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N N ` 02 January 2007 AGE-NC Project No. 95-0121 Page 2 of 10 PREVIOUS INVESTIGATIONS AND REMEDIATION The following is a brief summary of previous investigation for underground storage tanks (USTs) formerly located at the site: • In August1988, two1 ,000-gallon gasoline USTs were removed from the site. Followingtank removal, soil samples were collected beneath both tanks and submitted for analysis of petroleum hydrocarbons. Analytical results of soil samples collected from beneath the USTs indicated that substantial petroleum hydrocarbon concentrations were present in soil samples. • The tank area was re-excavated in November 1988. Soil was removed from the former UST area to a depth of approximately 19 feet below site grade (bsg). Soil samples were collected from the excavation and screened for organic vapors using a photo-ionization detector (PID). Organic vapor readings indicated that petroleum hydrocarbons were present at depths in excess of 19 feet bsg. As a result, the excavation was backfilled and a preliminary site assessment initiated. • Between May 1989 and December 1994, a total of nineteen soil borings were advanced at the site; nine of the soil borings were completed as two-inch diameter monitoring wells (MW4 through MW-9). Extensive petroleum hydrocarbon-impacted soil and groundwater, including free product, was encountered during assessment of the site. Analytical soil data is presented in Table Al . • In July 1995, one dual-purpose soil vapor extraction/ground water extraction well (EW- 1), and two soil vapor extraction wells (VW- 1 , VW-2) were installed at the site. hi August 1995, an 8-hour soil vapor extraction (SVE) pilot test was conducted at the site. The pilot test was performed to provide vapor extraction data for proper system design of a remediation system. • In November 1995, a Corrective Action Plan (CAP) was submitted to the San Joaquin Country Environmental Health Department (EHD), and included details of the SVE pilot test. Recommendations in the CAP included installation of an SVE system to remediate hydrocarbon-impacted soil and an in-situ air sparging (IAS) system to remediate hydrocarbon impacted ground water. • The EHD concurred by letter dated 06 June 1996 with AGE's recommendations that SVE and AS could effectively remediate petroleum-hydrocarbon impacted soil and ground water at the site. • In October and November 1998, AGE installed 7 soil vapor extraction wells (VW-3 through VW-9) and 5 air sparging wells (AW- 1 through AW-5). • Ground water elevation rose approximately 10 to 15 feet between August 1995 and August 1999; increasing the interval thickness of impacted soil in the saturated zone and reducing the volume of impacted soil in the vadose zone, where SVE is most effective.