The objective of this project was to reduce the uncertainty associated with assessing long-term concentration trends for use in remedy selection at sites with chlorinated solvent contamination in soil and groundwater. The hypothesis was that diffusion of contaminant mass into and out of low permeability geologic strata within a source zone provides a method of reconstructing the source history by obtaining detailed soil concentration profiles within these less transmissive layers, along with measurement of relevant transport parameters. Specific objectives of this project were to (1) demonstrate a sampling and analysis approach for collecting data appropriate for use in reconstructing source histories; (2) develop simple models to validate the data obtained during the field demonstration; and (3) create guidance documents for using this approach at Department of Defense (DoD) sites to aid source management decision making.
This project tested an innovative approach for reconstructing the “source history” at a site (i.e., the concentration trends over time) by using high-resolution soil coring within low permeability (low-k) zones. Essentially, soil cores in these zones serve a similar role as tree rings, in that the cores store information about historic environmental conditions. For contaminants that have migrated into low-k zones via diffusion and slow advection, the concentration versus depth profile can be used to determine if attenuation of the contaminant source in the overlying transmissive zones has occurred. The results can provide an important line of evidence for evaluating the viability of monitored natural attenuation (MNA) at a site.
The project developed a simple transport-based spreadsheet tool—the Source History Tool—to generate source history estimates from high-resolution soil core data. The basis for the modeling approach was the one-dimensional (1-D) diffusion equation using Fick’s second law. This law defines the diffusion of a chemical in solution in response to a concentration gradient. The analytical solution allows for the concentration at any depth to be determined based on the concentration at the interface. Results were compared to prior source history reconstructions based on detailed numerical modeling.
The Source History Tool calculates the concentrations at the high-k/low-k interface over time that would best represent the vertical concentration profile measured in the soil cores. It develops this pattern by systematically adjusting the interface concentration at various time intervals until a representative “best” fit is obtained.
To validate the tool, data were collected from two different source areas located at the Naval Air Station (NAS) Jacksonville, and soil core data from other sites were used to supplement the evaluation. An evaluation of project results yielded the following key conclusions:
- The tool proved easy to use, and successfully modeled data from all source areas tested. A total of 17 source histories were generated during this project (for different compounds and sites), and all of these proved similar to existing estimates and/or site information. As described below, the tool was successful at both of the source areas at NAS Jacksonville. Results showed that using the Source History Tool can provide valuable information for developing and/or refining the conceptual site model by enhancing our understanding of the likely “style” of source history at a site.
- Source history was successfully reconstructed at a site with a constant source area concentration over time. A constant source was predicted for chlorinated ethenes at the first source area (Building 106). This prediction was consistent with the measured soil concentration profiles, where the maximum concentrations were measured at the interface, and the concentrations decreased with distance into the low-k clay. The release date predicted for the near source location, as well as the subsequent arrival dates at downgradient locations, were consistent with site characteristics. In addition, the results confirmed existing evidence of attenuation along the plume flowpath.
- The model was also used successfully at a source area with a declining concentration. Decreasing chlorinated ethene concentrations at the high-k/low-k interface were predicted at the Building 780 source area, and this prediction was consistent with the soil coring data showing peak concentrations occurring several feet into the low-k clay. This result provided strong evidence for source decay over time. With the exception of 1,2-dichloroethane (DCA), data from co-located cores showed no evidence for decay of chlorinated solvent sources at this site. Again, the release and arrival dates that were predicted by the model were consistent with the available site information.
- Understanding the potential impact of degradation is an important component of the source history approach. The Toolkit incorporates degradation within the low-k zone by including a constituent half-life as an input parameter. While the default assumption is that degradation is minimal in low-k zones, a focused sampling and analysis program was performed to assess biodegradation in the low-k zones at the Building 106 source area. This assessment included use of molecular biological tools, compound-specific isotope analysis, geochemical analyses, and measurements of the relative distribution of parent compounds versus degradation byproducts. The assessment supported the assumption that, at this site, the majority of degradation was occurring in the high-k zone and not the low-k zone.
- Several characterization tools proved useful for screening locations for soil coring. The tools tested during this project included membrane interface probes (MIP), the WaterlooAPS TM, and the Geoprobe Hydraulic Profiling Tool (HPT). These methods are well-suited for determining relative permeability distributions and locating interfaces between low-k and high-k zones.
- The uncertainty analysis included in the Toolkit can provide valuable information. The analysis is not only helpful for understanding the relative sensitivity of various input parameters, but it also can be used to simulate alternative scenarios. For the sites investigated during this project, the results were most sensitive to porosity and tortuosity. The constituent half-life has little impact until values fall below approximately 10 years.
- Proper sample handing, preparation, and analysis methods are critical. A thorough evaluation confirmed that the protocol used in this project, including methanol preservation in the field, generates high-quality data and should be adopted to the extent possible.
- The costs for high-resolution soil coring may be justified if it allows use of MNA. For a site where the source history method was applied at a scale similar to that used for this project, the stand-alone costs were approximately $161K, or $1150 per vertical foot (cored), with $35.8K of this cost due to project-specific reporting. For a full-scale application of this approach where it leads to selection of MNA, the estimated life cycle cost is 23% less than enhanced in situ bioremediation (which may not be effective in low-k zones), and 75% less than pump-and-treat.
This project will help at DoD sites where natural attenuation has been proposed, but not accepted by regulators due to concerns that the source is not being treated fast enough. A long-term source history from the beginning of releases at a site to present time can help confirm the site conceptual model that shows attenuation is a significant process for both the source and the plume, and it generates data that are well-suited for predicting future attenuation trends. This approach has the potential to eliminate source removal as a necessary step at some sites prior to adopting natural attenuation as a long-term remedy, which reduces the complexity of remedy selection and accelerates implementation.