Natural Attenuation as a Remedial Strategy

Natural attenuation should not be perceived as a permanent remedy or as a means to achieve certain cleanup levels, but rather as (1) an interim measure until future technologies are developed, (2) a managerial tool for reducing site risks, and (3) a bridge from active engineering (i.e., pump-and-treat, vapor extraction, etc.) to no further action. 

No further action may be preferable to natural attenuation in certain instances. Very low risk situations may be better served by no further action since it eliminates the need for continued monitoring and further documentation. Sites with low levels of contaminants or nondiscemible plumes may be better candidates for no further action. Furthermore, very minor releases of hydrocarbons to the subsurface may not be sufficient to support bioremediation. Alternatively, sites with elevated levels of contaminants in nonpotable aquifers may be better addressed through conduct of a risk assessment. 

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Primary Parameters for Evaluation of Natural Attenuation as a Remedial Strategy... 

Affected communities want contamination to be controlled quickly and natural attenuation is unlikely to be perceived as being part of the solution. There are many factors that may influence the perception of the acceptability of natural attenuation as a remediation strategy for a contaminated site. The NRC (36) has outlined many of the problems that confound community confidence in the use of MNA as determined by a panel of six community leaders representing four states affected by large industrial waste sites. Among their concerns ... 

Other U.S. EPA reports explain what MNA means when the term is used to describe a potential strategy to remediate a contaminated site. They also describe the various physical, chemical, and biological processes of natural attenuation that may occur at a site. Other informational materials are in preparation and will provide more specific details and scientific depth for the evaluation of MNA as a remedy at specific sites. 

A scientific basis must be provided before natural attenuation can be accepted as an effective remediation alternative, because the public and communities often suspect this to be just a do nothing or walk away strategy by the polluters (National Research Council, 2000). The specific natural attenuation processes must be identified and quantified, and their long term sustainability must be evaluated (National Research Council, 2000). The geochemical modeling work, described in this section, provides the conceptual framework, which can help document and interpret the footprints (National Research Council, 2000) of the natural attenuation processes that are actually occurring in the aquifer. 

Because of the heterogeneous spatial distribution of physical and chemical parameters such as hydraulic conductivity, Fe(II)/Fe(III)-mineral content models of aquifer systems are mostly limited by an insufficient amoimt of input data. The idea of virtual aquifers is to engineer a data set describing a virtual aquifer system in the computer as realistically as possible in order to simulate different monitoring and remediation strategies. The quality and limitations of such strategies can be evaluated without measurement of concentrations in the field, which is costly and not very helpful for this task. The capabilities of virtual aquifers to identify and solve problems in evaluation of natural attenuation processes will be presented in three examples. 

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