Development of a Quantitative Framework for Evaluating Natural Attenuation of 1,1,1-TCA, 1,1-DCA, 1,1-DCE, and 1,4-Dioxane in Groundwater

Abstract

A series of quantitative decision frameworks were developed to aid in the evaluation of Monitored Natural Attenuation (MNA) as a remedy for 1,4-dioxane, chlorinated ethanes, and chlorinated ethenes. The end product is an updated version of BioPIC, a successful, existing decision framework for MNA of chlorinated ethenes. In evaluating MNA, the goal is to follow existing protocols but also incorporate more recent insights that have increased the potential viability of this technology for a wider number of contaminants. This includes data supporting the first line of evidence for MNA (decreasing concentration trends over time at appropriate sampling locations), as well as data that will serve as secondary and tertiary lines of evidence for MNA. For the BioPIC update, a similar lines of evidence approach was developed, using information from literature and project-specific data to identify the relevant lines of evidence. As part of the update, a model for predicting contaminant trends over time and distance, including a method to estimate site-specific biodegradation rate constants for chlorinated ethenes, chlorinated ethanes, and 1,4-dioxane, has been included within BioPIC. The quantitative software tool provides step-by-step technical guidance for collecting and evaluating other lines of evidence, including data associated with targeted ¹⁴C assays, isotope fractionation, biomarkers for degradation, geochemical parameters, and co-contaminant concentrations, to improve the MNA evaluation process.