Perchlorate Remediation Project Case Study

ARCADIS was contracted in 2000 to lead the development, evaluation and selection of a final groundwater remedy for this 94-acre site in California. As part of this work, ARCADIS is also participating in the evaluation of innovative technologies with the potential to treat soil impacts in the multiple source areas across the site. Specifically, the project is focused on the remediation of seven source areas and their associated groundwater plumes, some of which are commingled and are comprised of multiple contaminant types. The complexity of the contaminant distribution and the need to avoid the disruption of site operations has presented a significant remedial challenge.

Background
The site has produced small explosives and vehicular safety products since 1957. As a result of the historic operations, soil and groundwater at the site has been impacted by perchlorate, trichloroethene (TCE) primarily, and hexavalent chromium (Cr6+).

The site is underlain by alluvial/colluvial deposits consisting of interbedded sand, silt, and clay. There are three water bearing zones at the site; a shallow unconfined aquifer, and two deeper confined aquifers. Groundwater in the shallow aquifer is encountered at approximately 40 feet below land surface (bls) with perched conditions present in some areas. The most significant impacts to groundwater are present in the shallow aquifer. Consequently, much of the remedial effort completed to date has focused on this aquifer. A strategy is currently being developed to address impacts in the deeper aquifers, which extend up to 400 feet bls.

Source area soils have been found to contain perchlorate concentrations as high as 1,500 milligrams per kilogram (mg/kg), TCE as high as 6.5 mg/kg, and Cr6+ as high as 0.084 mg/kg. Concentrations of these same contaminants in shallow groundwater have been detected up to 500 milligrams per liter (mg/L), 90 mg/L, and 0.25 mg/L, respectively.

Previous groundwater remediation efforts have been limited in scope with modest results. A two-well shallow groundwater pump-and-treat system was installed in 1994. Performance data indicates this system is providing marginal mass removal and only localized hydraulic containment. In addition, a bioremediation pilot test was conducted using hydrogen release compound (HRC®) to treat TCE. The HRC® failed to remediate TCE within the test area, but resulted in significant reduction of perchlorate and Cr6+.