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' SUBSURFACE INVESTIGATION RESULTS AND CLOSURE REPORT 909 Hammer Lane Stoddon California 2 0 BACKGROUND The site, a former Exxon service station, is currently vacant and no structures remain ' The underground storage tanks and piping were removed in 1986 and 1990, respectively Petroleum hydrocarbons were noted in soil samples collected from excavation and soil boring activities at the site since 1986, and vapor samples collected during 1987 and 1990 soil gas surveys A soil vapor extraction system was operated on the site from March through December 1991 by Evax Technologies, Inc An estimated 1,439 pounds of petroleum hydrocarbons and 60 pounds of benzene were removed from the subsurface Vapor extraction was conducted via the existing groundwater monitoring wells and perforated steel probes temporarily installed for this purpose A schematic of ' the system is incluaed in Appendix D When remediation was considered complete, the steel probes were removed and the holes sealed per the diagram in Appendix D Groundwater has varied from a depth of approximately 47 feet in 1987 to approximately 60 feet in September 1992 Five groundwater monitoring wells (MW-1 through MW-5) were drilled in 1986 and 1987 In 1990, wells MW-6, MW-7, and MW-8 were drilled in ' the vicinity of wells MW-3, MW-4, and MW-5, respectively, because the latter went dry The wells have been sampled periodically since 1986 None of the groundwater samples from the wells has contained detectable petroleum hydrocarbon concentrations since 1991 The most recent groundwater sampling event occurred in June 1993 EA Engineering submitted a workplan in November 1992 to drill and sample three soil ' borings to confirm soil remediation The County approved the workplan, and agreed to recommend site closure to the Central Valley Regional Water Quality Control Board (CVRWQCB) if the boring soil samples confirmed remediation Aegis Environmental Inc (Aegis) drilled and sampled the borings in June 1993 Petroleum hydrocarbons were only detected in one boring (SB-9 on Figure 2) at depths ranging from 16 to 36 feet below ' grade The highest concentrations were noted at 21 feet below grade This worm was documented in Aegis' "Remediation Confirmation Sampling Results and Closure Recommendation" report, dated September 9, 1993 MIn November 1993 Aegis supervised the excavation of soil containing petroleum hydrocarbons found in boring SB-9 in June 1993 This work was documented in Aegis' ' "Soil Excavation Results Report", dated December 15, 1993 In March 1994, Fugro updated Aegis' September 9, 1993 report to include additional ' background information requested by the County The report and closure request were presented to the County and CVRWQCB by Fugro and Exxon As a result of this meeting, the County requested two additional soil borings in a letter dated April 15, 1994 ' (Appendix E) The letter stated that the two soil borings be advanced within five feet laterally of piping excavation sample S-3-TR5 (Figure 2) and monitoring well MW-1 It is Exxon's and Fugro's understanding that case c#osure will be granted by the County, 93473005 ARP z 1