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1 i ' Quarterly Report - 4th Quarter 2003 Former CHASE CHEVROLET (MADISON) Facility ' 423 North Madison Street, Stockton, California ' r 10• INTRODUCTION At the request of the City of Stockton,Advanced GeoEnvironmental, Inc (AGE)has prepared this ' quarterly report summarizing field activities performed during the fourth quarter of 2003 at 423 North Madison Street,Stockton,California The scope of work included conducting a quarterly r ' ground water monitoring event and preparation of this report The site setting is illustrated in Figure 1 Structures and monitoring well locations are illustrated in Figure 2 f ' Based on previous investigations,AGE has identified two potential hydrogeologic units informally identified as the Upper and Lower Water Zones These two zones are separated by approximately 5 to 10 feet of clay and silty clay During the previous six years of ground water monitoring,ground water flow direction in the Upper Water Zone has been consistently toward the north to northeast ' while the ground water flow direction in the Lower Water Zone has been toward the west-northwest, southwest and southeast Petroleum hydrocarbon-impacted ground water at the site is limited to the area surrounding the former UST No 8 location (Figure 2) The northern extent of impacted ground water is defined by ' minor,Intermittent detections of analytes in quarterly ground water samples from wells MW-13 and MW-20B South of the former UST No 8 location,minor concentrations of benzene,toluene, ethyl benzene and total xylenes (BTEX) constituents (less than 5 Ag/l) and steadily declining or non- detectable concentrations of total petroleum hydrocarbons as diesel(TPH-d)in ground water samples collected from wells MW-19A and MW-19B indicate that these wells mark the southern margin of Impacted-ground water at the site However, the cast, west and vertical extent of impacted-ground ' ;water is not well defined ' Additional site background information was summarized in Revised Corrective Action Plan - February 1997,Site Investigation Report- USTNo 8,dated 10 February 1998 and Quarterly Report -August 1998, prepared by AGE Data from previous ground water monitoring events is included in the tables of this report 1 ' 2.0 PROCEDURES 'Field activities were performed in accordance with guidelines Issued by the Central Valley Regional ' Water Quality Control Board (RWQCB) and by the San Joaquin County Environmental Health Department (EHD) for investigation of underground storage tank (UST) sites and sampling of ground water monitoring wells, respectively Advanced GeoEnvironmental,Inc ' 1