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Supplemental Preliminary Site Investigation for State Route 99/Hammer Lane Project No. H2O2 to Mark Thomas & Co., Inc. November 17, 1999 Abandoned Gas Station (Former Mobile Oil Station) 7840 North Highway 99 East Frontage Road, Stockton, California 95212 Site Description The Abandoned Gas station site is located on the northeast corner of Hammer Lane and North Highway 99 East Frontage Road (Figure 2, Site Map). The site is currently unused and the former service station structure has been removed. Weathered asphaltic concrete pavement and Portland cement concrete, which appears to be associated with the former station, still remains. Large bushes and a north-south trending swale border the west side of the property. A chain-link fence is situated along the south portion of the property and orchards surround the north And west side of the property. Site Investigation For our previous Preliminary Site Investigation for this project, dated February 26, 1999, we performed six (6) test trenches on February 25, 1999. The trenches were excavated by backhoe at the locations of magnetic anomalies encountered on-site during our geophysical investiiation. One trench was performed in an area that was suspected to be the location of the former underground storage facilities (based on location and lack of surface pavement). This trench revealed the presence of fill, comprised of silty sands, gravel and concrete rubble. The lack of surface paving and nature of the underlying fill suggested that this area was likely the site of the former underground storage facilities. As of the date of this current investigation, our research has not confirmed the exact location of the former underground storage tanks. The assumed location of the former underground storage facilities was therefore based on physical site evidence (adjacent to former service islands, lack of surface pavement and the presence of granular backfill). For the current investigation, Espana Geotechnical Consulting completed soil borings at three selected j locations within the suspected former underground storage:tank area on October 18 and 19, 1999 (Figure 4, Boring Location Map). Two of the borings Were performed at opposite ends of the suspected former tank pit area (north and south sides) and were drilled to depths of 21.5 feet. A third boring was advanced in the approximate center of the former pit area and drilled to a depth of 71.5 feet. Ground water was encountered at 67 feet. The soil cuttings were screened for hydrocarbons using a PID. The PID did not detect the presence of hydrocarbons in the cuttings of the three borings i performed at this site. I Laboratory Analysis Laboratory analysis for determination of the presence of benzene, toluene, ethylbenzene, total xylene, TPH as gasoline and TPH as diesel (EPA test methods 8015m and 8020) indicated that none of the acquired soil samples contained these constituents above the detection limit. The ground water sample, which was acquired from boring EGC-5 and analyzed in the laboratory, indicated the presence of unidentified diesel hydrocarbons in the C 10-'C24 range at 0.126 mg/l. The results of the analytical laboratory analysis are included in Appendix II. A summary of the laboratory results are presented on Table 2, below: 5 `W ESPANA GEOTECHNICAL CONSULTING