The property was used for retail gasoline sales and auto repair since the 1940s. Underground storage tanks (USTs) operated at the site for an unknown number of years until operations ceased in December 1998. The site is currently used for high-end automobile retail sales and represents a significant revenue base for the local borough. The area includes mixed land use, with commercial properties, residences, public lands, a middle school, and the local Water Authority near the site. The site is within the Allegheny Plateau Physiographic Province of southwestern Pennsylvania and lies on the northern bank of the northwesterly flowing Ohio River, with the river approximately 400 feet south of the site. The site overlies unconsolidated materials that comprise the Ohio River Sand and Gravel Aquifer, a prolific aquifer with hydraulic conductivities estimated at 600 feet per day. The water table is encountered at about 40 feet below ground surface (bgs), yielding a saturated thickness of about 60 feet near the site. Groundwater flow direction at the site was determined to be generally south, towards Ohio River at low gradients (0.0005 ft/ft). At distances removed from the site, the groundwater flow direction is influenced by operation of the Water Authority supply wells, resulting in southeasterly groundwater flow directions. The site has a large vadose zone and UST-related impacts have been investigated and extended to the deep portions of the Ohio River sand and Gravel Aquifer (approximately 90 feet). Land use issues further complicate remediation because the former gas station is presently used as a part of a profitable retail business, which limits potential remedial options. The site hydrogeology includes multiple water-bearing zones including high conductivity zones and fractured bedrock). Groundwater in the area of the site is used for potable well water supply (rural area).

• Fractured Bedrock

Contamination conditions described in 2009 identified conditions including:
ý Benzene, toluene, ethylbenzene, and xylene (BTEX) impacts were predominantly confined to the area immediately surrounding the former source area;
ý BTEX impacts were observed throughout the aquifer to depths of 90 feet below ground surface (bgs).
ý Methyl tert-butyl ether (MTBE) impacts were present throughout the saturated thickness of the aquifer and extending to a significant downgradient distance from the site, migrating under municipal supply wells.
ý The MTBE dissolved phase plume was determined to ýdiveý to greater depths along the off-site migration pathway, based on density and hydraulic conductivity effects.
ý The aquifer has sufficient biological capacity to support in-situ bioremediation; however, the depressed availability of dissolved oxygen (DO) in groundwater was determined to be a potential limiting factor.

• Benzene-toluene-ethylbenzene-xylene (BTEX) (var)
• Methyl-tert-butyl ether (var)
• Naphthalene (var)
• Trimethylbenzene (var)

• Chemical Oxidation (In Situ)
  • Ozone

Specific cleanup goals are not identified in the sources reviewed. URS has been implementing risk characterizations to develop a site-specific, closure approach for the site property, with a health-based cleanup approach for the downgradient area.

Since implementation of the ozone sparging technology, volatile organic compound (VOC) concentrations have been reduced significantly in the source area and downgradient of the active remediation area. More than 90% reductions were achieved in the source area for concentrations of BTEX, MTBE, naphthalene and trimethylbenzenes. These were achieved after implementation of the ozone-based remedy. Downgradient VOC concentrations have been similarly reduced.

Several previous pilot studies including pump and treat, dual-phase extraction, Fenton's reagent injection, and conventional air-sparge have failed to identify an adequate remedial approach for a timely site cleanup. Various remedial technologies have been applied over time to address the complex contamination, hydrogeology, and land use conditions at the site.

Reference:
Moore, R.A and Coll, Frederic R (URS Corporation). 2012. ýOzone-Based Remedial Action for Fractured Bedrock Site." 8th International Conference Remediation of Chlorinated and Recalcitrant Compounds. May 21-24.

Coll, Frederic R, and Moore, R.A (URS Corporation). 2009. In Situ Chemical Oxidation via Ozone at a Multiple-Remedy UST Site. Presented at WM 2009 Conference March 1-5, 2009, in Phoenix, AZ.