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

Reactive Media:
Fe⁰

Construction:
Funnel and Multiple Gate

Point of Contact:
J. H. Woll
Federal Highway Administration
Tel: 303-716-2106
Fax: 303-969-5903
Email: jhwoll@
road.cflhd.gov
555 Zang Street
Room 259
Lakewood , CO 80228

A permeable reactive barrier (PRB) system was installed in October 1996 at a site in Lakewood, Colorado. Contaminants at the site included 1,1,1-trichloroethane (TCA), 1,1-dichloroethylene (1,1-DCE), trichloroethylene (TCE), and cis-dichloroethylene (cDCE). Design concentrations for the PRB include TCE and 1,1-DCE concentrations up to 700 µg/L. No information is provided on the concentrations of cis-DCE or (TCA).

The contaminated area is an unconfined aquifer that is 15-25 ft thick and consists of unconsolidated gravelly sand overlying weathered (fractured) claystone. These units are in hydraulic connection and act as one aquifer. The geometry of the aquifer is irregular, with a local presence of clay lenses in the unconsolidated sand and sandstone lenses in the claystone. The aquifer is confined from below by unweathered (unfractured) claystone. No information is provided on porosity, transmissivity, hydraulic conductivity, etc.

The PRB system is comprised of a 1,040-ft funnel section and four reactive gate sections, each 40 ft wide. This was the first funnel and multiple gate PRB system using granular zero-valent iron. A high degree of lateral geologic heterogeneity and variation in volatile organic compound (VOC) concentrations led to varying iron thicknesses in each gate. The gates were constructed using a sheet pile "box." Native material was excavated from the box and the reactive material installed, separated from the aquifer materials by a layer of pea gravel.

The cost of the PRB system was about $1,000,000. This includes the cost of design, construction, materials, and the zero-valent iron.

Ground-water velocities through the gates were expected to range from 1 ft/day to 10 ft/day, depending upon the hydrogeologic conditions in the vicinity of the respective gates. Measurements in the cells using a heat-pulse flowmeter have ranged from < 0.1 ft/day to about 1.5 ft/day. Design concentrations include up to 700 µg/L of TCE and 700 µg/L of 1,1-DCE. Half-lives of about 1 hour or less were measured for these compounds in bench-scale design studies. The only VOC exiting the cells above the 5 µg/L reporting level is 1,1-dichloroethane, which has been measured up to 8 µg/L on the downgradient side of the cells. There is some evidence of the precipitation of calcite and siderite in the cells based on decreases in calcium and inorganic carbon in the treated ground water. This is estimated to result in a potential porosity loss of 0.5% of the porosity per year of operation.

Hydraulic head has increased upgradient of the barrier, with up to 10 ft of head difference measured across the barrier. This increases the possibility for contaminated water to move around the barrier. Indeed, VOC concentrations are increasing in ground water moving around the south end of the barrier and there is some evidence of VOCs moving under the barrier in one location.

Lessons Learned

Fractured aquitard materials and ground-water chemistry have greatly affected this remediation attempt. Permeation infilling of the basal claystone is required along with geochemical awareness of the ground-water chemistry.

SITE-SPECIFIC REFERENCES