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Project No. E6576.001 �t tional Groundwater SamplingAV. Clover 22 March 1995 1 i BACKGROUND After three underground fuel storage tanks (USTs) were removed from the subject property ("Site") in 1987, three monitoring wells were installed by others and sampled periodically in 1987 and 1988. The previous owner reported that he would receive a closure after one more round of testing. However, when he attempted to close in 1990, he was told by the County Health Department that they had lost or misfiled the file and, therefore, he would have to do additional work. These wells were also sampled in 1994, and the analytical results were negative; as were the previous tests. These results were reported in Reference 1, along with a request for closure. After reviewing the referenced report, the San Joaquin County Public Health Services, Enviromnental Health Division (PHS-END), requested additional investigation in order to obtain data within 10 feet of the site of the excavated USTs. This was requested because the rpreviously installed monitoring wells had been located in a triangular pattern averaging about 100 feet apart. The location and installation of an additional monitoring well (MW-4) was done in accordance with a work plan dated 13 October 1994 (Reference 2). INSTALLATION OF MONITORING WELL In accordance with the work plan, the first step was estimation of the center of the prior tank excavation. This estimation was enabled by distinctive features on aerial photographs and ra sunken ground surface feature. The shallow exploratory borings were centered on this point L.� which is indicated by B-2 on Figure 1, attached. The first boring (13-1) was purposely located in an area where native soils were anticipated. The logs of some of the exploratory borings are given in Appendix A. B-1 penetrated light tannish brown native silty clay, as expected. B-2, located in the center of the suspected excavation site, exposed soils with markedly different color and character and obvious hydrocarbon stain and odor. This material appeared to be a mixture of fill and contaminated backfill sand. The remaining, surrounding five borings rexposed soils with little or no obvious contamination, and much of the soils appeared to be i•• native. Groundwater was encountered at depths of 6 to 7 feet in the soil borings. Soil samples taken at 9.0 and 9.5 feet in B-2 appeared to be from uncontaminated native material. (Note: the soil samples from B-2 were incorrectly labeled B-3). B-3, located 13 feet in the previously ff J L' TERRASEARCH, INC. Page 3 �i