Determining Source Attenuation History to Support Closure by Natural Attenuation
Abstract
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 for this project 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 sol 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.
Document Details
- Document Type
- Technical Report
- Publication Date
- Nov 01, 2013
- Accession Number
- ADA610552
Entities
Organizations
- Environmental Security Technology Certification Program