Underground Storage Tanks

EPA recognizes that determination of what timeframe is "reasonable" for attaining remediation objectives is a site-specific determination. The NCP preamble suggests that a "reasonable" timeframe for a remedy relying on natural attenuation is generally a "...timeframe comparable to that which could be achieved through active restoration" (USEPA, 1990a, p.8734; emphasis added). The NCP preamble further states that "[t]he most appropriate timeframe must, however, be determined through an analysis of alternatives" (USEPA, 1990a, p.8732). To ensure that these estimates are comparable, assumptions should be consistently applied for each alternative considered. Thus, determination of the most appropriate timeframe is achieved through a comparison of estimates of remediation timeframe for all appropriate remedy alternatives.

If restoring groundwaters to beneficial uses is a remediation objective, a comparison of restoration alternatives from most aggressive to passive (i.e., MNA) will provide information concerning the approximate range of time periods needed to attain groundwater cleanup levels. An excessively long restoration timeframe, using the most aggressive restoration method, may indicate that groundwater restoration is technically impracticable from an engineering perspective (USEPA, 1993a). Where restoration is technically practicable using either aggressive or passive methods, the longer restoration timeframe required by the passive alternative may be reasonable in comparison with the timeframe needed for more aggressive restoration alternatives (USEPA, 1996a).

It should be noted that the timeframe required for MNA remedies is often longer than that required for more active remedies. As a consequence, the uncertainty associated with the above factors increases dramatically. Adequate performance monitoring and contingency remedies (both discussed in later sections of this Directive) should be utilized because of this higher level of uncertainty. When determining reasonable timeframes, the uncertainty in estimated timeframes should be considered, as well as the ability to establish performance monitoring programs capable of verifying the performance expected from natural attenuation in a timely manner (e.g., as would be required in a Superfund five-year remedy review).

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