The site includes three water bearing zones consisting of alluvium/weathered bedrock (Zone 1) and two additional fracture zones of alternating claystones and sandstone at approximately 25 (Zone 2) and 35 (Zone 3) feet below ground surface (bgs)
Targeted Environmental Media:
 Dense Non-aqueous Phase Liquids (DNAPLs) - Fractured Bedrock
During the period of operation of the former mall, some of the business operations resulted in tetrachloroethene (PCE) impacted soil and impacted ground-water in an area over 3000 feet downgradient of the site.
Major Contaminants and Maximum Concentrations:
- Tetrachloroethene (4,000 µg/L)
No technologies selected.
- Fracturing
- Bioremediation (In Situ)
- Reductive Dechlorination (In Situ Bioremediation)
Comments:
Due to the presence of tetrachloroethene (PCE) degradation products and overall remedial goals enhanced reductive dechlorination via an anaerobic in-situ reactive zone (IRZ) was selected as the ground-water remedy.
Pilot testing of the IRZ technology indicated the relatively low hydraulic conductivity typical of the Denver Formation limits the ability to effectively deliver the electron donor amendment necessary to establish the IRZ. To enhance electron donor delivery, the bulk hydraulic conductivity was increased via a hydrofracturing program. In addition, the hydrofracturing program was designed to quickly release organic carbon into the subsurface, establishing the proper environment necessary to enhance the reductive dechlorination rate and promote the complete dechlorination of the chlorinated ethenes. The hydrofracturing was performed using a pneumatic straddle-packer assemblage and fracturing rig capable of producing up to 450 pounds per square inch (psi). A total of 57 locations were fractured and two to three zones were fractured per location. Surface monitoring (tilt meter and depth to water measurements) indicate the radius of influence from each fracture ranged from 35 to greater than 70 feet.
Increased total organic carbon (TOC) and anaerobic and reducing conditions were observed within approximately one month of the fracturing. The increase in TOC and anaerobic and reducing conditions created the IRZ, enhancing the reductive dechlorination rate. The IRZ has been maintained via an amendment injection program.
This site is a Brownfields site being remediated for redevelopment. The goal of this project is mass removal. The regulatory cleanup goal for this site is the maximum contaminant level (MCL) (5 micrograms per liter [µg/L])
Performance monitoring results indicate the reductive dechlorination rate has been enhanced. PCE concentrations have been reduced by at leased an order of magnitude and some areas to below MCLs within six months following the hydrofracturing. PCE degradation product were either not observed or observed at concentrations two orders of magnitude less than PCE. These degradation products have been observed at increased concentrations, although the current volatile organic compound (VOC) trends indicate decreasing concentrations of dichloroethene (DCE) and vinyl chloride (VC) and increasing concentrations of ethene.
References:
Jacobs, Daniel L.; Scott D. Andrews. 2004. Utilization of Fracturing to Enhance Permeability and Reductive Dechlorination in Fractured Bedrock. The Fourth International Remediation of Chlorinated and Recalcitrant Compounds Conference, Monterey, California. May 24-27.
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