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• INTRODUCTION GHH Engineering, Inc (GW is currently providing JAMAR Oil Distribution(JAMAR) professional engineering services to assist them with environmental issues pertaining to their site located at 4075 East Main Street, Stockton, California, as shown on Figures I and 2 Mr Jay Mcllrath has authorized GHH to prepare this Fourth Quarter, 2002, Groundwater Monitoring Report and Additional Assessment Report(Report) This Report presents the results of the quarterly groundwater monitoring and sampling completed at the site on December 19, 2002 and the results of a Geoprobe and CPT investigation completed between January 13-16, 2003 During the Geoprobe investigation, three cone penetrometer test (CPT) borings and 3 geoprobe bonngs were completed to provide data to evaluate the extent of impacted groundwater and soil to the east-southeast of the subject site The work was conducted in accordance with the April 2002 Additional Assessment Workplan (AAWP) Results from the CPT and geoprobe investigations have been used to select proposed locations for two additional groundwater monitoring wells These wells will allow for continued monitoring of the east and southeast edges of the plume of impacted groundwater This report has been prepared for submittal to the San Joaquin Environmental Health Department (County), the lead regulatory agency overseeing environmental issues at the site . BACKGROUND AND SITE HISTORY Site Description The site is currently an Exxon retail gasoline service station that is located on the northwest corner of East Main Street and Oro Avenue at 4075 East Main Street, Stockton, CA The property houses one rectangular-shaped office/store building The property is asphalt paved except in the area of the dispenser islands where it is paved with concrete There is also a strip of dirt surface on the northern portion of the site The site contains one underground storage tank (UST) and two dispenser islands as shown on Figure 2 The UST is a split 20,000 gallon tank that stores 12,000 gallons regular, unleaded gasoline and 8,000 gallons premium, unleaded gasoline No diesel fuel tanks have been located on this site An underground waste oil tank (300 gallon)was located behind the store building to the north and it was removed in 1995 It has been reported that this waste oil tank had been filled with diesel fuel for testing purposes approximately - three times since it was installed There were also three previous UST's that stored unleaded regular, leaded regular, and unleaded premium gasoline These three UST's were removed in 1998 The location of the former waste oil tank and the previous UST's are shown on Figure 3 Geology and Hydrogeology Geologically, the property is underlain by the Quaternary Modesto Formation and the deeper • Quaternary Riverbank formation, both of which consist mainly of arkosic alluvium derived predominantly from erosion of the Sierra Nevada Mountains to the east