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<br /> ERI 205513 OSR19 Tosco Service Station 5886 Stockton caiiforma Jgnmry 30 2002
<br /> i • September 1996 - Unocal drilled four additional soil borings, and constructed groundwater
<br /> monitoring wells in three of these borings (Delta, December 5, 1996)
<br /> • March 1997 --Tosco submitted a Work Plan for Supplemental Environmental Investigation
<br /> (ERI, March 28, 1997) to the County
<br /> • April 1997 -Tosco submitted an Addendum to Work Plan (ERI, April 3, 1997) to the County
<br /> • April 1997 -Tosco performed a soil and groundwater investigation at the site The investigation
<br /> included hand-auguring three soil borings adjacent to catch basins, drilling four soil borings, and
<br /> constructing dual-completion groundwater monitoring wells (MW4S/D through MW7S/D) at
<br /> discrete depths in the borings(ERI, June 3, 1997)
<br /> • July 1997 -Tosco submitted a Work Plan for Supplemental Environmental Investigation
<br /> (ERI, July 14, 1997) to the County
<br /> • August 1997 -Tosco submitted the Addendum to Work Plan (ERI, August 8, 1997) to the County
<br /> • July 1997 through December 1998 -Tosco performed an underground utility search
<br /> (ERI, October 6, 1997) (Plate 2), continued to perform quarterly groundwater monitoring and
<br /> sampling, obtained access agreements to drill soil borings at select properties located south and
<br /> southwest,of the site, and requested that Sequoia Analytical Laboratories (Sequoia) review
<br /> chromatograms of groundwater samples collected during the September 1995 Geoprobe®
<br /> investigation(Delta, February 6, 1996) and provide an estimate of methyl tertiary butyl ether
<br /> (MTBE) concentrations in groundwater(ERI, March 2, 1999)
<br /> • February 1999 -Tosco advanced three off-site cone penetrometers (CPT1 through CPT3) and
<br /> drilled three off-site soil borings (138 through B10) (ERI, March 2, 1999)
<br /> • April 1999 --Tosco installed nine off-site groundwater monitoring wells, consisting of three nested
<br /> wells (shallow, intermediate, and deep) at each of three well locations (ERI, June 9, 1999)
<br /> • April 1999 --Tosco submitted the Work Plan for Supplemental Evaluation (ERI, April 16, 1999) to
<br /> the County
<br /> • May 1999 - ERI submitted the report entitled Air Sparge/Soil Vapor(AS/SVE) Extraction Test,
<br /> dated October 18, 1999, presenting the data collected during ERI's May 1999, ASISVE feasibility
<br /> test
<br /> • December 28, 1999 - ERI submitted the report entitled Corrective Action Plan (CAP) Tosco
<br /> received a letter from the County dated February 22, 2000, approving the remediation
<br /> recommendations provided in ERI's December 28, 1999 CAP as an interim remedial (IR) action
<br /> • During second quarter 2000, ERI installed one UST cavity well in general accordance with the
<br /> CAP (ERI, December 28, 1999) and Tosco initiated periodic purging of groundwater from the UST
<br /> cavity well and from groundwater monitoring well MW3 Tosco purged approximately 46,000
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