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:
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.