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Ms. Laurie A. Cotulla San Joaquin County Public Health Services April 29, 1993 Page 2 respectively) were detected in'a single sample from a depth of 25 feet bgs. ;Xylenes were detected in three sample between 20 ar d 30 feet bgs at a naximum'concentration`of•73 ppb.. Based on these results, SEACOR concluders that significant lateral migration of petroleum hydrocarbons released from the former USTs had not occurred, and that the extent of petroleum hydrocarbon-affected soils was limited to directly underlying the former USTs. These findings and conclusions were presented in a SEACOR technical report dated June 11, 1991 which was submitted to your agency in June 1991. In your response letter dated August 14, 1991', your staff agreed with our conclusions as no additional soil assessment work was requested. In September 1991, SEACOR submitted to your agency a Soil Remediation Work'Plan dated September 13, 1.99 1. The Work Plan included a proposal to extract soil vapors from.a soil vapor extraction well completed beneath the former,USTs, near boring location MW-1A, where elevated TPHg and BTEX concentrations had been detected. In a response letter dated October 7, 1991, your staff stated "the soil remediation work plan prepared and-submitted by SEACOR.offers a logical approach to the cleanup of the'contaminated'subsurface soils". In April 1992, SEACOR initiated soil vapor extraction from a single 41nch .diameter extraction well (EW=1) located 5.feet east from MW-1A. The soil vapor extraction system operated until January 15, 1993, at which.time influent vapor concentrations had decreased to below 50 ppm. An estimated 1070 pounds of gasoline were removed from subsurface soils based on the measured flow rates and vapor stream concentration data collected during system operation. ,I In December 1992, SEACOR submitted to your agency a Work Plan dated December 18, 1992 which included SEACOR's proposed approach and scope for confirming the effectiveness of the soil vapor extraction. The Work Plan included a.proposal to advance a single confirmatory soil boring within 5 feet from,boring_ location MW-lA, where elevated concentrations of petroleum hydrocarbons had been detected during the initial assessment. In a phone conversation with Ms. Mary Meays, she provided a verbal conditional approval of the Work Plan. The conditions of the approval were unrelated to the confirmatory soil boring and pertained to the collection and chemical analysis of soil samples from two additional groundwater monitoring wells. At that time, Ms. Meays did not raise a concern regarding the number or location of proposed confirmatory borings. In January 1993, a confirmatory soil boring (CSB-1) was advanced between the vapor extraction well .A. (EW-1) and boring location MW-lA as proposed in our Wol&Plan. ' Analytical testing results from the confirmatory soil boring showed that soil vapor extraction was very effective in removing adsorbed_phase gasoline from subsurface soils in the vicinity of the former USTs. With the exception of a single detection of TPHg (1 ppm) and trace levels of xylenes (7 and 100 ppb) in two samples, TPHg and BTEX were not detected-in soil samples collected from confirmatory soil boring. SEACOR believes that these results combined with the initial assessment results provide. adequate evidence that petroleum hydrocarbon-affected soils have been adequately assessed and remediated. Furthermore, it is SEACOR's professional opinion that the trace levels of TPHg and xylenes.remaining no longer pose a threat to underlying groundwater quality. KmD.LTR 150014003-01