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Work Plan • ARCO Facility No.6100 Tracy,California Page 2 were drilled; 20 borings were completed as vapor extraction wells. Groundwater was not encountered during any of the investigations. A water supply well was installed onsite in 1969, in the southwest corner of the property, and supplies water to the operating service station. The well was constructed of 8-inch casing to 540 feet bgs, screened from 396 to 416 and from 496 to 536 feet bgs, and gravel packed from 35 to 540 feet bgs (The IT Group, September 10, 1999). Reportedly, a sanitary seal was not installed. A water sample from this well was extracted and analyzed in August 1999. All analyzed constituents were reported as non detect. Depth to water was recorded in the well at the time of sampling to be approximately 40 feet bgs. In October, 1998, Pinnacle Environmental Solutions submitted Hydrocarbon Mass Evaluation, presenting the estimated mass of TPHG and BTEX constituents. An addendum to this report was prepared by the IT Group and submitted in September 1999. San Joaquin County PHS/EHD submitted a Case Closure Summary to CRWQCB on February 1, 2000 requesting "no further action" of environmental work at the site. The CRWQCB declined the request for no further action (March 14, 2000)and required that a groundwater investigation be conducted at the site. Scope of Work Based on the analytical results of soil samples collected from the previous investigations, the distribution of residual petroleum hydrocarbons appears to trend southeast from the USTs with increasing depth. Soil samples collected from B-26/VW-17 contain the highest concentrations of hydrocarbons detected at the greatest depth. Therefore, to investigate the vertical extent of petroleum impacted soil and the possible impact of hydrocarbons to groundwater, we are proposing to drill one boring in the right-of way of Patterson Pass Road, in the vicinity northwest of B-26/VW-17 (see Figure 2). The boring will extend to groundwater or until two consecutive soil samples, over a distance of 20 feet, have been collected that do not contain petroleum hydrocarbons. If groundwater is encountered, a 4-inch diameter monitoring well will be installed, developed and sampled. Task 1: Prefield Activities • Update the Site-specific Health and Safety Plan • Secure permits from San Joaquin County to drill one boring and install one groundwater monitoring well(if warranted) • Prepare and submit an encroachment permit application to the City of Tracy to drill in the shoulder of Patterson Pass Road S • Mark the proposed boring location and notify Underground Service Alert, ARCO, and San Joaquin County of the scheduled field activities