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George's Service Remedial Action Work Plan Project No.425.2 June 30,2011 3.5. Chemicals of Concern The investigation of the release has identified gasoline range petroleum hydrocarbons as the chemicals of concern (COC) at the site. The analysis of gasoline components is usually limited to benzene, toluene, ethyl benzene and xylenes (BTEX), total petroleum hydrocarbons as gasoline(TPH-G) fuel compounds because: (1)they are readily adaptable to gas chromatographic detection, (2) they pose a serious threat to human health (benzene is carcinogen), (3) they have the potential to move through soil and contaminate groundwater, (4) their vapors are highly flammable and explosive (Leaking Underground Fuel Tank Field Manual, State of California Leaking Underground Fuel Tank Task Force, October 1989), and (5) a high percent of gasoline is composed of these compounds. These COC have been identified at the site and are included in the monitoring and analytical protocols. GTI has also monitored and sampled the soil and groundwater at the site for fuel oxygenates (MTBE, DIPE, ETBE, TAME and TBA) as well as lead scavengers(1,2-DCA and EDB). 3.6. Contaminant Distribution 3.6.1. Soil Plume Soil analytical data suggests that the vertical extent of the soil plume is concentrated in the 10-15.5 foot interval as all soil samples (from the Additional Site Characterization work conducted in January, 2010) collected above 10 feet and below 15.5 feet were reported to be non-detect for TPH-g and BTEX constituents with two localized exceptions. The lateral extent of the heaviest soil contamination was estimated to cover an area approximately 30-feet by 40-feet in size, a portion of which was excavated in 2002 as discussed below. 3.6.2. Groundwater Plume Recent groundwater monitoring data indicates that the groundwater plume is present in the shallow aquifer (Zone A) centered near MW-2, which was reported to have a TPH-g concentration of 25,000 pg/L during the second quarter 2011 groundwater monitoring event. 3.7. Mass Estimate Based on the information available and on-going investigations conducted by GTI: • The initial soil contamination mass estimate that was prepared prior to excavation indicated that approximately 5,275 kg of TPH-g was present in the subsurface. • During the excavation (2001), it was estimated that approximately 42% of the contaminant mass was removed, or 2,216 kg of TPH-g. • The current (2010) estimate of mass remaining in the subsurface is 3,060 kg of TPH-g. • Recent groundwater monitoring analytical data was used to estimate the mass of contamination present in the groundwater. The calculations show that there are 4eological7ecknicsluc. 7 4252 RAP.docx