|Site Characterization to Support Use of Monitored Natural Attenuation for Remediation of Inorganic Contaminants in Ground Water (EPA/600/R-08/114) November 2008
The term "monitored natural attenuation," as used in the following discussion and in the Office of Solid Waste and Emergency Response (OSWER) Directive 9200.4-17P (hereafter referred to as the 1999 OSWER Directive; USEPA, 1999), refers to "the reliance on natural attenuation processes (withinthe context of acarefullycontrolled and monitored site cleanup approach) to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods." When properly employed, monitored natural attenuation (MNA) may provide an effective remedy for ground water where a thorough engineering analysis informs the understanding, monitoring, predicting, and documenting of the natural processes. In principle, MNA provides a reasonable remedy for attaining groundwater cleanup objectives for some sites with inorganic contaminants (typically metals and radionuclides). Due to potential limitations in attenuation capacity within an aquifer, MNA is likely to be more applicable as a polishing step and/or under more dilute plume concentrations as compared to situations encountered in source zones or in more concentrated regions of a ground-water plume.
The objective of site characterization for assessing the viability of MNA as a component of ground-water cleanup is determination of the performance characteristics of the subsurface system with respect to achieving cleanup goals. As stated within the 1999 OSWER Directive, one of the primary processes that may result in natural attenuation of an inorganic contaminant in ground water is the transfer of the mobile contaminant into an immobile form within the aquifer solids; this process is generally referred to as "sorption", inclusive of adsorption, co-precipitation, and precipitation reactions (See page 8 of USEPA, 1999; illustrative reactions shown in Table 1). The presumption for sites where "sorption" (hereafter referred to as immobilization) appears to result in contaminant attenuation is that a specific mechanism (or mechanisms) controls contaminant partitioning to aquifer solids. Thus, in order to reliably evaluate the capacity for and stability of contaminant immobilization within the aquifer, the mechanistic characteristics of the partitioning process and the identification of the subsurface components that influence the extent of the immobilization reaction need to be understood. This requires information on the abundance and chemical speciation of solid phase reactants and products that participate in the immobilization reaction. The purpose of this Issue Paper is to highlight at what stage of the process solid phase characterization techniques need to be implemented in the site characterization process and to describe two case studies where the results of such techniques were critical to evaluation of MNA as a potential component of ground-water cleanup.
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