At many Department of Defense (DoD) sites where an active remedy has been used, the performance of the active remedy has appeared to “hit a wall” in terms of being able to actually decrease risk and/or achieve closure criteria, in part due to site heterogeneity and the role of matrix diffusion in enhancing contaminant persistence at these complex sites. In 2012, a key National Research Council (NRC) report identified the need to “transition from active remediation to more passive strategies and provide more cost-effective and protective long-term management of complex sites,” including conducting formal transition assessments. While the NRC effectively describes the importance of a transition assessment, it understandably did not attempt to prescribe how such an assessment should be performed. In the intervening years, there has been continued interest in understanding and applying the concept, but little has been provided in terms of guidance or tools for conducting transition assessments. Without a clear understanding of how to proceed, it may be viewed as not worth the effort. These are the knowledge gaps that this research intends to address, specifically by developing a framework for performing transition assessments that includes monitored natural attenuation (MNA) as a key transition technology.

The objective of this research is to provide an easier way for site managers to answer several specific technical questions that are important during a transition assessment, such as:

  1. Is the plume at my site stable (and thus a good candidate for transitioning)?
  2. What is the likelihood that my site has a “persistent source” that will be resistant to further active treatment?
  3. How can I establish if performance of an active remedial technology has plateaued?
  4. What type of contaminant removal rates can I expect after transitioning to MNA?

Home page for the web-based software tool (TA2 Tool) being developed for this project. It contains 10 different modules that are designed to provide different quantitative and qualitative information to support a site-specific transition assessment.

Technical Approach

This project will leverage methods to calculate attenuation rates using site-specific data, “big data” remediation performance databases, and the next-generation groundwater models to give site stakeholders a reliable, transparent process regarding if and when a transition from active remediation to MNA is appropriate. The end product will be an integrated web-based tool that will contain:

  1. Easy-to-use calculators that transform historical site monitoring data into rate constants and trend data to compare active vs. passive site management alternatives and help identify transition points and the appropriateness of MNA;
  2. A statistical forecasting process based on ESTCP’s extensive data mining and rebound databases; and
  3. Next-generation matrix diffusion modeling tools that can, for the first time, provide accurate representation of how well active remediation will perform when combating matrix diffusion at real sites.

The web-based tool will consider multiple types of sites, such as sites where pump-and-treat is on-going vs. sites where active source zone remediation is being contemplated. It will also provide a detailed description of enhanced MNA alternatives at sites where MNA alone may not be sufficient to control risk, and will include automated “report writers” that condense, display, and illustrate the science and logic behind the transition assessment findings. These findings thus will address what have been identified as the key components of a transition assessment:

  1. A description of site complexities and their implications for achieving remedial objectives;
  2. A quantitative assessment of concentration trends that may include projecting the remediation time-frame based on the current remedial approach, demonstrating asymptotic behavior and plume stability, and estimating the attenuation rates/remediation time-frame if the current remedial approach was discontinued; and
  3. Identification of alternative approaches for managing the site along with their expected performance.


The research will generate a quantitative tool for Remedial Project Managers (RPMs) to use to support a site-specific transition assessment. The goal is to establish a protocol for evaluating sites where MNA could serve as an effective transition technology for longer-term management. Consequently, it is intended to improve the technical basis for decision making and thus foster consensus among stakeholders. (Anticipated Project Completion - 2023)