Bedrock is Triassic sedimentary rocks comprised of fine- and medium-grained pale brown to pale red, gray, and pale orangearkosic sandstone and interbedded red shale. In places, thick accumulations of shale can be found. The approximate depth to weathered bedrock is 7 to 12 ft below ground surface (bgs). Bedrock strikes northeast to southwest and dips approximately 7 degrees to the northwest.
There are three units; a surficial and two confined separated by shale and mudrock. Downward vertical gradients were measured in the unconfined and confined units. Ground-water in the unconfined is found 15 to 30 ft bgs. The lateral gradient of flow in the unconfined unit is approximately 0.09.
Aquifer testing indicated tranmissivity values of 377 gallons per day per foot (gpd/ft).
Targeted Environmental Media:
 Fractured Bedrock - Light Non-aqueous Phase Liquids (LNAPLs)
In the spring of 1995, a catastrophic release of several thousand gallons of gasoline occurred when a gauging stick punctured the bottom of an 8,000 gallon tank. About 20 feet of free phase gasoline was observed in an on-site water supply well.
Major Contaminants and Maximum Concentrations:
- Benzene-toluene-ethylbenzene-xylene (BTEX) (48,940 µg/L)
- Methyl tert butyl ether (5,900 µg/L)
- Pumping Tests
- Pump and Treat
- Bioremediation (In Situ)
- Reductive Dechlorination (In Situ Bioremediation)
- Other (Oxygen Infusion)
Comments:
In the fall of 1999, a bioreactor pump and treat system was installed extracting 1.5 to 2.0 gallons per minute (gpm). Free phase gasoline was also being extracted. After approximately 2 years of operation, the decreasing trend of concentrations reached static conditions. The pump and treat system was also used to hydraulically control the ground-water systems. The extracted ground-water was treated in aerobic bioreactors and discharged into leachfields located upgradient of the groundwater extraction wells.
In September 2002, oxygen infusion units were installed in order to expedite the cleanup of the shallow aquifer. Oxygen infusion technology was pilot tested as a means to increase the dissolved oxygen (DO) in the shallow system, and thereby enhance in situ bioremediation. The in situ oxygen infusion technology utilizes microporous hollow fiber to provide a large surface area for contact between oxygen and ground-water to allow for the efficient mass transfer of the oxygen to the ground-water. Oxygen infusion wells were initially installed in three monitoring wells as a part of the pilot test.
The objective of installing the oxygen infusion technology was to increase the amount of DO throughout the shallow ground-water system to increase the number of fracture pathways through which DO could be distributed.
The pump and treat system has been effective in the cleanup of the deeper ground-water systems and the shallow off-site ground-water system. After four years of operation, the ground-water contamination periodically exceeded 25,000 parts per billion (ppb) of benzene, toluene, ethylbenzene, and xylenes (BTEX) and 8,000 ppb methyl tertiary butyl ether (MTBE).
Immediately following the installation of the oxygen infusion units the DO concentrations increased. After six months, DO concentrations increased from 0.57 parts per million (ppm) to 40 ppm, heterotrophic colony units increased two to four orders of magnitude and gasiline concentration decreased. As a result of the pilot test, six additional oxygen infusion units were installed. The oxygen infusion units are still operating. There is a significant decreasing trend in the concentrations of dissolved gasoline compounds.
Total BTEX concentrations in the source area are less than 1,000 ppb after 16 months of oxygen infusion. MTBE concentrations were reduced to less than 200 ppb. There are small areas of on-site contamination resisting cleanup. The oxygen infusion units are being redeployed to these locations in order to subject the contamination to more directed oxygen enhancement.
Oxygen infusion technology is capable of increasing heterotrophic colony units by several orders of magnitude.
Reference:
Morrow, James; David Side. 2005. In Situ Remediation of Highly Contaminated Groundwater in Fractured Bedrock with Oxygen Infusion. The Eighth International In Situ and On-Site Bioremediation Symposium, Baltimore, Maryland. June 6-9.
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