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MR_ a ...s. _ _ tou WESTERN +GEO-ENGINEERS SECTION 8.0 DISCUSSION AND SUMMARY Desert Petroleum has owned and operated the project site as a gasstation fility sor to Desert ince 1979, this site has been an active service station ownership. On April 19, 1988 the station was undergoing remodeling fortDesert ac Petroleum, Inc. The existing tanks, piping, islands, and overall staticxn was to be upgraded and remodeled by Walton Engineering of West Sacramento, CA. Upon tank removal hydrocarbon odors ware noticed and,the construction aon was discontinued, except for remov,r�i of known o�'taminated soil from the tank excavation area; a^tiI assessment of the possible contamination to soil and water Could be performed(see figure 4A and Appendix J). Tank removal was performed on this date in which four underg',ound storage tanks were removed(one regular gasoline, one unleaded gasoline, one super unleaded gasoline and one waste oil). 6hortly after tank removal, Western Geo-Engineers along with Walton Engineering Proceeded with the initial phase of the investigation by installing three 2 inch ground water monitoring wells (MW 1 through MW 3) on May 5 and 8, 1888 on site (see figures 4A and Appendices A, H, K and L). These wells were installed to a total depth of 29 feet. Monitor well 1 was installed north and closest to the tank excavation and sampling showed ground water contamination with hydrocarbons. Monitor well 2 was installed southeast of the excavation behind the station builiding and Monitor well 3 was installed east of the excavation and pre-existing pump islands. Sampling of monitor wells 2 and 3 indicateno ground water contamination exists in this area Soil samples taken from the base and sidewalls of the fuel tank excavation and from boring 1 WW 1) indicate that all soil contamination associated excavation had to be re- excavated, a soil sample taken at the base of the original with the fuel tanks was removed during initial tank removal. The waste oil tank excavation indicated that soll contaminated with oil type hydrocarbons still existed. The re-excavation of the waste oil site was performed June ti, 1988 under the supervision of Western Geo-Engineers and San Joaquin Local Health inspectors The excavation at this time was expanded in anorth-south and east- Westdirection' and deepened approximately 1112 to 2 feet. The four soil samples taken at the base of each wall of me excavation indicate that all contaminated soil has been removed. Total hydrocarbon concentrations that were present in the soil at the fuel tank excavation ranged from non-detected in the native soil left after excavation to 33 parts per miilon, gasoline range, in the spoil soil pile. Total hydrocarbon concentrations that were present in the :waste oil excavation ranged from nor:-detected in the re-excavation to 32 parts per million, motor oil range, in the original excavation. Hydorcarbon concentrations in the ground water ex ----------------fue_!_tank_excavation-ran a from 20.5 arts _po ..- .11—the -----------__- -� -- g p per_-million-ire-tfie north end and 77.7 parts per million in the south end, gasoline range. Hydocarbon concentrations in the soil taken from.the borings (MW 1, 2 and 3) are non-detected. Hydrocarbon concentrations in the groundwater sampled from the monitor wells range from non-detected in MW 2 & 3 and non-detected to 4.0 arts iri MW I. No floating P per million,-gasoline range, :J g product was observed in the excavations or the monitor wells during Western Geo-Engineers investigation (other than the thin fluorescent skim from boring of MW#1). page 36