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4 F) f tR eat i Fitt 30.2070-2 - 19 Dccemb", 1991 Page 2 of 30 feet for MW-2 and a depth of 35 feet for MW-3. ESE determined the direction of groundw,eter flow(groundwater gradien•) beneath the subject site by utilizing hydrostatic water level measurements obtained following completion of the initial three monitoring wells. However, due to the near linear placement of the three monitoring wells relative to one another, an inaccurate determination of the groundwater gradient was calculated. s Anderson Consulting Group (formerly Anderson Gwtechnical Consultants, Inc,) was retained by Anheuser-Busch Companies in January 19888 to install twc, additional monitoring Wells and further assess the extent of potential groundwater contamination. In February 1'988, Anheiser-Busch instructed Anderson Censulting Group (ACG) to perform the completion of the groundwater contamination investigation as well as compose a Problem Assessment Report for the site. In September 1989,both the eastern most tank of Oei -tank cluster as well as the single tank located in the front parking lot of the bakery were excavated and removed. Three soil samples were obtained by another consultant following removal of the tanks. One soil sample (AA-1563) was obtained from beneath the product Iine which ran from the eastern tank of the tank cluster..to the dispenser. The other two samples (AA-1561 and AA-I562) were obtained from each end of the eastern tank of the tank cluster. Laboratory analysis of the soil samples indicated the presence of 609 paw per million total petroleum hydrocarbons for sample AA-1563. No petroleum hydrocarbons were detected in the other two samples. Soil samples it"_r ^.ot obtained from the tank excavation near the front of the bakery.,_ /, 1.3 Geologic and Hydrogeologic Sattirig The Rainbo Bakery site is located in the north-central portion of the geomorphic province known as the Central Valley. The Central Valley is bounded by the Coast Ranges to the west and the Sierra Nevada to the east. Material derii ed from erosion of the adjace-It mountains has filled the valley with up to several thousands of feet of consolidated and unconsolidated sedimentary dei wits. These deposits overlay older, Mesozoic (-100-200 million years old) granitic bas^ment rock, 71-.esedirr.entary deposits of the valley thin out to several feet in thickness both westward and eastward near the bases of the Coast Ranges and the Sierra Nevada, respectively. ~' Tate subject property is situated directly atop Holocene (<10,000 years old) Lasin deposits which j formed in response to erosional and transportational processes of the San Joaquin River and its tributaries (Departrn:nt of Water Resources, 1974). The thicknesses of the Holocene deposits vary from approximately 5 feet near the edges of the Central Valley to several hundred feet in the center of the valley (United States Geological Survey, 198.5). These deposits are ger!erally .. horizontal layers of unconsolidated, fine-grained sand, silt and clay which grade basin-ward into organic mud. _. The Holocene Basil) deposits are underlain by Pleistocene (up to 2 million years old) alluvium: Three formations of alluvial deposits are stratigraphically present in the vicinis.y of tha ect property (United States',Geological Survey, 1985). These formations are, :n order of inereas:ng age, _ the Modesto Formatio , Riverbank Formation and Turlock.Lake Formation. All three formations were deposited as stream channel and/or overbank deposits in response to fluvial processes. Due { ANDERSON CONSUCONSULTING - f I