqueous phase liquids (DNAPL) would require at least 100 years to dissolve completely (Feenstra et al. 1996). Feenstra et al.
(1996) concluded that source zones with considerable DNAPL mass “ . . . will persist for many decades or centuries.”

In 2003, a USEPA expert panel, based on their experience and judgment, stated that “the current default assumption is
that DNAPL sites will require several decades to centuries of plume management with significant cost and future uncertainty” (Kavanaugh et al. 2003) and that contaminant mass in source zones can (except in rare cases) only be partially depleted (removed or destroyed)by active remediation. The expert panel’s definition of a “DNAPL source zone” was the groundwater volume where DNAPL is in a separate phase or the volume where once-present DNAPL is now present only in the dissolved or sorbed phases or diffused into the matrix in fractured systems.

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