Within the last decade, the effects of natural attenuation processes have been integrated into hundreds of groundwater remedies for chlorinated aliphatic hydrocarbon (CAH) plumes across Department of Defense (DoD) facilities, either as stand-alone monitored natural attenuation (MNA) remedies or as part of multicomponent remedial designs (e.g., engineered source reduction and MNA). In recent years, several groups have concluded that available technical guidance documents do not provide for the type of analyses required for defensible, long-term, proof-of-principle determinations for groundwater remedies that include MNA. There is a need for supplemental technical guidance that focuses on using more advanced and/or refined methods to quantitatively evaluate the long-term performance (i.e., sustainability) of MNA.

 The objective of this project was to identify and compile advanced analysis approaches, including both predictive and interpretive models, that may be applied to site-specific characterization and long-term monitoring (LTM) data to more thoroughly evaluate the sustainability of CAH natural attenuation processes at achieving reasonable regulatory goals or endpoints.

A list of data analysis requirements and recommendations to support credible long-term evaluations of groundwater remedies that include an MNA component was first developed, in part through a survey of professionals, regulators, and researchers. Information was then collected from several sites where MNA was a component of remedial designs to document the current state-of-the-industry concerning MNA assessment and performance evaluations and long-term sustainability estimates. In addition to analyzing actual site data, this effort reviewed the appropriateness of model selections and/or simplifying assumptions that were made during initial assessment activities. The usefulness of alternative predictive models or statistical approaches to address limitations in predicting MNA sustainability also was assessed.

The following general observations were developed from a review of data from multiple military sites where MNA is either a primary or secondary remedy component for remediation of chlorinated solvents in groundwater:

• Closure has remained elusive at a large number of sites where chlorinated solvents have been detected in groundwater at concentrations above applicable standards;
• A wide variety of in situ treatment approaches have been tested and implemented for chlorinated solvents, but no single technology or combination of technical approaches has emerged as the most effective approach to achieve site closure in terms of overall cost or performance;
• MNA is likely to remain an important component of low-cost remediation alternatives because there are significant technical and/or cost limitations encountered with active remedial strategies that attempt to achieve typical site clean-up objectives; and
• The technical performance and cost evaluation associated with LTM of MNA-based remedies can benefit from LTM programs that are designed to support site-specific closure strategies and data analysis methods that quantify the long-term sustainability of MNA-based remedies.

The three principal components of the sustainability assessment framework described in the final report are (1) analysis of plume stability, (2) estimation of remediation timeframes, and (3) estimation of the longevity of specific CAH degradation processes. Based on the site data reviews performed as part of the study, sufficient data were available from multiple sites to support development of recommended methods for advancing the state of practice for quantitative assessments of plume stability and low-cost estimates for remediation timeframe. However, a review of available data from 35 candidate sites did not yield any sites with existing data sets that contained all of the data required to perform the type of analysis that is believed to be needed to make a quantitative assessment of MNA sustainability where biodegradation is a significant contributor to contaminant mass degradation and protection of site-specific receptors.

 The purpose of evaluating various techniques for assessing plume stability was to advance the state of practice toward quantitative, rather than purely qualitative, metrics of plume stability. As part of the effort, both concentration-based and mass-based metrics of plume dynamics were evaluated. The findings of the study indicate that concentration-based and mass-based analyses of plume dynamics provide complementary information that can not readily be quantified by either method alone. For example, results confirm that statistical trend analysis of compliance well concentrations will continue to be important for documenting protectiveness of downgradient receptors. When looking at different areas within the dissolved plume, however, statistical trends in CAH concentrations may produce conflicting results that make it difficult to assess the overall ‘strength’ and stability of the CAH plumes. By visually or statistically analyzing changes in dissolved CAH plume total mass and mass distribution by CAH species over time, an approach for categorizing dissolved CAH plumes as ‘strengthening’ (dissolved mass increasing), ‘sustaining’ (dissolved mass stable), or ‘weakening’ (dissolved mass decreasing) was developed. In this context, the main plume assessment advantage gained by performing mass-based analyses is quantification of changes in dissolved plume ‘strength’ over time.

 In addition, this project provides multiple observations and recommendations that summarize lessons learned on the mechanics of applying the various methods described in this study to actual site data. These lessons learned can be broken into the following three general categories: (1) handling of below detection and low concentration measurements; (2) understanding of method limitations and how off-the-shelf programs implement these methods; and (3) criteria for choosing which data to include in a particular data analysis method. Of these three categories, the recommendations on how to select data for inclusion in a particular analysis method have the largest impact on the design of future LTM programs and on the data that should be entered into ‘black box’ plume stability assessment programs (e.g., Monitoring and Remediation Optimization Software [MAROS]). Specifically, the recommendations on how to select data for each analysis method provide a rationale for collecting more samples at regular, but infrequent events, and less samples during interim compliance monitoring events that are specified in site-specific regulatory decision documents.

Over the next 5 to 10 years, DoD will commit significant resources to evaluating and defending the performance of various groundwater remedies that have been implemented to address dissolved CAH contamination. Groundwater remedies that include an MNA component must be reviewed to determine the long-term sustainability of treatment and/or containment by natural processes. This project will provide guidance to DoD environmental managers on how to evaluate the sustainability of MNA and how to promote and select MNA remedies when appropriate. (Project Completed – 2008)