The overall objective of this project is to repurpose a commercially available geotechnical technology, the “Grout Bomber” (or alternatively the “VibroCat”), for enhanced delivery of granular activated carbon (GAC) or other sorbent material for in situ treatment of the following:

• Per- and polyfluoroalkyl substance (PFAS) source zones via a network of hundreds of closely spaced sorbent columns throughout the source zone to act as a “sponge” to sorb PFAS in both high-k media (via advection) and low-k media (via diffusion), and
• PFAS plumes via a series of hundreds of closely spaced vertical sorbent columns positioned in several transects perpendicular to groundwater flow to sorb dilute PFAS downgradient of the source zone. These transects form a cost-effective permeable sorbent barrier, analogous to a permeable reactive barrier.

The geotechnical industry offers a variety of well-established techniques for quickly and efficiently accessing the subsurface for the purposes of ground stabilization, foundation rehabilitation, porewater drainage, and structural support. The speed and efficiency of these techniques are advantageous to the field of environmental remediation, particularly for emplacement of remedial amendments into the subsurface at PFAS-impacted sites. Two promising techniques with potential environmental applications are:

• The “Grout Bomber”, a soil stabilization technology that was successfully “repurposed” for treatment of chlorinated solvents in low-k media for ESTCP project ER-201627.
• The “VibroCat”, another soil stabilization technology that utilizes specialized equipment (down-hole vibrator suspended from a crane or specially built rig) for emplacement of aggregates in 2-3 ft diameter vertical columns.

Both technologies utilize specialized machinery to rapidly access the subsurface via a direct push technique. For example, the Grout Bomber can install hundreds of closely spaced vertical columns containing remediation amendments like activated carbon per day to depths greater than 50 ft below ground surface. The project team plans to field-test the most appropriate of these technologies for the selected site as a new delivery system for GAC or other sorbent media to manage PFAS source zones and dilute PFAS plumes at a fire-training area.

With these technologies, the project team can deliver more sorbent per unit area than injection-based sorbent barriers, increase the reliability of these barriers by using closely spaced sorbent columns and reduce the cost of sorbent barriers at many sites. For example, it is estimated that 150,000 pounds of carbon can be installed in 2-3 weeks at a cost of $25/yd³ of source treatment volume (contractor + materials).

The Grout Bomber (or VibroCat) can efficiently deliver large quantities of activated carbon (or equivalent) into the subsurface and offer substantial cost savings compared to conventional injection methods and pump and treat systems. More importantly, the sorbent barrier concept, enabled by the Grout Bomber and potentially the VibroCat, will provide the Department of Defense (DoD) with a second method to deliver sorbents in the ground to stop PFAS plumes, doubling the technologies from the current single approach (injecting particulate sorbents such as PlumeStopTM). This will significantly increase the flexibility, types of vendors, and choices for DoD remedial project managers considering constructing PFAS sorbent barriers at DoD PFAS groundwater sites. (Anticipated Project Completion - 2025)