Determining Source Attenuation History to Support Closure by Natural Attenuation
ER-201032
Objective
The objective of this project is 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 is 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 are 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.
Technology Description
At a series of two or three sites, soil and groundwater samples will be collected at discrete depths within high and low permeability zones within and below the source zone and also the downgradient plume, with soil sampling targeted predominately at defining the mass distribution in low permeability zones. When combined with modified 1-D and/or 2-D transport models that account for diffusion processes, this sampling approach enables historical reconstruction of the source loading over time. In cases where the characteristic back diffusion profile from locations within the source and in the downgradient plume are encountered in the low permeability zones, they can be used to demonstrate a concentration trend resulting from source decay that has occurred in the time elapsed following release. This is done by back-calculating the source loading (i.e., concentration history at the low permeability interface) that would have resulted in the measured soil concentrations in the low permeability layers as the result of diffusion-dominated transport in these layers. Performance objectives include development of site-specific source history models that capture the style of the detailed profiles collected during this study and a comparison of the sampling and analysis techniques with conventional methods.
Benefits
This project will help break the logjam of 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. This project will address sites where chlorinated solvents have impacted soil and groundwater, but it also has potential applicability at sites where other constituents of concern are present, such as fuels, MTBE, perchlorate, and explosives. (Anticipated Project Completion - 2012)
Points of Contact
Principal Investigator
Dr. Charles Newell
GSI Environmental Inc.
Phone: 713-522-6300
Fax: 713-522-8010
Document Types
- Fact Sheet - Brief project summary with links to related documents and points of contact.
- Final Report - Comprehensive report for every completed SERDP and ESTCP project that contains all technical results.
- Cost & Performance Report - Overview of ESTCP demonstration activities, results, and conclusions, standardized to facilitate implementation decisions.
- Technical Report - Additional interim reports, laboratory reports, demonstration reports, and technology survey reports.
- Guidance - Instructional information on technical topics such as protocols and user’s guides.
- Workshop Report - Summary of workshop discussion and findings.
- Multimedia - On demand videos, animations, and webcasts highlighting featured initiatives or technologies.
- Model/Software - Computer programs and applications available for download.
- Database - Digitally organized collection of data available to search and access.