A Framework for Evaluating Monitored Natural Recovery of Metals-Contaminated Wetland Sediments

摘要

Background/Objectives. Monitored natural recovery (MNR) is often included as a componentof sediment remedies due to the cost and impact of active remediation. Theevaluation of MNR as a remedial approach is difficult because it does not rely on thestraightforward comparison of contaminant concentrations to a single clean-up criterion.Rather, as described in the “Technical Guide: Monitored Natural Recovery at ContaminatedSediment Sites” (Magar et al., 2009), the effectiveness of MNR for reducing risk tohuman health and the environment requires an evaluation of multiple lines of evidence.Collectively, the evidence must provide confidence that natural processes will reduce riskover time. Unlike organic compounds that can degrade to inert substances, metals are notsubject to degradation. Thus, for metals, the lines of evidence need to be particularlyconvincing.Approach/Activities. The fate, transport, and toxicity of metals in wetlands sedimentsdepend greatly on redox conditions and pH. A framework that captures this geochemistryas well as other important factors for natural recovery is necessary to properly evaluateMNR as a remedial technology. As part of a comprehensive evaluation of remedial alternativesfor a project site with metals-contaminated sediments in a wetland, CDM Smithdeveloped a conceptual framework to evaluate monitored natural recovery. The frameworkconsiders geochemistry in sediments, hydraulics in the wetland, sediment stability,as well as bioavailability and public acceptance. The framework was incorporated intothe study to evaluate the effectiveness of MNR as a standalone technology and in conjunctionwith other remedial technologies.Results/Lessons Learned. The data types and site understanding necessary to evaluateeach factor will be discussed and presented. The results of a comparative analysis of thefactors will be presented to demonstrate the relative importance of each factor in variouswetland settings. Previous guidance for MNR from USEPA and others will be referenced.This work demonstrates the usefulness of the lines of evidence approach and expands ourunderstanding of how to evaluate MNR as a component of an overall site remedy.