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ASSESSING THE ROLE OF NATURAL ATTENUATION FOR INORGANIC CONTAMINANT REMEDIATION IN GROUND WATER
Citation:
Ford*, R G., R W. Puls*, AND R T. Wilkin*. ASSESSING THE ROLE OF NATURAL ATTENUATION FOR INORGANIC CONTAMINANT REMEDIATION IN GROUND WATER. Presented at NIEHS Superfund Quad-University/EPA Region 9 Conf, Berkeley, CA, October 08 - 10, 2003.
Impact/Purpose:
To inform the public.
Description:
Monitored natural attenuation (MNA) has been applied as a knowledge-based remediation technology for organic contaminants in ground water. The application of this technology is being considered for remediation of inorganic contaminants in ground water at hazardous waste sites. In order to provide regulators and the regulated community with the appropriate information for assessing the viability of this technology, the Ground Water and Ecosystems Restoration Division has initiated an effort to develop an Agency framework document to serve as a point of reference and is carrying out site-specific evaluations for the application of MNA for ground-water restoration at contaminated sites.
The Ground Water and Ecosystems Restoration Division is leading an effort to develop a USEPA publication that provides the policy, scientific and technical framework for assessing the viability of MNA for inorganic contaminants in ground water (hereafter referred to as the Framework Document). Initial guidance on the application of MNA for all ground-water contaminants was first provided by the USEPA through the OSWER Directive, entitled, Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites (9200.4-17P). The purpose of this Directive was to clarify USEPA policy regarding the use of MNA for remediation of contaminated soil and ground water. However, this publication is limited in scope with respect to providing detailed requirements for the technical assessment of inorganic contaminant MNA at hazardous waste sites. The behavior of inorganic contaminants in the subsurface differs significantly from organic contaminants in that microbial degradation is not a viable process for contaminant removal. The primary processes effective for natural attenuation of inorganic contaminants (radionuclides and non-radionuclides) are immobilization and radioactive decay. The assessment of contaminant immobilization requires establishing contaminant removal onto immobile aquifer solids and verification of the stability of this removal process towards remobilization due to changes in site geochemistry. This necessitates knowledge of the hydrologic and biogeochemical processes that control contaminant immobilization. Tasks to be performed as part of a site-specific assessment will be apportioned within a tiered process in which a progressively increasing level of knowledge is required to successfully demonstrate MNA viability. Candidate sites that do not meet the requirements of a given tier will be removed from consideration for application of MNA. The Framework Document will outline requirements and techniques for acquisition of site-specific data. It will also provide a summary of the potential success of MNA for a select number of radioactive and non-radioactive contaminants based on the current state of knowledge with respect to contaminant geochemistry and alternative remedial technologies that can be implemented as a contingency in the event of MNA failure.
Development of the Framework Document is being carried out in conjunction with site-specific assessments of the viability of MNA at sites with ground water contamination. For one of these field sites, the Ground Water and Ecosystems Restoration Division is assessing the potential for natural attenuation of arsenic within a contaminated ground-water aquifer. Based on the current state of knowledge, arsenic is considered to be a contaminant for which application of MNA may be of marginal success. The mobility of arsenic in ground water is strongly dependent on partitioning to immobile aquifer solids. However, arsenic is susceptible to changes in chemical speciation due to shifts in redox chemistry resulting from abiotic and biotic processes. These potential changes in chemical speciation require that detailed information for assessing the stability of immobilized arsenic be collected in space and time. This observation has been confirmed as part of the field study, where it has been established that partitioning to sediments results in significant removal of arsenic from the aqueous phase. However, the partitioning process is readily reversible under reducing conditions, indicating that MNA cannot be used as a sole remedy for site cleanup.