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ell I • z•0 INTRODUCTION SECOR International Incorporated (SECOR) has prepared this work plan on behalf of Conoco Incorporated (Conoco) to conduct confirmation soil sampling at 4325 East Washington Street in Stockton, California(Figure 1) The objective of this investigation is to evaluate if the site qualifies for no further action status The proposed investigation involves the drilling of two soil bonngs, the collection and chenucal analysis of soil samples, and the preparation of a technical report detailing the findings of the investigation All work is to be conducted under the direction of a California registered geologist Z 0 BACKGROUND The former Econo Gas Station is located at the northwest corner of the intersection of Washington and Sinclair Streets in Stockton, California It is bounded on the east by Sinclair Street, on the south by Washington Street, and the north and west by private residences The only remaining structure on the site is the farmer cashier's office The pump islands, canopy, underground storage tanks and lines were removed in 1987 To date, a total of 48 wells have been drilled on and near the site to investigate and remediate soil and groundwater impacted by petroleum hydrocarbons Thirty-one of these wells were originally dnlled as groundwater monitor wells, three were drilled as "bio" wells and fourteen were drilled as vapor extraction wells The monitor wells at the site which were originally screened below the water table began going dry in July of 1989 The bottom of the screen interval in these wells ranges from 50 to 70 feet below ground surface (bgs) By January of 1990, all of the groundwater monitor wells had gone dry due to a dechmng water table Four additional monitor wells (MW-27 to MW-30) were installed to depths between 90 and 98 feet bgs in October 1991 Groundwater samples collected from these wells have had no detectable concentrations of total petroleum hydrocarbons as gasoline (TPHg) or benzene, toluene, ethylbenzene, and xylenes (BTEX) since May 1993 MTBE was detected under 5 ug/L during the June 1996 sampling event A vapor extraction and thermal oxidation treatment system was installed at the site to remediate the vadose zone which increased in volume as a consequence of the declining water table The remediation system operated at the site from August 1989 to February 1996 During this time, approximately 106,000 pounds (17,000 gallons) of gasoline were removed and treated 4325stoc wpl Job No 1070MIM2 SECOR December 2 19% 1