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S E C 0 R Ms. Lori Duncan ARCO Station 4493 Site Closure Report October 31, 2006 Page 3 MtBE have been reported below laboratory detection limits since the second quarter of 2003 (SECOR, 2002). In November 2002, URS supervised the installation of dual-completion groundwater monitoring well set MWA 1 MID downgradient from the site. Groundwater analytical data suggest that groundwater in the vicinity of MW-11 M is impacted with TPHg and MtBE. MtBE has been detected in well MWA 1 D during four of five sampling events since the fourth quarter of 2002, but none of the detections have been above the MCL of 13 micrograri�s per liter (pg/L) (URS, 2003). In December 2003, one deep groundwater monitoring well (MW-12D) was installed under the direction of URS in the planterwest of North Center Street to assess the vertical extent of petroleum hydrocarbon contamination in groundwater immediately downgradient from the former pump islands (URS, 2004x). On May 3, 2004, one dual-completion groundwater monitoring well set(MW-13M/D)was installed under the direction of URS in the State of California parking lot at 31 East Channel Street,west of EI Dorado Street, to assess the lateral and vertical extent of petroleum hydrocarbon contamination in groundwater downgradient from the former ARCO station pump islands. Groundwater samples collected from these wells were non-detect for gasoline-related compounds and MtBE (URS, 2004b). Quarterly groundwater sampling data is summarized in Tables 1 and 2. Boring logs for the t_ groundwater monitoring wells are presented in Attachment C. Well construction details are summarized in Table 3. SITE GEOLOGY AND HYDROGEOLOGY Soil encountered during previous investigations consisted of silt, silty sand, sandy clay, sandy gravel, and clayey silt. Geologic cross sections of the site are included in Attachment D. Boring logs for the monitoring wells at this site are included in[ Attachment C. Depth to grou dwate beneath the site has ranged from approximately 10.85 (1212001) to 27.26 (9/2004)feet top o casing (TOC). Based on quarterly water level measurements at this site from February '999 t ugh June 2006 the inferred groundwater flow direction is generally to the northeast at a gradient of 0.003 to 0.01 feettfoot. Groundwater gradients and flow directions are summarized in Table 4. DISCUSSION In 1999, relatively low petroleum hydrocarbon concentrations in groundwater were reported in the samples collected from the site monitoring wells. TPHg was reported at a maximum concentration of 15,000 micrograms per liter pg/L in February 1999 (Well MW-4). During the second quarter 2006 TPHg, BTEX, MtBE, and TBA were detected above laboratory reporting limits in groundwater samples collected from site wells. Concentration contour maps for TPHg, MtBE,and Benzene are included as Figures 3,4, and 5, respectively. Calculations for mass of contaminate remaining in groundwater and soil areprovided in Table 5 and 6,respectively. From �-' data presented in this report,SECOR estimates 90 pounds(lbs)of TPHg remain in the groundwater