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' Well Installation Report 5606 Pacific Avenue Stockton Cahfornca(CAO No S-p0707) PAf Project No 1232 40 DISCUSSION 41 Distribution Of Hydrocarbon Impacted Soil Hydrocarbon impacted soil was not identified in boring MW-181) (see Table 1) and well MW- 24D, located on the Delta College property, only contained minimal hydrocarbons at depths ranging between 80 to 95 feet bgs ' Well MW-23D contained hydrocarbon concentrations at depths ranging between 45 and 60 feet bgs The highest TPIT-G concentrations detected during installation of the wells was in boring ' MW-25 @ 60 feet bgs (1,200 milligrams per kilogram [mg/kg]) Figure 3 presents north-south cross-section A-A' and Figure 4 presents east-west cross-section ' B-B' Data on these cross-sections indicate hydrocarbon-impacted soil below the water table exists as follows • At depths of approximately 80 to 90 feet bgs in well MW-24D • At depths of approximately 45 to 60 feet bgs in wells MW-I 1 and MW-23D • At depths of approximately 60 to 65 feet bgs in wells MW-10 and MW-25D ' Well MW-24D This well is near the Robinhood Irrigation Well (RIW) located on the Delta College campus Operation of the RIW was terminated on August 22, 2000, due to the presence of hydrocarbons in the well water Typically, ground water monitoring wells near the RIW have had an increase 1 in hydrocarbon concentration during summer months when the RIW was in operation Wells MW-1 I and MW-23D These wells are located in the sidewalk adjacent to the Site that was formerly occupied by the Unocal service station These wells are between, and in line with, the former Unocal USTs and the RIW Since ground water flow direction at the Site is generally easterly when the RIW is not in operation soil and ground water in these wells may have been impacted due to the operation of the RIW Wells MW-10 and MW-25D These wells are located near the existing building and are in the area of the former USTs Approximately 6,000 cubic yards of soil were excavated in this area in 1988 Soil was excavated to the top of the water table, which at the time was located at a total depth of approximately 50 feet bgs Fill material was encountered in boring MW-255D to a dcpth of approximately 33 feet bgs (see Appendix B) Therefore, based upon lrthology encountered during drilling, this boring was located on the edge of the former excavation Hydrocarbon impacted soil was encountered in borings MW-10 and MW-25D at a depth of approximately 60 • feet bgs (see Figure 3) 6HM 1212R0121 5 ENVIRONMENTAL AUDIT, INC