Installation Date:
01/11/1998

Contaminants:
1,1,1-Trichloroethane, 1,2-Dichloroethane, 1,1-Dichloroethene, Vinyl chloride

Reactive Media:
Fe⁰, iron filings

Cost:
$1,065,000

Construction:
Continuous Wall Trenches

Point of Contact:
Richard Roller
Shaw Air Force Base Environmental Flight
Tel: 803-895-9991
Fax: 803-895-5103
Email: richard.roller@
shaw.af.mil
345 Cullen Street
Shaw Air Force Base
Sumter , SC

A full-scale permeable reactive barrier (PRB) system was installed at Shaw Air Force Base in Sumter, SC, in 1998. Parallel continuous wall trenches were chosen based on favorable site conditions for chain trenching, the relatively shallow depth of contaminants, timely installations (within 5 days), and the immediate effectiveness of the system. Tricholoethane (TCA), Dichloroethane (DCA), dichloroethylene (DCE), and Vinyl chloride (VC) are the contaminants of concern at the site. Initial concentrations of these analytes were 18,100 µg/L for TCA, 4,554 µg/L for DCA, 2,500 µg/L for DCE, and 180 µg/L for VC.

The site was formerly a fire training area where combustible wastes (jet fuel, spent solvents, waste oils, and hydraulic fluids) were burned in an unlined and bermed pit. Fluids that were not ignited during the fire training exercises drained through the fine to medium-grained sandy soil impacting ground water. Contaminants migrated eastward with the ground-water flow, toward the lower-lying plain of Long Branch Creek. Depending on the season, depth to ground water ranges from 8-14 ft below ground surface (bgs) in the area of the pit and 3-6 ft bgs in the flood plain. Total depth of the shallow aquifer is approximately 60-70 ft bgs. Ground-water flow velocity is 1.5 ft/day, and porosity is 10-20%.

The PRB consists of four parallel continuous wall trenches, each measuring 8 in wide, 270 ft long, and 24 ft deep. The trenches are approximately 10 ft apart. The saturated sands and hydrostatic pressure prevented the chain trenchers from attaining the originally planned 30 ft depth and 14 in width. This fact was realized at the time of installation. Design costs were $123,000. Installations costs including site preparation, construction, materials, and patent royalties totaled $942,000.

Samples taken from the same location at which pre-design concentrations were observed and at the down gradient edge of the PRB show reductions from 18,100 to <1 µg/L for TCA, from 4,554 to 340 µg/L for DCA, and from 2,500 to 40 µg/L for DCE. VC increased from 180 to 290 µg/L. VC is a daughter product of the dechlorination processes and is produced at higher concentration than the PRB was originally designed to destroy. However, monitored natural attenuation processes at the down gradient edge of the PRB have indicated sufficient biodegradation rates to reduce the VC levels at a proposed compliance boundary prior to entering a creek. Long term sampling is conducted quarterly.

Lessons Learned

The production of daughter products within the PRB zone should be considered in specifying the width and retention time required to treat contaminated ground water to remedial levels.

SITE-SPECIFIC REFERENCES