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Work Plan for Additional Subsurface Investigation @ Former Unocal Station 40187 November 12, 1999 TPHg(up to 420 ppm) and benzene(up to 3.0 ppm)were detected in soil samples from well NP-1,mainly in the interval from 32 to 80 feet bgs. Discrete groundwater samples collected from NP-1 had concentrations of TPHg and benzene of up to 73,000 and 4,200 ppb, respectively. MtBE by EPA Method 8260 was not detected in either soil or grab groundwater samples from this boring. Soil samples from well NP-2 contained concentrations of TPHg and benzene of up to 1.5 and 0.18 ppm, respectively, and discrete groundwater samples contained TPHg and benzene at concentrations up to 4,000 and 1,200 ppb,respectively. MtBE by EPA Method 8020 was not detected in soil or grab groundwater samples from this boring. Results of this subsurface investigation are presented in ARCADIS Geraghty & Miller's Supplemental Site Characterization Report dated June 3, 1999. Quarterly monitoring and sampling of the groundwater monitoring wells began in August 1994. Dissolved concentrations of TPHg have been detected in all wells except U-5 and U-6,and benzene has been detected is all wells except U-5. MtBE has been detected is wells U-1, U-3, and U-4. During the most recent sampling event on February 5, 1999,TPHg was detected at concentrations ranging from 73 to 73,000 ppb, and benzene concentrations ranged from 0.52 to 7,000 ppb. The highest concentrations of both TPHg and benzene were detected in well NP-1. Quarterly sampling and monitoring of the wells is performed by GR. Historical groundwater chemical analytical data are summarized in the most recent GR report (Third Quarter 1999 Groundwater Monitoring and Sampling Report, dated October 1, 1999). PROPOSED COPE OF WORK In SJCPHS lettr dated September 13, 1999, SJCPHS stated that quarterly monitoring and sampling must continue and tl e wells be sampled for the oxygenates by EPA Method 8260, a work plan be prepared to define the veil ical and lateral extent of impacted soil and groundwater, and recommended the onsite installation of apor-extraction wells and air sparging wells for feasibility testing. GR has review d the onsite soil data and the most recent groundwater monitoring report. Groundwater is currently at ap 3roximately 25 feet below surface grade (bsg). The majority of soil impact is submerged beneath the st allow groundwater table. The basis for implementing vapor extraction as a remedial alternative is to reduce hydrocarbon concentrations in the unsaturated zone. Based on this,vapor extraction would not be a viable remedial alternative at this time. With regards t air sparging, SJCPHS has determined that soil and groundwater have not been delineated laterally or vertically. Until SJCPHS and Unocal agree that delineation has been accomplished, the installation of air sparge wells is premature at this time. Thus, possible feasibility testing for remedial activities is net proposed at this time. The first generation USTs located in the southwestern comer of the subject site were removed in 1967. Based on the soil data from this location, it appears that this former UST complex is the source for soil and groundwater i pact. Since these USTs were removed in 1967,at a time before oxygenate compounds were roundwater with the used in petrol um fuels,any release from these USTs would not have impacted soil or g oxygenate compounds. This was confirmed when the groundwater monitoring wells were sampled for the oxygenate cot npounds by EPA Method 8260 in March 1998. The data indicated that the samples did not 4 240042.05