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May 24, 1996 Quarterly Monitoring Report Chapin Brothers Automobile Repair Facility Stockton, California 1.0 INTRODUCTION a This report summarizes the analytical results generated during a quarterly groundwater monitoring event conducted by Lush Geosciences at the Chapin Brothers Automobile Repair site located at 1766 Monte Diablo Avenue in Stockton, California. The property is located on the southeast corner of the intersection of Monte Diablo Avenue and Buena Vista Avenue in the City of Stockton, California(Figure 1). A service station operated on the property from the late 1930s until 1990. An automobile repair facility has operated on the site since that time. The quarterly monitoring was conducted to collect and analyze representative groundwater samples to help evaluate concentrations of total petroleum hydrocarbons as gasoline (TPHg) and purgeable aromatic hydrocarbons (benzene, toluene, xylenes and ethylbenzene M (BTEX)) in uppermost groundwater at the site. PREVIOUS WORK Because of a suspected release from the underground fuel storage tanks, a single monitoring well was installed at the site on December 5, 1990, by Roger Foott Associates, Inc. of West Sacramento, California. Subsequent laboratory analysis of a sample from the well determined that shallow groundwater beneath the site had been impacted with gasoline. On August 4, 1993, one 550-gallon waste oil tank, two 4,000-gallon unleaded gasoline tanks, and two 8,000-gallon unleaded gasoline underground storage tanks were removed by Jim Thorpe Oil of Stockton, California. Three additional smaller sand-filled tanks were also discovered and removed during the removal operation. Laboratory analysis of soil samples collected from beneath the tanks indicate that gasoline was present in soil underlying the 4,000-gallon tank located at the northwest corner of the site. In December 1993 Science and Engineering Analysis Corporation (SEACOR) conducted a subsurface investigation of the property. According to the SEACOR report dated July 22, 1994, fifteen soil borings were completed at the site. Three of the borings were converted to groundwater monitor wells, one was converted to a vapor extraction well, and three were used as temporary vapor extraction/observation',wells. The remaining eight borings were drilled to groundwater where groundwater grab samples and soil samples were collected. Based on the laboratory data generated during their subsurface investigation, SEACOR concluded "the highest concentrations of contamination are confined mainly to the area directly beneath the site, and that subsurface contamination, apparently from the site, is present to a lesser degree between 100 and 200 feet west and southwest of the site.' A soil vapor extraction test and a aquifer pump test 530-001 '' - 1 - LUSH GEoscll;NcEs