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SECOR ' Request for No Further Action Chevron Station 9-2396 December 16 2004 Page 7 ' 4 2 Estimated Mass of Hydrocarbons Remaining in Soil ' This mass calculation is based on the historical soil data presented in the "Tank Removal Report" dated September 20, 1993, and "Tank Removal and Excavation Report" dated February 10, 1994 For this estimate, the following conservative assumptions were made ' • The area of Impacted soil at the site consists of one contiguous area, • Impacted soil in the area of the former UST pit was excavated to an approximate depth of 16 feet bgs, ' • Soil concentrations of TPHg were averaged across the site, one detection (OX-8-16, 410 mg/kg) was not used because the sample area was Included in a 1,500 square foot excavation in 1993, ' • Sall concentrations of TPHg do not extend to a depth greater then 20 feet bgs, and, • A porosity of 25 percent was used, which corresponds to a "poorly graded sand with silt" (American Society for Testing Materials [ASTM] — D2488) ' The equation used for estimating the mass of hydrocarbons remaining in soil is presented in Appendix G The results of this calculation indicate that 312 pounds of TPHg could remain in soil beneath the former USTs ' 4 3 Petroleum Hydrocarbons Dissolved in Groundwater Groundwater samples were collected from SB-1 through SB-3 on April 26 and 27, 2001 using Hydropunch ® MtBE was detected in the sample from S13-1 at a concentration of 13 0 µg/l_ During quarterly groundwater monitoring in wells MW-1 through MW-3, MtBE was detected at a maximum concentration of 1 µg/L in MW-1 and 7 µg/L in MW-2 MtBE ' was not detected in groundwater samples collected from MW-3 The historical extent of hydrocarbons dissolved in groundwater is presented in Figure 8 Currently, hydrocarbon analyte concentrations are below method detection limit (Table 3 and Appendix D) 4 4 Estimated Mass of Hydrocarbons Remaining in Groundwater ' To calculate the mass of MtBE remaining in groundwater, the following assumptions were made ' . To conservatively estimate the mass of MtBE in groundwater a site-wide concentration of 13 0 µg/L (0 013mg/L) was assumed This value corresponds to the highest concentration of MtBE detected from onsite groundwater sampling (SB-1), ' • Concentrations of constituents in the dissolved phase are assumed to vary linearly with distance between Isocontour intervals This assumption was made to facilitate a calculation by removing the logarithmic diffusion rate Because concentrations typically ' decrease exponentially with respect to distance, a linear relationship is likely conservative, and, 1 I1Chevron192396IReports\FINAL_NFAR_REPORT doc