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fir f Monitoring Well Installation Report-Tosco(Former BP)S5# 11195,Lathrop California , f July 24,2002 in September 1998. Soil samples collected during the investigation at depths of 4 to 6 feet bgs contained petroleum hydrocarbon concentrations of up to 1.5 ppm of TPHg and 0.0034 ppm of benzene. Oxygenating compounds including methyl tert-butyl ether(MBE)were not detected(Alisto, 1999). A sensitive receptor survey and a 1/2-mile radius water well survey were performed by Alisto. No basements or subways were found within a 300-foot radius of the subject site. An off-site subsurface utility survey revealed the presence of a sanitary sewer trench adjacent to the subject site that may influence the transport of petroleum hydrocarbons in groundwater. A search of California Department of Water Resources (DWR)and Banks Information Solutions files indicated that seven water wells had been installed within a t/2-mile radius of the subject site,but only two could be definitively located. A visual reconnaissance of the area did not identify any additional water wells (Alisto, 1999). One 10,000-gallon, one 8,000-gallon, and one 6,000-gallon single-wall fiberglass gasoline USTs,two dispenser islands,related product lines,and one canopy were replaced with two 12,0007gallon double- wall gasoline USTs, two dispenser islands,related product lines, and one canopy in December 1998. The new USTs were installed in an excavation located immediately adjacent to the former UST complex. These excavations are hydraulically connected with pea gravel backfill material encountered =� in the northeast corner of the former UST pit. Soil samples collected from the capillary fringe of the former and new UST excavations at depths of 11 to 12 feet bgs contained petroleum hydrocarbons at concentrations of up to 130 ppm of TPHg, 50 ppm of MtBE,290 ppm of ethanol, and 51 ppm of tertiary butyl alcohol(TBA). All of the soil samples were non-detect for benzene and the oxygenate compounds. Soil samples collected from beneath the former product lines at depths of 3 to 4 feet bgs were non-detect for TPHg,benzene,MtBE, or the six oxygenate compounds. A set of first generation product lines were discovered during construction activities and removed. Soil samples were collected from beneath these product lines at depths of 2.5 to 4 feet bgs. Concentrations of TPHg (50 ppm), MtBE (0.17 ppm), and ethanol (64 ppm) were reported in these samples. The area surrounding these samples was overexcavated and four sidewall soil samples and one bottom sample were collected at depths ranging from 6 to 11.5 feet bgs. These samples contained TPHg concentrations up to 950 ppm,but were non-detect for all other analytes. Groundwater was encountered in the excavations at a depth of approximately 11 to 12 feet bgs. Two grab groundwater samples were collected from the former UST excavation and analyzed for TPHg, BTEX,MtBE,and six oxygenate compounds. Grab groundwater samples SW and CW,collected from. beneath the former south and center USTs, contained 8,400 and 60,000 ppb of TPHg, 310 and 1,500 ppb of benzene, 100,000 and 130,000 ppb of MtBE, and 460 and 870 ppb of tert-amyl methyl ether (TAME),respectively(Gettler-Ryan Inc., UST and Product Line Replacement Report, dated April 23, 1999). 140193.12-2 3 i I