Fractured Bedrock Aquifer Hydrogeologic Characterization for a Bioaugmentation Pilot Study

A hydrogeologic characterization was performed to evaluate the nature of groundwater flow in an alluvial and underlying fractured bedrock aquifer in the area between an unlined, inactive landfill and a river in southern California. The hydrogeologic characterization was used to enhance the site conceptual model for the site, and to design a bioaugmentation pilot test to evaluate the use of enhanced in-situ bioremediation (EISB) to treat chlorinated VOCs, primarily PCE, TCE, 1,1-DCA, cis-1,2-DCE, and VC in the fractured rock aquifer beneath the site. The pilot test area (PTA) is characterized by a thin veneer of alluvium overlying fractured tonalite (bedrock). Groundwater beneath the site is unconfined within the alluvium and fractured bedrock and flows from the landfill and through the PTA toward the river. Chlorinated VOCs have been detected in shallow, intermediate and deep monitor wells installed in the PTA at depths of up to 300 feet below ground surface (bgs). To date, VOCs have not been detected in surface water or shallow groundwater samples along the northern river margin. Using lineament analyses and geophysical surveys, subsurface bedrock fracture zones and potential zones of higher groundwater flow rates were identified. Three multi-level monitoring well clusters were installed in the PTA in areas believed to be within significant fracture zones. The upgradient and intermediate monitoring clusters consist of a shallow well screened in the alluvium, an intermediate well screened in the upper highly fractured bedrock aquifer, and a deep well screened in the lower fresh, slightly fractured bedrock aquifer. The downgradient monitoring cluster includes an additional deep bedrock well. Downhole geophysical logging conducted in select boreholes prior to well construction included caliper, electrical logging, acoustic televiewer (ATV), and borehole image processing system (BIPS) logging. Additional testing performed on selected borings included thermal flow meter testing and hydrophysics. A 48-hour continuous rate pumping test and a 72-hour recovery test were performed to evaluate the hydraulic connection between well clusters and different screened intervals within the PTA. Aquifer test analyses indicated that hydraulic conductivities (K) ranged from 25 to 40 feet per day (ft/day) in the fractured rock aquifer, and 13 to 17 ft/day in the alluvium. However, due to the low effective porosity of the fractured rock aquifer, groundwater velocities as high as 40 ft/day per day were estimated, compared to velocities of less than 2 ft/day for the alluvium. A qualitative dye tracer study using three distinct dyes was performed in the PTA to evaluate hydraulic communication between the three clusters and intervals under passive, non-pumping conditions. Results of the tracer study confirmed that groundwater within the fractured rock aquifer was moving rapidly under passive conditions, and confirmed communication between the upgradient and downgradient wells within the pilot test area. The hydrogeologic characterization demonstrated that sufficient hydraulic communication is present within the PTA and that plans for a passive EISB pilot test should proceed.