Installation Date:
1999

Contaminants:
Trichloroethene, trans-1,2-Dichloroethene, cis-1,2-Dichloroethene

Reactive Media:
Fe⁰

Cost:
$220,000

Construction:
Vibrated caissons filled with Fe⁰, followed by deep-soil mixing.

Point of Contact:
Debra R. Reinhart
University of Central Florida
Tel: 407-823-2156
Fax: 407-823-5483
Email: reinhart@
mail.ucf.edu
PO Box 162993
Orlando , FL 32816-2993

A pilot-scale permeable reactive barrier (PRB) system was installed in 1999 to remediate the heavily contaminated subsurface at Cape Canaveral Air Force Station Launch Complex 34 (LC34). This site, which was used for Saturn rocket launches between 1959 and 1968. Cape Canaveral Air Force Station is located on an island on the eastern coast of central Florida, bordered by the Atlantic Ocean on the east and the Banana River on the west. The soil immediately below the surface consists of medium to fine sands with some shell and silt. The soil is primarily sandy with shell fragments and traces of silts to depths up to 30 ft. Below 30 ft, the soil is stratified with layers of clay and fine sands. Beyond a depth of 100 ft, limestone fragments ranging from 20-50% are found within the clay and sandy soils. Ground-water level is 3-7 ft below the surface. Major ground-water contaminants of concern are trichloroethylene (TCE) (0.005-29 mg/L) and daughter products of biological degradation of TCE: trans-dichloroethylene (0.25 to 0.8 mg/L) and cis-dichloroethylene (14-44 mg/L).

The soil immediately below the surface consists of medium to fine sands with some shell and silt. The soil is primarily sandy with shell fragments and traces of silts to depths up to 30 ft. Below 30 ft, the soil is stratified with layers of clay and fine sands. Beyond a depth of 100 ft, limestone fragments ranging from 20-50% are found within the clay and sandy soils. Ground-water level is 3-7 ft below the surface. The gradient for the entire site ranges from .010-.001 ft/min. Additional gradient measurements, calculated from water levels measured in piezometric wells installed at LC34, concluded that the gradient does not vary significantly with time in both direction and magnitude. Tests were also conducted to examine the effect of the ocean tide on ground-water levels. The results show no signs of local tidal influence on the ground-water level.

The PRB system consists of a series of 11 overlapping columns (each about 4 ft in diameter) that contain a mixture of Fe⁰ (16% by weight), native soil (79% by weight), and gravel (5% by weight). The total barrier length is about 40 ft. The PRB was installed to a depth of about 40 ft below ground, ending just above an impermeable clay stratum. and is keyed into a clay layer that is about 40 ft below ground.

After considering several alternatives, the Deep Soil Mixing technique (DSM) was selected to construct the PRB. DSM, which is typically used for soil improvement or contaminant containment, offers a method of PRB construction that reduces the volume of contaminated soil requiring special disposal and minimizes construction worker exposure to hazardous materials. In addition, the DSM can readily reach depths of 150 ft and has proven to be a cost effective alternative to PRB construction. Use of the DSM presented some construction challenges related to iron placement, since this was the first application of the technique. These were solved in field. In addition, some distribution problems with iron within the wall leading to breakthrough of VC and cis-DCE were noted.

Quarterly sampling at the site continues. To date, monitoring data suggest that removal of the chlorinated solvents challenging the wall is successfully occurring. TCE and its daughter products are at non-detectable levels within the wall and are declining in downstream wells, with the exception of vinyl chloride. Values for breakthrough VC and cis-DCE are continuing to decline.

Lessons Learned

A PRB installed using the deep-soil mixing technique offers several advantages over other construction techniques and treatment methods. The DSM technique produced no contaminated excavated soils that required special disposal. Exposure of workers to hazardous chemicals was also minimized since the mixing occurred below grade. This is a passive, in situ remediation technique and aboveground treatment equipment is unnecessary during routine operation. Sufficient mixing during PRB construction is recommended to provide sufficient iron throughout the treatment barrier capable of complete destruction of TCE and all daughter products.

SITE-SPECIFIC REFERENCES