Assessment of the Natural Attenuation of NAPL Source Zones and Post- Treatment NAPL Source Zone Residuals

摘要

This project demonstrated a paradigm for assessing source zone natural attenuation (SZNA) at chlorinated aliphatic hydrocarbon (CAH) impacted sites. 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 CAH-impacted sites. Thus there is a need for a well-defined and accepted assessment approach. The data-driven method anticipates that decision makers will be interested in the following questions: 1-Is SZNA occurring and what processes contribute; 2-What are the current SZNA rates; and 3-What are the longer-term implications of SZNA. 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. Application of the approach was demonstrated at three CAH impacted sites through four events per site over about three years. The mass loss rates were relatively consistent over time for each site, but varied from site to site, ranging between about 1 10 kg/y at two sites and as high as about 600 kg/y at the third site. When applying the generalized CAH-SZNA method, it is likely that different practitioners will choose the number and locations of samples in different ways. For example, this could happen at a site over different sampling events. This then raises the question: Is the calculated SZNA mass loss rate likely to be dependent on the sampling strategy, and if so, how should sampling plans be designed to ensure consistency in results across practitioners. As a result, the high spatial-density data collected from the demonstration sites in this project were used to examine the effect of different sampling strategies on the quantification of mass loss rates at those sites. That experience, and lessons- learned from previously published studies on this topic, were used as the basis for new proposed heuristic sampling guidelines. The hope is that these can increase sampling efficiency and confidence in the quantification of SZNA rates.