Installation Date:
06/01/1998

Contaminants:
Tetrachloroethene, Trichloroethene

Reactive Media:
Fe⁰

Construction:
Hydraulic Fracturing

Point of Contact:
Robert W. Gillham
University of Waterloo
Tel: 519-888-4658
Fax: 519-746-1829
Email: rwgillha@
sciborg.uwaterloo.ca
2400 Univeristy Avenue West
Waterloo
, Ontario N2V 1T4 Canada

Installation of a permeable reactive barrier (PRB) system to remediate ground water contaminated with chlorinated solvents was completed by University of Waterloo researchers at the Massachusetts Military Reservation (MMR) near Falmouth, MA, in 1998.

The uniqueness of the project was the great depth of the site—the Chemical Spill 10 (CS-10) plume extends to about 120 ft below ground surface (bgs) near its source area. The demonstration program was pilot-scale in width, but full-scale in depth. The primary contaminants of concern at this site are perchloroethylene (PCE) and trichloroethylene (TCE), for which initial maximum concentrations of 300 µg/L and 15 µg/L, respectively, were identified. A 600 ft-wide contamination plume resulting from the maintenance of BOMARC missiles and related equipment during the 1960s exists in the area of MMR's Buildings 4642 and 4601, now known as the UTES site.

The CS-10 demonstration site is located in an area of glacial outwash sand and gravel, where the water table is located approximately 80 ft bgs. Ground-water flow velocity in the area is approximately 1 ft/day, and the horizontal hydraulic conductivity is approximately 200 ft/day. Maximum contaminant concentrations were identified at about 100 ft bgs.

Two iron walls approximately 20 ft apart were installed perpendicular to the contaminant plume using vertical hydrofracturing with a guar-based slurry. In the preliminary design for this project, installation methods were selected for their ability to emplace granular iron to the required depth. This installation technique required the drilling of 1-ft-diameter boreholes at 15-ft intervals along the wall. The "frac wells" were installed from ground surface to below the base of the contamination zone, and a specially-designed frac tool was used to cut a vertical notch for initiation of the fracture. A fracture was then induced and filled with granular iron suspended in a hydrated and cross-linked guar slurry. The propagating fracture from one frac well coalesced with the emplaced material from the adjacent well, thus forming a continuous vertical wall. The upgradient wall contains 44 tons of fine- to medium-granular iron (Master Builders GX-027), averages 3.3 inches in thickness and 48 ft in width, and extends from approximately 78 ft to more than 120 ft in depth.

A second wall, of similar dimensions, but consisting of a mixture of 5 tons of sand and 5 tons of granular iron, was emplaced to demonstrate the possible use of sand as a filler and permeability-increasing amendment for more highly reactive enhanced-iron materials. The upgradient, 100%-iron wall was verified by active resistivity and borehole radar tomography, hydraulic pulse interference testing, and borehole deviation measurements. More than 30 monitoring wells have been installed to monitor performance of the demonstration project.

Installation cost for this demonstration is estimated to be $160,000. This includes design, construction, materials, and the reactive media.

Although cleanup goals were not specified for this demonstration, cleanup to levels below maximum contaminant levels (MCLs) served as the target. Sampling of the ground-water upgradient and downgradient of the PRB system is conducted every 2-3 months. Results of the demonstration will be available upon its completion in mid-2000.

Lessons Learned

It was recognized early in the demonstration process that, depending upon the initial contaminant concentrations and flow velocity, this type of PRB system may require multiple walls to achieve a sufficient thickness. For the CS-10 source area plume, three walls with commercial iron of 3-inch thickness were expected to be needed for full treatment with an adequate factor of safety.

The 100% iron wall was installed successfully. During the installation of the second wall, however, fracturing control was lost when the propagating fracture came close to two screened monitoring wells deviating as much as 7 ft horizontally over their 150-ft length. Use of the system to remediate deep plumes such as this requires that the proximity (3-dimensional coordinates) of screened monitoring wells to the wall installation be carefully planned and checked with borehole deviation testing. As a result of an unanticipated delayed break of the cross-linked guar during construction of the system, more time was required for reestablishment of ground-water flow through the wall. Accordingly, it was determined that an improved guar-iron mix design was needed to establish flow through reactive zones soon after installation of the walls.

SITE-SPECIFIC REFERENCES