Contaminants:
1,1,1-Trichloroethane, Tetrachloroethene, Trichloroethene

Reactive Media:
Fe⁰

Construction:
Continuous Trench

Point of Contact:
Stephen Tappert
RTC VECTRE Corporation
Tel: 973-383-2500
Fax: 973-579-0025
Email: stappert@
trccos.com
15 Route 15 South
Lafayette , NJ 07848-0930

A full-scale permeable reactive barrier (PRB) was installed at a site in Fairfield, NJ, to treat chlorinated solvent contamination. The site, a former electromechanical product manufacturing, assembly, and testing facility is currently in operation as a school. It consists of a single one-story slab foundation brick building and paved parking lot covering 60% of a 2.8-acre plot of land. Environmental investigations at the site identified a plume of chlorinated solvents, with an apparent source in the vicinity of a former dry well and septic system. Contamination was limited to the shallow sandy aquifer. The total VOC concentration at the plume front was approximately 4,500 µg/L. Key contaminants included 1,200 µg/L trichloroethane (1,1,1-TCA), 19 µg/L tetrachloroethylene (PCE), and 110 µg/L trichloroethylene (TCE). A pool of dense nonaqueous-phase liquid (DNAPL) was also identified with significant concentrations of solvents in saturated soils below 15 ft. Underground utilities in place at the site included two storm drains and a sewer line at 13 ft below grade.

The site is underlain by 15-20 ft of silty sand with some gravel, overlying a lacustrine clay 10-15 ft thick. The clay unit varies in depth from 15-23 ft below grade. Ground water at the site occurs under water-table conditions within the glacial sediments above bedrock, and under confined conditions in the deeper sand aquifer. Shallow ground-water flow is moving toward a nearby creek at an average hydraulic gradient of 0.005 ft/ft. Depth-to-water in the shallow zone has been as high as 4 ft below grade. An upward vertical ground-water gradient exists between the shallow aquifer and the silty sand unit underlying the clay, with a head difference of almost 6 ft in some areas.

Prior to installation of the PRB, the DNAPL pool was excavated. As a remedial measure, the excavation was partially backfilled with a 1:1 mix of zero-valent iron and sand. For the PRB, conventional sheet piling construction was selected as the most reliable approach with the most predictable timeframe for completion. The PRB was constructed as a continuous barrier located ahead of the highest plume concentrations to prevent offsite migration. The bottom portion of the barrier used a 4:1 iron/sand mixture and the upper portion of the barrier used a 3:2 iron/sand mixture. A total of 720 tons of iron were used. The final barrier was 127 ft wide, 25 ft deep, and 5 ft thick. After the barrier was installed, the site was graded and seeded, and the parking lot was repaved. Construction was generally straightforward with the only major problem being the below-grade sewer line that permitted a large volume of water to enter the excavation. Construction ultimately required subaqueous excavation to complete that section of the wall.

Design costs for the barrier, including a licensing fee, were $150,000. Installation costs (which include construction, materials, and reactive media) totaled $725,000.

Cleanup goals for chlorinated solvents at the site were New Jersey Ground Water Quality Criteria: 1 µg/L for PCE, 1 µg/L for TCE, and 30 µg/L for 1,1,1-TCA. Monitoring wells were installed upgradient, downgradient, and within the PRB and samples have been collected on a quarterly basis since system installation. VOC concentrations at the center of the plume have decreased to near detection limits within the PRB. Quarterly sampling results have also reported an increase in pH from approximately 6.5 to 9.5, a decrease in Eh from -50 mv to -400 mv, and concentrations of VOCs at or near detection limits in the central portion of the wall. Quarterly monitoring of selected wells will continue for one more year, then with reduced frequency after that.

Lessons Learned

Detailed knowledge of the site and detailed planning were critical to making this technology work. Also, it was important to get the state agency on the team early to expedite the project.

SITE-SPECIFIC REFERENCES