Residual contamination was found along relic foliation fractures within the saprolite soil, however the bulk of the free phase was found within a transition zone and within the fractures in the shallow granite bedrock. The primary zone targeted for remediation was 45-75 ft. deep.
Targeted Environmental Media:
 Fractured Bedrock
Free product ranges from 45 to 75 ft bgs.
Major Contaminants and Maximum Concentrations:
- Tetrachloroethene (DNAPL)
- Borehole Geophysics
- Vertical Chemical Profiling
- Coring
- Fracturing
- Pump and Treat
- Soil Vapor Extraction
- In Fractured Bedrock Vadose Zone
- Multi Phase Extraction
- Bioremediation (In Situ)
- Reductive Dechlorination (In Situ Bioremediation)
Comments:
Pneumatic fracturing was very effective in opening up dead end fractures, providing interconnection between wells and greatly increasing the yield. At some locations, up to 400 psi. were needed to create crosshole flow between wells. Many wells yields were increased from less than 1 gpm. to greater than 10 gpm. To prevent potential drag down of contaminants from shallow zones, the DNAPL source area was dewatered in stages. After several months of free phase pumping and hi-vacuum dewatering, the main portion of the DNAPL mass was removed and localized drawdowns of up to 80 ft. were achieved. Forced hot air injection and vapor extraction were very effective. Crosshole hot air/water flushing through fractures was very effective in recovering immobile DNAPL, especially when using air preheated to >1000 degrees F. By increasing the temperature of the target zone from 60 degrees to >250 degrees,increased PCE vapor concentrations, and thus recovery rates by 20 times. In less than 30 days PCE concentrations in one of the primary wells decreased 97% from greater than 4,000 ug/L to less than 100 ug/L. Temperatures of purge water from the primary well remained at or near 240 degrees for several days and 160 degrees for several weeks folowing the test.
None provided
PCE concentrations in the source area have dropped below the 1 mg/L target level. To evaluate the potential for rebound, a three month shutdown test of the pump and treat system was conducted. Dispite a full recovery of water levels and complete flooding of the source zone, PCE concentrations have remained below 200 ug/L and are similar to those in the dissolved plume.
As of January, 2004, there has been no rebound at the source zone and molasses has been injected both at the source zone and at the plume boundry to enhance anaerobic bioremediation. Monthly injection (100 gal of 20% molasses) has been conducted for over two years at the source area and concentrations have been fluxuating around 200 ppb VOCs. These injections ended in 2003. Injections are still occuring at the property boundry where concentrations are around 5 ppb.
The use of multiple wells, as opposed to single well remediation technologies, induces stron pressure gradients between wells and throughout the zones targeted for remediation. The pressure gradients increase the mobility and recovery of DNAPL in low-permeability zones.
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