Monitored natural attenuation (MNA) has emerged as a preferred remedial option at many sites impacted by chlorinated solvents because it offers a cost-effective and practical approach for cleanup of solutes in groundwater. However, existing MNA protocols do not include 1,4-dioxane and commonly co-occurring chlorinated solvents like 1,1,1-trichoroethan (1,1,1-TCA), 1,1-dichloroethan (1,1-DCA), and 1,1-dichloroethene (1,1-DCE). The objectives of this project were to:

1. Develop a modified model and framework for evaluating natural attenuation of these compounds.
2. Develop and validate a protocol to directly measure rate constants for natural biodegradation of 1,4-dioxane using ¹⁴C- labeled 1,4-dioxane and groundwater from 10 different field sites.
3. Use the field and laboratory data to establish if there is consistency between various lines of evidence for 1,4-dioxane attenuation.

An evaluation of MNA relies on establishing various lines of evidence, including secondary and tertiary lines of evidence that help demonstrate degradation processes and associated rates that are responsible for the primary line of evidence (decreasing concentrations of the target compound(s)). This project developed a new fate and transport model to easily evaluate historical monitoring data to predict biodegradation rate constants as well as new decision matrices (flowcharts) that serve as a guided tour on how to interpret potential lines of evidence for MNA. These were then integrated into an existing software platform (BioPIC) that allows users to access both the model and the decision matrices. Several approaches also were used to generate input data to support and validate the model and framework. First, rate coefficients and lines of evidence for attenuation were calculated and/or measured at multiple sites using a focused sampling program at 10 field sites. Second, degradation and the associated rate constants for 1,4-dioxane at these same sites were determined using a ¹⁴C-labeled 1,4-dioxane assay developed for this project.

The ¹⁴C assay was successfully validated and used to establish 1,4-dioxane natural attenuation capacity at 9 of the 10 field sampling sites. Similarly, the model was able to extract rate constants for 1,4-dioxane biodegradation at 8 of 9 sites that had sufficient data to evaluate. Several project performance objectives focused on ensuring that the decision framework was functional and captured all relevant processes for the targeted compounds. The success criteria for all these objectives were successfully achieved, in part because the model provides quantitative support for the decision framework. Multiple performance objectives related to the sensitivity and robustness of the ¹⁴C assay in quantifying 1,4-dioxane rate constants were also achieved. Performance objectives that were partially achieved or not achieved were related to: i) consistency between the model-predicted rate constants and the ¹⁴C-based rate constants (achieved at 3 of 9 field sites with sufficient data); and ii) correlations between biomarker abundance and rate constants (no clear relationship could be established).

A cost model was developed that included collecting the necessary data and performing a comprehensive MNA evaluation using the project deliverables. This resulted in an estimated total cost of approximately $30,000 for assessing MNA at a single site. While not incorporated into the model, this assessment could result in substantial savings if the use of MNA is justified versus more aggressive remedies.

Application of the decision matrices and BioPIC-TCA/Dioxane likely will increase the number of sites where MNA is implemented. These decision matrices will include logic for evaluating/selecting remediation strategies based on A Guide to Selecting Remedies for Subsurface Releases of Chlorinated Solvents and NAS. This approach will reduce capital and O&M expenses, and will help minimize impacts associated with invasive remediation options. No regulatory or procurement issues are anticipated. Implementation involves standard equipment and no special permitting. End-user concerns are minimal since data can be collected during a single mobilization without disruption to site activities. 

Adamson, D.T., G. Uhlir, S.R. Rauch, T. Klein, and A.S. Danko. 2021. Trends in 1,4-Dioxane Analyses: Implications for Identification and Characterization of Contaminated Groundwater Sites. Groundwater Monitoring and Remediation, 41(3):29-40. doi.org/10.1111/gwmr.1242.

Adamson, D.T., J.T. Wilson, C.J. Newell, B.A. Strasert, P.C. de Blanc, D.L. Freedman, C. Lebrón, and A. Danko. 2022. State of the Practice Worldwide: 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. Groundwater Monitoring and Remediation, 42(4):78-84. doi.org/10.1111/gwmr.12509.

Adamson, D.T., J.T. Wilson, D.L. Freedman, A.A. Ramos-García, C. Lebrón, and A. Danko. 2022. Establishing the Prevalence and Relative Rates of 1, 4-Dioxane Biodegradation in Groundwater to Improve Remedy Evaluations. Journal of Hazardous Materials, 424(D):127736. doi.org/10.1016/j.jhazmat.2021.127736.

García, Á.A.R., D.T. Adamson, J.T. Wilson, C. Lebrón, A.S. Danko, and D.L. Freedman. 2022. Evaluation of Natural Attenuation of 1, 4-Dioxane in Groundwater using a 14C Assay. Journal of Hazardous Materials, 424(C):127540. doi.org/10.1016/j.jhazmat.2021.127540.