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S [EC0R Conceptual Site Model March 24, 2004 Page 13 4AvE B pounds (approximately 1,600 gallons) of TPHg from January 1997 to February 1998 The SVE system was shut down due to decreasing concentrations (Delta, 2000) 633 Historical Report Summary In July 1991, a subsurface investigation consisting of four soil borings, was conducted at former ; Arco service station 2168 to evaluate the extent of hydrocarbons in the soil and groundwater Samples were analyzed for TPHg and BTEX TPHg concentrations ranged from 40 mg/kg (S-25-131) y` to 2 1 mg/kg (S-30-131), benzene ranged from 0 46 mg/kg (S-25-131) to 0 0059 mg/kg (S-40-132) f These activities are summarized in Resna's April 7, 1992 Addendum to Work plan for Subsurface y Investigation sit In 1993, Resna reported that dissolved hydrocarbons in the deeper zone were the result of off-site migration and proposed to conduct an off-site investigation as reported in Resna's June 9, 1993 Status of Environmental Investigation, First Quarter 1993, and Addendum to Work Plan k In October of 1993, A Resna field geologist supervised Westex Drilling, as they completed two offsite b� soil borings (B-20 and B-21) Soil borings B-20 and B-21 were converted to groundwater monitoring a wells and renamed MW-6 and MW-7 Resna reported that analytical results obtained from samples collected from MW-6 and MW-7 indicated a possible source of hydrocarbons up-gradient of the ARCO site Groundwater samples collected from MW-6 reported maximum TPHg and benzene concentrations of 93,000 pg/L and 15,000 pg/L, respectively TPHg and benzene were reported at concentrations of 42,000 pglL and 3,100, respectively in MW-7 These activities are summarized in Resna's February 15, 1994 Status Report Fourth Quarter In January 1997 EMCON installed and began operating a soil vapor extraction (SVE) and treatment system Influent and effluent samples were collected and submitted for chemical analysis Vapor samples were analyzed for TPHg and BTEX TPHg and BTEX influent concentrations were 55,000 milligrams per cubic meter (mg/m), 980 mg/m3, 1,400 mg/m3, 200 mg/m3, and 890 mg/m3, Y' respectively These activities are summarized in EMCON's February 13, 1997 Source Test Results for the Soil Vapor Extraction and Abatement System report In February of 1998, EMCON shut down the SVE and treatment system for a three week period to allow for the soil vapor concentrations to rebound Prior to the system being turned off background ; influent vapor samples were collected The SVE system was restarted three weeks after system shut-down and influent vapor samples were collected and submitted for chemical analysis Continued monitoring indicated a decrease in TPHg and benzene levels over a few days Based on field and analytical data EMCON shut down the SVE system These activities are summarized in fi EMCON's April 8, 1998 letter, Results of Soil Vapor Extraction and Assessment Between July and August 2000, Delta environmental installed five groundwater monitoring wells on r site (MW-1B, MW-1C, MW-8, MW-86, and MW-8C) MW-113 and MW-1C were installed adjacent to MW-1 MW-8, MW-8B, and MW-8C were installed up gradient of the former UST's The reported purpose of the clustered wells was to establish vertical characterization of groundwater quality by collecting samples from screened intervals at discrete depth intervals These activities are summarized in Delta Environmental Consultants, Inc November 22, 2000 Soft and Groundwater 9 Investigation Report 11Chevron1920331REPC)RTSICSM%CSM 92033(REV B) doc 1 i