ARCHIVED REPORTS XR0004919 - Laserfiche WebLink

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Ms Lori Duncan Page 2 San Joaquin County Environmental Health Department ARCO 6080 February 20, 2006 In June 2000, three onsite wells (MW-1, MW-2, and MW-3) were installed to assess if groundwater was impacted at the site No analytes were reported at or above the detection limit in the sol] samples collected from MW-3 Only TPHg was detected in MW-2 at a concentration of 6 49 mg/kg at 11 feet bgs, the sample collected from MW-2 at 21 feet bgs did not detect TPHg The 11-foot sample from MW- 1 contained 536 mg/kg TPHg, 290 µg/kg toluene, and 2,250 µg/kg total xylenes The 21-foot sample of MW-1 did not detect any contaminants (Attachment C) Groundwater was first encountered in this boring at 11 feet bgs, therefore, the contaminant results at 11 feet may be due to groundwater smear and not be reflective of soil impacts Quarterly groundwater monitoring activity utilizing groundwater monitoring wells MW-1 through MW- 3 has been conducted at the site since June of 2000 (Tables 1 and 2) As of December 2005, all analytes were not detected at or above the method detection limits, except for one detection of MtBE in MW-2 (0 90 micrograms per liter [pg/L])that was below the maximum contaminant level (MCL) Site Geology and Hydrogeology Soils encountered during drilling indicate the presence of sands from ground surface to approximately 26 feet, the total depth of the borings Clayey silt was identified in MW-2 from 16 to 20 feet bgs and sandy silt in MW-3 from 16 to 20 feet bgs Soil boring logs and a geologic cross-section of the site are included in Attachment D F Based on quarterly water-level measurements from June 2000 through December 2005, the inferred groundwater flow direction is generally to the east or southeast at a gradient of 0 0004 to 0 009 feet/foot Groundwater gradients and flow directions are summarized in Table 3 REMEDIAL ACTION SUMMARY Approximately 50 cubic yards of impacted soils were over-excavated from beneath the dispensers and piping during upgrade activities conducted in September 1999 (47 cubic yards of soil were generated during the initial upgrade activities) Soil samples collected after over-excavation was completed indicated that impacted soils remained in these areas No additional soil or groundwater remediation has been conducted at the site CONTAMINANT MASS REMAINING AND DISTRIBUTION Mass calculations for TPHg and MtBE in soil and groundwater are presented in Attachment E and summarized below K 1Wprocess\J5 BP ARCO\060801Closure Rpts16080 Closure Report doe