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Subsurface Investigation Report-Tosco(Former BP)Service Station No. 11195 Lathrop,California
<br /> April 11,2002
<br /> In conjunction with the April 9, 1999 groundwater purging and sampling, GR conducted a performance test
<br /> to qualitatively analyze the radius of influence generated during removal of 5,000 gallons of groundwater from
<br /> the conductor casing. During pumping, GR personnel collected depth to water (DTW) readings from the
<br /> conductor casing and on-site monitoring wells MW-3, MW-4, and MW-6 (Figure 2). DTW readings were
<br /> collected from immediately prior to initiation of groundwater pumping until 125 minutes after completion of
<br /> pumping activities, for a cumulative total of 160 minutes. The conductor casing was purged for 35 minutes
<br /> to fill the 5,000-gallon vacuum truck, at which time the casing was beginning to dewater. The maximum
<br /> observed drawdown in the conductor casing was 1.92 feet. Well MW-3,located approximately 45 feet from
<br /> the conductor casing, showed a drawdown approximately 0.70 feet it) 55 minutes before recovering.
<br /> Drawdown in well MW-4, located approximately 8 feet from the conductor casing, was approximately 0.38
<br /> feet in 40 minutes, before recovering. Well MW-6, located approximately 114 feet from the conductor from
<br /> the conductor casing, showed a drawdown approximately 0.20 feet in 75 minutes before recovering (GR,
<br /> Remedial Action Plan, dated May 26, 1999).
<br /> Weekly groundwater purging events are continuing at the site. Approximately 3,500 to 5,000 gallons of
<br /> groundwater per event are removed from the conductor casing and.monitoring wells MW-3 and MW-5 by Onyx
<br /> Industrial Services(Onyx)of Benicia,California. The groundwater is purged directly from the wells into an Onyx
<br /> vacuum truck and transported to the Tosco Refinery in Rodeo,California for disposal. As of February 5,2002,
<br /> approximately 609,261 gallons of groundwater have been removed from the site during the weekly purging
<br /> events,and approxunately 66.00 pounds of MTBE have been removed from the site by the groundwater purging
<br /> program.
<br /> The eleven existing on and off-site groundwater wells have been monitored and sampled on a quarterly or semi-
<br /> annual basis since 1993. Historically,the groundwater samples collected from the wells at the site have contained
<br /> TPHg in concentrations ranging from not detected to 130,000 ppb and concentrations of MtBE ranging from not
<br /> detected to 110,000 ppb. The most recent groundwater samples,collected on February 7,2002,contained MtBE
<br /> at concentrations ranging from not detected to 4,900 ppb in MW-10(GR,2002,First Quarter Event of February
<br /> 7, 2002, Groundwater Monitoring and Sampling Report,dated March 15,2002).
<br /> 3.0 FIELD WORK
<br /> Field work was conducted in accordance with GR's Field Methods and Procedures (Appendix A), the GR
<br /> Health and Safety Plan and the Site Specific Safety Plan dated February 15, 2002. Geoprobes® were
<br /> advanced under drilling permit numbers 28962 and 28966, issued February 21, 2002, by the PHS-EHD.
<br /> Underground Service Alert (USA) was notified prior to probing at the site. As a precautionary measure, a
<br /> private utility locator was contracted to identify utilities near the proposed boring locations.
<br /> 3.1 Geoprobe®Advancement
<br /> On February 25, 2002,a GR geologist observed Gregg Drilling Inc. (C57 #485165) advanced five off-site
<br /> Geoprobe® soil borings (B-1 through B-5). The borings were hand excavated for the first five feet bgs to
<br /> insure that no utilities were disturbed. The borings were advanced with a truck-mounted Geoprobe rig utilizing
<br /> direct push technology. The Geoprobe locations are shown on Figure 2.
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