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® GEOMATRIX ® Groundwater has also been sampled from an existing off-site monitoring well (WSFMW-1) by Geomatrix in 1999 (Geomatrix, 2000). Historical groundwater results indicate the presence of total petroleum hydrocarbons (TPH) quantified as diesel (TPHd) in on-site monitoring well MW-22 (2,300 micrograms per liter ® [µg11]); in general, no petroleum constituents have been detected in on-site wells MW-3 and MW-4 or off-site well WFSMW-1.3 The groundwater sample collected from MW-1 contained elevated concentrations of TPH quantified as gasoline(TPHg; 61,000 4g/1) and TPHd (3,000 µg/l). Benzene,toluene, ethylbenzene, and xylenes(collectively referred to as BTEX)report- edly were not detected above the laboratory quantitation limit of 1,000 µg/l. A thin layer of i separate-phase petroleum has historically been present in MW-1. These elevated concentra- tions are likely the result of separate-phase petroleum entrained in the sample and likely do not represent dissolved concentrations of petroleum constituents in groundwater. ! Additionally, on-site production well W-1,which reportedly has been used for facility water, ® was tested for volatile organic compounds (VOCs) and BTEX in 1995 and 1996; no com- pounds were detected and the well water met applicable drinking water standards (ERM 1995a and GTI, 1996). f Concentrations of VOCs(i.e., BTEX and cis-1,2-DCE) in groundwater samples collected from the CPT borings located within the estimated area of petroleum impacted soil were less than water quality objectives4 (WQOs). The groundwater sample collected from the off-site and downgradient location GMX-6 contain low concentrations of TPHg, TPHd, and benzene. Based on previous site reconnaissance activities and discussions with facility personnel, GMX-6 is located adjacent to a former gasoline station.5 The constituents detected in the GMX-6 sample may be related to this off-site source and not from residual crude oil originating from the OVP (Geomatrix, 2000). 2 Sample extracts were not treated with silica-gel prior to analyses for the samples collected from MW-2,Mw-3, and Mw'4. 3 The sample collected from Mw-3 in 1998 contained 300 µgll of TPHd without silica gel preparation. Water Quality Objective(WQOs)for petroleum hydrocarbons are presented in A Compilation of Water Quality Goals,California Environmental Protection Agency,Regional Water Quality Control Board,Central Valley Region,by Jon B.Marshack,August 2000.;WQO for chlorinated VOCs are based on United States Environmental Protection Agency Maximum Contaminant Levels(MCLS). s Based on a communication from Mr.Don Guzenhauser,facility representative for the 35275 Welty Road Site, a Union Gasoline Station was present from at least 1972 to the mid-1980s. 1loaklldeptdataDoc_Safe16OOGs16447.0031Repon\US Can SoiIGW Rpt.doc 4 ....... ............