The objective of this project was to demonstrate and document the use of a source zone natural attenuation (SZNA) assessment approach at three chlorinated aliphatic hydrocarbon (CAH)-impacted sites over a multi-year assessment period. The data-driven approach leads to confirmation that SZNA is occurring and site-specific quantification of the source zone mass loss rate.
The CAH SZNA assessment paradigm is built on the SZNA conceptualization and calculation approach described by Johnson et al. (2006) and is structured around three groups of data collection and analyses. In brief, Group I measurements provide evidence that SZNA is occurring, Group II measurements and analyses are focused on quantifying current SZNA mass loss rates, and Group III measurements and analyses are focused on answering longer-term questions concerning the longevity of source zone impacts. This project focused mainly on Group II measurements and analyses, as often the immediate question of greatest interest is “what is the SZNA rate?”
As a data-driven, macroscopic, multiple-lines-of-evidence approach, the paradigm is consistent with the National Research Council (NRC) philosophy. Furthermore it is complementary to existing dissolved plume natural attenuation protocols and makes use of dissolved mass flux techniques and the source zone evolution with time modeling work discussed above. Lundegard and Johnson (2006) demonstrated the SZNA assessment approach at a multiple-source hydrocarbon spill site. Their paradigm was adopted by the Interstate Technology Research Council (ITRC) and reframed as “source zone natural depletion” in their guidance document.
In this project, the approach was demonstrated over four assessment events at three CAH-impacted sites. The SZNA loss rates estimated were: 2.9, 8.4, 4.9, 2.8 kg/y as tetrachloroethene (PCE) at Site 1;1.6, 2.2, 1.7, 1.1 kg/y as PCE at Site 2; and 570, 590, 250, 240 kg/y as trichloroethene (TCE) at Site 3. There were no clear temporal trends in the results and the differences in results for Site 3 are attributable to a change in groundwater sampling procedures. It was concluded that different practitioners likely will sample sites in somewhat different ways, especially regarding sampling density on a groundwater transect. The effects on discharge estimates from varied sampling densities and spacings were examined and sampling guidelines with practical site sampling densities were developed to reduce the variability in discharge estimates and to improve confidence in SZNA rates.
The approach used in this project for the assessment of SZNA at CAH sites uses fairly standard and readily available sampling and analytical tools. No barriers to the collection of the necessary data are anticipated other than those presented by unique site conditions. Many of the data needs and lines-of-evidence are similar to those appearing in dissolved plume natural attenuation guidance, with the exception of the assessment of effective vadose zone diffusion coefficients in Group II (to characterize gas transport processes) and the use of “bench-scale weathering tests” to provide Group III data. No special permits are required for implementation of the approach.