Zero-Valent Iron Media
East Helena, MT: Permeable Reactive Barrier Pilot Study, Smelting Plant
A former ore smelter had arsenic contamination from historical operations that now was threatening the nearby community water supply. US EPA had determined that the arsenic contamination on this site was one of the most severe in the country. In addition, the design team was concerned that boulders and cobbles in the subsurface could make deep excavation for a PRB (permeable reactive barrier) very difficult or impossible. There was also a concern that while laboratory studies had determined that ZVI (zero valent iron) could be used for a PRB treatment, a pilot project could be invaluable in determining field performance. Accordingly, a pilot project was implemented to test the ZVI and difficulty of excavation.
Geo-Solutions teamed with Shaw Environmental for the PRB installation. Geo-Solutions provided the long stick attachment for a local hydraulic excavator to permit excavations up to 50 ft deep. Geo-Solutions also provided the bio-polymer slurry, slurry mix plant, tremie system as well as the expertise and direction for the installation.
The installation was complicated because the PRB need to be at least 6 ft wide for treatment but only 20 ft long to minimize the cost of the ZVI. Geo-Solutions helped devise a system for the excavation that utilized end stops to permit the proper depth and width while minimizing the trench length when excavating with the large power hydraulic excavator needed to remove boulders and cobbles.
The actual PRB excavation and backfilling required only a few days which included the installation of 7 wells in the PRB for study and monitoring. The excavation and backfilling of the PRB went as expected with many cobbles removed by the large excavator.
As a result of the team effort and with Geo-Solutions involvement the installation of the pilot study was a success for the design team and the contractor.