Carbon tetrachloride, Chloroform, Freon
Granular cast iron
Continuous PRB constructed using vertically oriented hydraulic fracturing
Point of Contact:
Stephen H. Shoemaker
6324 Fairview Road
Charlotte , NC 28210
A full-scale permeable reactive barrier (PRB) demonstration is being conducted at a DuPont site in Oakley, CA. Contaminants, including carbon tetrachloride (CCl4), 20-40 ppm, chloroform (CHCl3), 1-3 ppm, trichlorofluoromethane (Freon 11), 10 ppm, and trichlorotrifluoroethane (Freon 113), 3 ppm, are present.
The PRB was emplaced in a stratified, alluvial aquifer system consisting of fine to medium sands. The upper aquifer extends to a depth of 55 ft below ground surface (bgs), underlain by a 10-ft thick, leaky clay aquitard. The lower aquifer extends from 65-120 ft bgs, underlain by a very thick and competent clay aquitard. The PRB was emplaced across the entire thickness of the lower aquifer.
The goal was to prove the feasibility of emplacing a 6-in thickness of iron to a depth of 120 ft bgs using hydraulic fracturing.
A probabilistic design model was used to make use of all of the available design data and optimize the barrier thickness at a 90% confidence level. Flowing sands and the required
120-ft depth necessitated the use of hydraulic fracturing as the method of construction.
The PRB is 110 ft long and 6 in thick. The PRB begins at 65 ft bgs and extends to a depth of 120 ft below ground surface. Total design cost for the PRB system is estimated at $150,000. Installation cost, including construction materials and reactive materials totaled $1,000,000.
No problems were encountered except that DuPont's attempts to recover an intact core of the emplaced PRB at a depth of
120 ft in flowing sands were unsuccessful. Alternative methods of direct physical emplacement verification are currently being explored.
The lack of success at recovering an intact core from the emplaced PRB does not impact the efficacy of the emplacement technique or the performance of the PRB.
A 1-year monitoring program has begun to verify treatment performance before completing the remainder of the full-scale PRB.