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Subsurface Investigation Report-Tosco(Former 8P)Service Station No. 11195 Lathrop,California G,�f April 11,2002 5.3 Groundwater Analytical Data and Mass of MtBE Removed MtBE concentrations from the grab groundwater samples and groundwater samples from quarterly monitoring events were used to estimate the mass of MtBE removed in groundwater purged from the UST pit, CC-WS, MW-3, MW-5 and MW-10 at the site. An estimated MtBE concentration for each purging event was derived from averaging MtBE concentrations from groundwater samples collected on the dates prior to and following the corresponding purging event. The estimated concentrations were used to calculate the MtBE mass removed during each event. Based on this method, the estimated total MtBE mass removed in groundwater purged from the CC-WS, MW-3,MW-5 and MW-10 between May 5, 2000 and February 7, 2002 are 1.16, 1.84, 11.39 and 5.16 pounds, respectively. The estimated amount of MtBE removed from the site since the start date of January 19, 1999 is 66.00 pounds. The estimated amount of MtBE removed for each event, the cumulative MtBE removed, and the calculation methods are summarized in Table 4. 6.0 DISCUSSION AND RECOMMENDATIONS Based on the results of the.recent installation of five Geoprobe borings at the site, the lateral extent of petroleum hydrocarbon impact to soil has been defined. The lateral extent of hydrocarbon impact to groundwater downgradient of the site is defined for all constituents except methyl tertiary butyl ether(MtBE). Grab groundwater concentrations from the Geoprobes (Table 1) show that MtBE was detected in four of the five Geoprobe borings at concentrations ranging from 3 to 10,000 parts per billion (ppb) by EPA Method 8260. Therefore, in an attempt to provide lateral delineation of the hydrocarbon impact to groundwater, GR recommends the installation of four groundwater monitoring wells at the proposed locations shown on the attached Figure 2. GR recommends that the new monitoring wells be added to the existing monitoring and sampling program, and that the new wells be sampled for a minimum of four consecutive quarters, over the course of one hydrologic cycle. In addition, GR recommends continuation of the current monitoring and sampling program at the subject site. GR proposes the following schedule for the installation of additional groundwater monitoring wells. 1. A Work Plan detailing the field methods, specifications and analytical procedures for the proposed monitoring wells will be submitted by April 26, 2002. 2. Offsite access agreements have been obtained, but final approval must be obtained from the offiste owners prior to installation of the monitoring wells. 3. The proposed monitoring wells will.be installed by May 31, 2002,pending approval of the offsite owners and the PHS-EHD. 4. A monitoring well installation report will be submitted by July 26, 2002. 5. A Corrective Action Plan (CAP)which includes a Feasibility Study will be submitted by August 30,2002. 140193.11-2 7