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S ECOR ' Request for No Further Action Chevron Station 9-2396 December 16 2004 Page 8 ' • Porosity of the various soil types beneath the site range from 10-20% for silts and clays to 20-40% for sand A conservative 35% site-wide porosity was assumed to estimate ' the available water in saturated soils The equation used for estimating the mass of MtBE remaining In groundwater is presented ' in Appendix G The results of this calculation indicate that approximately 0 011 pounds of MtBE could remain in groundwater beneath the site However, the most recent analytical data (Appendix D) show all constituents below method detection limits ' 5.0 SITE REMEDIATION ' Remedial actions at the site consist of the removal of the six underground storage tanks and associated piping, and the removal of 1,500 ft3 of soil associated with past site assessment activities The primary sources of contamination have been removed and hydrocarbon contamination at the site is limited to shallow soils near the former USTs and around the station building (Figures 7 and 8) Therefore, concentrations of hydrocarbons at the site are expected to decrease with time due to natural attenuation Typical applications ' of Best Available Technology (BATS) to similar hydrocarbon plumes include oxidation of the plume or thermal treatment Oxidation of the plume appears to be the more efficient use of materials since 1) due to the close proximity to the ground surface oxygen concentrations lie are not expected to be deficient, and 2) close proximity to the surface makes thermal treatment of the soil impractical No formal risk assessment has been performed for the site However, assuming that the site will be continued to be used for industrial purposes, the risk associated with the 1 contamination described is considered low While no fate and transport modeling has been performed, the mobility of the contamination identified at the site is expected to be low Factors causing low mobility of hydrocarbon contamination include • adsorption of the remaining hydrocarbons to particulate surfaces, • lack of rainfall or atmospheric moisture to transport the hydrocarbons past the top ' layers of shallow soils, paved surface in the site vicinity prevents downward movement of surface water, • high surface tension of water in silt and clay in the shallow soils inhibit downward ' movement of solutes, • depth to groundwater of 50 feet or more ' 6.0 CONCLUSIONS MtBE was reported in monitoring well MW-2 at a concentration of 7 pgll Detected ' concentrations of MtBE in well MW-2 are below the California EPA primary (13 pg/1) maximum contaminant level (MCL) Detected concentrations of MtBE in well MW-1 are below both the California primary and secondary MCLs ' 11Chevron1923961Reports\FINAL_NFAR_REPORT doe