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h 1.0 Site Location and Description The JC Trucking property is located in the southern section of the city of Stockton, San Joaquin County, California, between Jackson Street on-the north, Clay Street on the south, and South Aurora Street on the east (Figure 1). Private residences border the property on its west side, and primarily commercial enterprises line South Aurora Street to the east. The property presently consists of an unpaved rectangular lot with a 50 by 30 foot concrete slab at its northeast comer, the site of the former office building, and a four foot.high 100 by 40 foot foundation for the former loading dock situated along the center of the east side of the property. The site was formerly a truck terminal and was closed in February 1991, after a fire. Two 3000 gallon Underground gasoline storage tanks were located in the northwest comer and two 3000 gallon underground diesel storage tanks were located in northeastern comer of the site. The two former tank location sites are indicated by depressions in the ground surface. Figure t 2 shows the site layout and the location of the tank excavations. 2.0 Site Conceptual Model 2.1 Groundwater E Analytical results-from the eleven monitor wells that are currently being sampled indicate that the highest groundwater contamination is in the upper most groundwater zone as expected. The groundwater level for the last 3 112 years has remained approximately between 31 and 35 feet below the ground surface (bgs). The acontamination in the groundwater at the site is essentially gasoline and diesel which is�6 light non-aqueous phase Iiquidi(LNAPL). As a result, there should be relatively little diffusion into the groundwater and the bulk of contamination is in and on the top layers of the groundwater. A cross section running approximately west to east through the site (figure 3) show the interpolated petroleum hydrocarbon contamination in the groundwater (figure 4). The highest gasoline values in the groundwater are from under the former gasoline UST location with values of 70,000 µg/L of TPH-gasoline and 4800 fcg/L of benzene in the I water collected at the last quarterly groundwater sampling event in September 2002. The macro-trend for benzene values recorded during the last 3 112 years is downward. The only diesel values currently noted are from under the former diesel UST location with a value of 1600 �Lg/L of TPH-diesel from the last quarterly groundwater sampling event. There are fourroundwater monitor wells on the site deeper:that are screened at g y intervals ranging from 100 to 130 feet bgs which is 70 to 100 feet below the groundwater surface. Low values of TPH-gasoline and related aromatic volatiles have been found in these wells. This is not typical for sites that have LNAPL contaminants such as this site has. Explanations may include contamination from leaking USTs during periods of much lower groundwater to off-site contamination. Previous analyses from up-gradient sampling .at or below 100 feet indicate that itis unlikely that the deeper contamination is f from an off-site, upgradient source. �. JJW GEOSCIENCES lnc.