The bedrock beneath the site consists of two major types: a Jurassic igneous diabase and a Triassic metamorphosed siltstone/sandstone (metasediments). The diabase is located throughout most of the facility, with the metasediments primarily in the north-central portion of the facility. The diabase exhibits minimal fracturing, is relatively impermeable, and acts as an aquitard. The metasediments are more permeable and exhibit substantial fracturing. The results of a deep aquifer test within the metasediments indicated aquifer responses consistent with confined or leaky (semi-confined) conditions that are fracture-controlled and have a radial flow component. The metasediment aquifer is generally shown to be anisotropic and heterogeneous, with two primary fracture zones connecting the wells located in the deep ground water treatment area.

• Fractured Bedrock

Both historical and recent sampling data have shown plumes of perchlorate and volatile organic compounds (VOC) centered in the north-central and central portions of the facility. The perchlorate plume in deep ground water is located 250 feet below ground surface (bgs) and extends 2,250 feet in a direction parallel to the natural deep hydraulic gradient and to a maximum width of 1,750 feet in the cross-gradient direction. The smaller VOC plume in deep ground water lies within the horizontal extent of the perchlorate plume.

• Perchlorate (11.7 µg/L  8293 µg/L)
• Halogenated VOCs (1.0 µg/L  327.9 µg/L)

• Borehole Geophysics
  • Other
• Pumping Tests

Comments:
The bedrock was characterized using aquifer pump tests and downhole geophysical testing of selected wells.

Concentrations of dissolved oxygen (DO) in the deep ground water ranged from 0.22 milligrams per liter (mg/L) to 1.19 mg/L. These low DO concentrations indicate an environment that is anoxic to anaerobic. These low DO concentrations are typical of deep semi-confined to confined aquifers.

• Pump and Treat
• Other (anaerobic bioremediation)

None provided

At the conclusion of the pilot test in May 2003, 3,123,000 gallons of water had been extracted, treated for VOCs, and reinjected. The influent was sampled periodically to evaluate tracer movement. Samples were monitored for perchlorate, chlorate, acetate, and iodide. Influent samples exhibited perchlorate at concentrations ranging from 1.84 mg/L in May 2003 to 6.24 mg/L in October 2002. The iodide tracer was not detected in the influent, possibly as a result of the low concentration of iodide in the substrate solution. However, acetate was detected in the influent sample collected in November 2002 at a concentration of 1.02 mg/L. This detection indicates that acetate has been transported from the injection well to the extraction well. The presence of acetate indicates that a substrate capable of stimulating the anaerobic microbial population has been distributed within the deep ground water treatment zone. Chlorate has not been detected in the influent, suggesting that insufficient time and likely inadequate amounts of electron donor have been allotted to initiate the anaerobic reduction of perchlorate.

The most recent sampling event indicated that the perchlorate concentration at the extraction well was reduced by 71 percent after 8 months of operation.

A modified bioremediation system has been developed based on the results of the pilot test. This modified system will expand on the current extraction and injection well. The major differences between the proposed and pilot systems are the replacement of the air stripping tower with the carbon adsorption system to treat VOCs and the use of a more suitable water-soluble substrate in lieu of CMA as an electron donor.

The substrate amendment system was operated for 2 weeks in October 2002 and 1 week during November 2002, but was discontinued because of injection well fouling and backpressure issues. The water at the facility is naturally hard. Use of the CMA electron donor contributed to fouling by mineral scale. Geochemical sampling indicated that the natural ground water exhibits low potential for mineral scaling. However, after aeration via the air stripping process, the ground water becomes susceptible to mineral scaling. In addition, small amounts of the CMA amendments increase the susceptibility to mineral scaling. Under typical operating conditions, the mineral precipitation could produce 10 pounds of solids per day, leading to significant scaling in the equipment and fractures. A possible solution to the scaling problem is the periodic addition of an organic acid (citric acid), which may yield multiple positive effects. These effects may include acting as a solvent for mineral scale, as an oxygen scavenger to maintain low DO concentrations, and as an electron donor substrate to promote the anaerobic biodegradation process.

Reference:
Smith, William; Jason Early; Kevin Morris; Christopher Underwood. 2004. Fractured Bedrock Pump-and-Treat Conversion to In Situ Bioremediation. The Fourth International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, California, May 24-27.