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SITE CONCEPTUAL MODEL Former Texaco Service Station/ARCO 440 West Charter Way, Stockton, California Site Address 440 West Charter Way incident Number- 88870583 City. Stockton, California Regulator San Joaquin County Environmental Health Department Item Evaluation Criteria Comments/Discussion 1 Hydrocarbon Source 1 1 Identify/Describe Release Hydrocarbon impacted soil was observed beneath two dispenser removed Source and Volume (if known) during construction activities in October 1988 Unknown quanity or source of hydrocarbons 1 2 Discuss Steps Taken to Stop No direct repairs or action was taken to stop the release as there was no Release specifically identified equipment failure USTs, product dispensers, and piping were removed during June 1999 The new property owner installed USTs in the same location as the previous UST at the subject site 2 Site Characterization 2 1 Current Site Use/Status The site is currently an operating ARCO Service Station 2 2 Sod Definition Status The lateral extent of TPHg, BTEX, and MTBE in sod is defined by CPT-1, CPT-3, CPT-5, and CPT-6 to the east, CPT-4 to the northeast, CPT-7 to the west, CPT-8 to the southwest, and CPT-9 to the south The vertical extent of TPHg, BTEX, and MTBE in soil is defined at CPT-1 to the east, CPT-4 to the northeast, CPT-7 to the west, CPT-8 to the southwest, and CPT-9 to the south Sod tables and maps are included in Attachment 1 2 3 Separate-Phase Hydrocarbon Separate-phase hydrocarbons (SPH) was detected in well MW-1 prior to Definition Status March 1993 SPH has not been detected at the site since 1993 2 4 Groundwater Definition Status TPHg and BTEX in shallow groundwater beneath the site is defined lateral) (TPHg and BTEX) by wells MW-2, MW-6, MW-8, and MW-14 to south/southwest, MW-13 to the west, MW-15 to the northeast, and MW-17 to the east (TPHg and benzene isoconcentration contour maps are provided in Attachment 6 TPHg is defined vertically at well MW-11-122 to the east Groundwater analytical results are included in Attachment 2 2 5 BTEX Plume Stability and TPHg, BTEX, and MTBE concentration trends appear to be decreasing Concentration Trends across the site Graphs included with the February 26, 2004 Site Investigation Work Plan and Bio-Parameter Data Analysis (see Attachment 2)clearly show TPHg and benzene concentrations declining over time 2 6 Groundwater Definition Status MTBE in shallow groundwater is defined laterally by wells MW-6, MW-7, MW- (MTBE) 13, MW-15, MW-17, MW-10 and MW-8 An MTBE isoconcentration contour map is presented in Attachment 6 MTBE is present in well MW-14 at levels higher than those found on the subject site Since this well is located on an adjacent property, which is also a LUST case, it appears likely the MTBE found in MW-14 is not from the subject site, but likely is from an offsite source MTBE is defined vertically beneath the subject site at wells MW-18- 120 and MW-11-122 and at borings CPT-8, CPT-9, CPT-1, CPT-2, CPT-3, CPT-4, CPT-5, and CPT-6 1399 Page l of 3 Date Modified 11/2/2004