Monitoring Performance of a Dual Wall Permeable Reactive Barrier for Treating Perchlorate and TCE

AMEC Geomatrix, through collaboration with Aerojet General Corporation and the University of California, Davis (UCD), has performed work leading to the installation of a dual wall permeable reactive barrier (PRB) system capable of treating perchlorate and chlorinated aliphatic hydrocarbon compounds (CAHs), including trichloroethylene (TCE), at Aerojet's Area 40 site in Sacramento, California. This unique system consisted of an upgradient zero-valent iron (ZVI) permeable reactive barrier (PRB) that is intended to not only degrade CAHs, but also, provide hydrogen generated from the ZVI corrosion process, to a downgradient bio-effective PRB (carbohydrate solution circulated through a gravel-packed trench) for destroying perchlorate. The subsurface was characterized during a site investigation, and numerous logistical and site-specific challenges of installation were addressed. The site-specific challenges included installation of a passive remediation system in a remote location with no access to electricity. The selected remediation system was keyed into the undulating bedrock 20 to 25 feet below the ground surface without the use of shoring. Under a collaborative effort, UCD provided initial bench testing. AMEC Geomatrix designed and installed the dual wall system consisting of two approximately parallel 50-foot long by 2-foot thick by 25-foot deep PRB segments which are separated by about 8 feet perpendicular to the approximate direction of groundwater flow. AMEC Geomatrix performed the installation of performance monitoring network, which consisted of 21 wells, and monitored these points for a 6-month period. Monitoring and sampling techniques were designed to measure water levels and water quality parameters in the subsurface during sampling events, to better assess the hydrologic and chemical processes. The monitoring results indicate that the upgradient ZVI PRB effectively treats groundwater with TCE concentrations approaching 60 mg/L, and in addition, may reduce concentrations of perchlorate from approximately 25 mg/L to less than 1 mg/L. A database was maintained to manage analytical results, water quality parameters, and water levels from each sampling event, and geographic information system (GIS) was used to generate plan diagrams of the treatment and performance. The small footprint of the monitoring network allowed for visualizations of the sample results in vertical segments, cross-gradient, within the PRBs, and upgradient or downgradient of the treatment zone. This method of monitoring treatment performance allowed for rapid assessment of the effectiveness of the dual walled PRB as new data became available. The initial time and cost to set up the data management structure (database to GIS) would allow for the ability to monitor the remediation performance more efficiently and to make more effective decisions, including assessment of the carbohydrate depletion within the bio-effective PRB.