A critical challenge preventing many chlorinated solvent (or chlorinated volatile organic compound [CVOC]) sites from reaching groundwater cleanup goals is the long-term and persistent release of residual contaminants from low permeability media (e.g., clays, silts). Sites dominated by matrix diffusion processes are very difficult to remediate and have the effect of significantly prolonging restoration timeframes and increasing long-term site management costs. One of the common side effects of matrix diffusion is “rebound” of contaminant concentrations after an in situ treatment is applied. This project aimed to “repurpose” a commercially-available geotechnical technology, the Grout Bomber, for rapid and efficient delivery of remediation amendments (e.g., zero-valent iron [ZVI], vegetable oil) into a low permeability, shallow aquifer contaminated with CVOCs.
The Grout Bomber is a subsurface stabilization technology that uses an excavator equipped with a specialized “stitcher” mast to quickly push a mandrel (3.5 inch diameter hollow cylindrical rod) into the subsurface and subsequently fill the hole and subsurface voids with cement grout (from bottom to top) using an in-line grout delivery system. For the environmental application, a remedial amendment mixture of ZVI, vegetable oil, and sand was used (instead of cement grout) to create hundreds of biotic/abiotic reaction columns for degradation of chlorinated solvents. This application relies on diffusion to draw the contaminants from low-k silt and clay strata into the reaction column, where the contaminants are degraded by abiotic and biotic reductive dechlorination. The diffusion of CVOCs is driven by steep concentration gradients between the treated pore water within the reaction column and the surrounding media. The “long tail” of contaminant flux from low-k units can be significantly reduced if the diffusion lengths associated with matrix diffusion are reduced. The hypothesis is that reaction columns spaced every 2-3 feet within the plume center of mass would greatly reduce the back diffusion time as the contaminant only has to travel 1-1.5 ft to the nearest treatment zone.
Eight hundred reaction columns (ZVI, vegetable oil, and sand) were emplaced in less than eight days (1-2 minutes per 30-foot column) over an approximately 4,700 ft2 source zone at the Site 17 North Plume at Naval Support Facility in Indian Head, Maryland. Groundwater data collected at seven post-installation monitoring events over a period of 26 months showed reductions in trichloroethene in site monitoring wells and detections of key degradation indicators for both abiotic and biotic mechanisms (acetylene, ethene/ethane). Applied to a typical Department of Defense middle-to-late-stage site, estimated treatment costs for the Bomber technology (including equipment, materials, mobilization, labor) are approximately $50-70 per cubic yard of source zone treated, which is less than the reported unit cost per cubic yard for comparable treatment technologies (i.e., excavation, thermal treatment, or soil mixing with ZVI). Results to-date are very promising and, although several operational improvements were identified, the Bomber technology appears to be a viable alternative for amendment delivery at low permeability contaminated sites.
Re-purposing of the Grout Bomber technology for installation of ZVI reaction columns in low permeability media presented a few implementation issues associated with i) preliminary pilot testing to determine appropriate remediation material mixture that satisfied both pumpability and treatability criteria, ii) site access and large area for equipment staging, iii) general mobility of large equipment in a tight work environment, and iv) need for detailed site characterization to minimize risk of vertical contaminant transport.
Richardson, S.D, J. Kolz, D.M. Hart, J.A. Long, N.W. Johnson, A.R. Denn, and C.J. Newell. 2022. A Novel Application of a Geotechnical Soil Stabilization Technology for Improved Delivery of Remedial Amendments. Remediation Journal, 33(1):25-35. doi.org/10.1002/rem.21740.