State of the Practice Worldwide: Developing Approaches to Transition from Active Remediation to Monitored Natural Attenuation

Abstract A landmark National Research Council report from 2013 emphasized the need to transition to passive or less resource‐intensive management strategies like monitored natural attenuation (MNA) at sites where pump‐and‐treat and active remedies are providing diminishing returns. While this report is now more than a decade old, the process for implementing this type of transition assessment is still not well understood by practitioners. The objective of this study was to help address this gap by developing a software tool (TA 2 Tool) that aids in gathering and analyzing data that are relevant for a site‐specific transition assessment. The implications of site complexities on achieving remedial objectives are a key component of transition assessment. This free web‐based tool has modules that perform quantitative assessment of concentration trends and project the remediation timeframe based on the current remedial approach. It has modules that predict how remediation timeframes are influenced by matrix diffusion to assess if additional remediation is warranted. Crucially, it includes modules that evaluate MNA as a transition technology, specifically by looking at plume stability, natural attenuation rates, and projections of plume concentrations at a downgradient point of compliance in the absence of further active treatment. This new tool complements existing resources on technology optimization and transitions, including ITRC guidance. The value of performing these types of assessments is highlighted through empirical data and case studies that show that it is not necessary to operate pump‐and‐treat systems in perpetuity and that many sites with these systems have either transitioned to other technologies or been closed.