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Ms. Laurie Cotulla July 25, 1989 Page 3 ' On February 6; 1987,.the San Joaquin County Local Health District (County) directed ARCO to conduct a soil and groundwater characterization at ARCO .Station 2093. Brown and Caldwell was authorized by ARCO in Task Order Number 2093-2-1, dated July 1, .1987, to L perform this investigation. The purpose of the:investigation was to determine the groundwater gradient, identify the presence or absence of product in the soil and/or groundwater beneath the site, evaluate the site geology, and obtain information necessary for evaluating potential remedial actions at the site. The investigation would include the drilling of four boreholes with groundwater monitoring wells installed in the boreholes. Figure 2 shows the:locations of the four monitoring wells (ATR-1 through.ATR-4), as well as three additional monitoring wells (ATR-5 through ATR-7), which were'insta_lled .during March 1989 and which will be explained in detail later in this letter report. I On March 16, 1987, the County approved Brown and Caldwell's March 2, 1987, work plan [included in Brown and Caldwell's September 28, 1987, Problem.Assessment Report (PAR)]. During April 1987, Brown and Caldwell conducted a site investigation in accordance with the a approved March 2, 1987, work plan. Monitoring wells were installed in each of the four boreholes .by inserting 2-inch-inside- diameter, schedule 40 polyvinyl chloride (PVC) casing with approximately 22 feet of 0.020- inch machine-slotted well screen beneath the blank casing to accommodate for seasonal groundwater fluctuations. The monitoring wells were designed by the on-site geologist to conform to the subsurface conditions. The geology underlying. the ARCO site was determined from the borehole lithology logs generated during drilling. Immediately underlying the site is a sequence of sand_, silt, and clay. A continuous layer of silt occurs from the ground surface to.approximately 20 feet below the ground surface. Groundwater occurs in this silt layer at approximately 11 feet below ound. surface. In April 1987, the direction of groundwater flow was determined to be to the northwest under a 0.13 percent gradient. Selected soil samples from each of- the four , boreholes were analyzed for benzene, L ethylbenzene, toluene, and xylene (BETX) using U.S. Environmental Protection Agency (EPA) Method 8020, total petroleum fuel hydrocarbons .(TPH) using EPA Method 8015, modified, and purgeable halocarbons using EPA Method 8010. One groundwater sample was collected from each of the four wells using approved methods. The groundwater samples were analyzed for BETX using EPA Method 602, TPH using EPA Method 8015, purgable halocarbons using EPA Method 601, and total lead using the California State Department of Health Services' (DHS) method of lead as described in the LUFT manual. The soil and groundwater were analyzed by a DHS-certified laboratory. Analytical results for the soil samples collected during the April 1987 investigation are L presented in Table L Table 2 presents a summary of analytical results from groundwater sampling from April 1987, as well as data from the quarterly sampling through the March _ 1989 (most recent) sampling period. L k: I BROWN AND CALDWELL C Li 723 S STREET SACRAMENTO,CALIFORNIA 95814-7092-(916)444-0123