Source zone natural attenuation (SZNA) is often used as a basis for assessing the performance and relative benefits of engineered remediation and is also a component of the cleanup strategy at most chlorinated aliphatic hydrocarbon-impacted sites. However, there hasn't been a well-defined and accepted assessment approach available until recently. ESTCP funded Dr. Paul Johnson and his team from Arizona State University to develop a data-driven method based on the assumption that decision makers will be interested in the following questions:

  1. Is source zone natural attenuation occurring and what processes contribute?
  2. What are the current source zone natural attenuation rates?
  3. What are the longer-term implications of source zone natural attenuation?
  4. Are the rates sustainable?

The approach uses multiple lines-of-evidence and macroscopic mass balances, and these lead to confirmation of SZNA and quantification of the total mass loss rate resulting from degradation, dissolved phase transport, and volatilization. The assessment paradigm is structured around three groups of data collection:

  • Group 1 measurements are to provide evidence that source zone natural attenuation is occurring.
  • Group 2 measurements are focused on quantifying current mass loss rates.
  • Group 3 measurements are focused on addressing the long term questions regarding the longevity of source zone impacts.

An important aspect of this study was to examine the impact of sampling strategies on the quantification of source zone natural attenuation rates, given that different practitioners will likely choose the number and locations of samples in different ways. Consequently, how should sampling plans be designed to ensure consistency in results across practitioners? Data from this study combined with lessons-learned from previously published studies on this topic were used as the basis for new proposed heuristic sampling guidelines. The intent is that these new sampling guidelines can increase sampling efficiency and confidence in the quantification of source zone natural attenuation rates.

Application of the source zone natural attenuation assessment paradigm was demonstrated at three chlorinated aliphatic hydrocarbon-impacted sites through four events per site over about three years, and the results of these investigations are available in the project’s Final Report.