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Four samples (two per borehole) collected from boreholes SWI and SW2 between 40 and 50 feet below ground surface were also analyzed for TPH-g and BTEX TPH-g and BTEX ' were found in all four of the samples collected from SWI and SW2 4.2 GROUNDWATER ANALYTICAL RESULTS LPH was not found in any monitoring wells on 18 May ' On 18 May 1993, groundwater samples were collected from wells MW1, MW8, MWIO, MW13, RW1—RW4, PI, P2, and V4 The samples were submitted to Pace Incorporated, where they were analyzed for TPH-g by Cal EPA-modified EPA Method 8015 and for BTEX by EPA Method 8020 The analytical results for the groundwater samples collected on 18 May are presented in boldface type in Table 2 with the results for previous efforts TPH-g and benzene concentrations are shown in Figures 3 and 4, respectively Copies of the laboratory analytical reports are included as Appendix C ' Petroleum hydrocarbons were found in samples collected from each of the sampled wells, including the new well MW13 installed downgradient at the Chateau Convalescent Hospital, where benzene was detected at 1 1 µg/L and TPH-g at 71 µg/L Samples from the wells located near and downgradient of the former tank field (MW 10, RW I, RW2, RW3, RW4, V4) contained hydrocarbon concentrations at their histoncal levels or slightly lower, ranging from 36,000 to 240,000 pg/L TPH-g and from 420 to 19,000 pg/L benzene. In the wells located upgradient of the former tank field (P1, P2), concentrations ranged from 1,400 to 3,200 µg/L TPH-g and from 43 to 65 µg/L benzene, which are at their historical levels One of the upgradient wells (MWI) contained 950 pg/L TPH-g and 19 pg/L benzene, which are an ' order of magnitude higher than the concentrations measured in any earlier effort, indicating that dissolved hydrocarbons from a source west of the Exxon site may be migrating eastward following the hydraulic gradient The sample from well MW8, located west (upgradient) of ' the Exxon site, contained 53,000 pg/L TPH-g and 850 pg/L benzene The concentrations of petroleum hydrocarbons in samples from well MW8, two orders of magnitude higher than in the on-site upgradient wells, suggests the possibility of a source of hydrocarbons to the west ' of former Exxon RS 7-4237 4.3 GRADIENT DETERMINATIONS ' Water levels were also measured on 18 May Depth to water Y p measurements and groundwater elevations for 18 May are presented in boldface type in Table 2 with previous data, and ' plotted as esocontours in Figure 5 The indicated direction of the groundwater gradient is to the east-northeast at an average magnitude of 0 01, which 1s consistent with previous data ' Groundwater elevations have increased 14 feet since February 1993 I 1 I 8373942(2YR01691TX 3 4