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AW fr Work Plan For limited Subsurface Investigation—Tosco(Former BP)Service Station No. 11195 October 15,2001 0.02-inch machine slotted well screen. The location of the conductor casing is shown on Figure 2. Between February 26 and March 26, 1999, approximately 41,760 gallons of groundwater were removed from the former UST backfill utilizing the conductor casing. Groundwater removal and transportation was performed by WMIS. The water was transported to the Tosco Refinery for treatment and disposal. A total of approximately 102,760 gallons of groundwater were removed from the former and.present UST pits and transported to the Tosco Refinery between January 1999 and March 26, 1999. In conjunction with the April 9, 1999 groundwater purging and sampling, GR conducted a performance test to qualitatively analyze the radius of influence generated during removal of 5,000 gallons of groundwater from the conductor casing. During pumping, GR personnel collected depth to water (DTW) readings from the conductor casing and on-site monitoring wells MW-3,.MW-4, and MW-6 (Figure 2). DTW readings were collected from immediately prior to initiation'of groundwater pumping until 125 minutes after completion of pumping activities,for a cumulative total of 160 minutes. The conductor casing was purged for 35 minutes to fill the 5,000-gallon vacuum truck, at which time the casing was beginning to dewater. The maximum observed drawdown in the conductor casing was 1.92 feet. Well MW-3, located approximately 45 feet from the conductor casing,-showed a drawdown approximately 0.70 feet in 55 minutes before recovering. Drawdown in well MW-4, located approximately 8 feet from the conductor casing, was.approximately 0.38 feet in 40 minutes, before recovering. Well MW-6, located approximately 114 feet from the conductor from // the conductor casing, showed a drawdown approximately 0.20 feet in 75 minutes before recovering. Weekly groundwater purging events have continued at the site. Approximately 3,500 to 5,000 gallons of groundwater per event are removed from the conductor casing and monitoring wells MW-3 and MW-5 by Onyx Industrial Services (Onyx) of Benicia,California. The groundwater was purged directly from the wells into a Onyx vacuum truck and transported to the Tosco Refinery in Rodeo,California for disposal. As of June 25,2001, approximately 468,260 gallons of groundwater have been removed from the site during the weekly purging events. As of May 5, 2000, approximately 43.88 pounds of MTBE have been removed from the site by the groundwater purging program. The eleven existing on and off-site groundwater wells have been monitored and samples on a quarterly or semi- annual basis since 1993. The most recently collected groundwater samples, collected on February 9, 2001, contained petroleum hydrocarbons at concentrations ranging from not detected to 66.6 ppb of TPHg(MW-4), not detected to 43,400 ppb MtBE(MW-10),and not detected for benzene. Depth to water on February 9;2001 ranged between approximately 7.61 and 9.99 feet below top of casing(TOC). The groundwater flow direction has historically ranged from northeast to southwest with an average gradient of 0.001 feet/feet or less (Ahsto, 1999;Blaine Tech Services, Fourth Quarter 1998 Groundwater Monitoring.Report, dated May 1999; and BP letter to the SJCEHD, dated May 3, 1999). PROPOSED SCOPE OF WORK To further define the extent of hydrocarbon-impacted groundwater downgradient of the site; GR proposes to install five GeoprobesOO at and in the vicinity of the site. Grab groundwater samples will be collected from each of the Geoprobes. GR Field Methods and Procedures are included in Appendix A. To perform this scope of work, GR proposes the following tasks. 140193.11-1 i 4 E i r f