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Feasibility Study/Corrective Action Plan 5 <br /> The Drake Properly,26485 Thornton Road Thornton California November 26 1996 <br /> 40 REMEDIAL ACTION ALTERNATIVES <br /> 41 Excavation <br /> This option addresses removing all potentially remaining hydrocarbon-Impacted soil from the former tank <br /> pit area Once excavated, the hydrocarbon-impacted soil is either transported from the site to a disposal <br /> facility or aerated on site to remove all volatile hydrocarbon constituents Clean soil imported to the site or <br /> soil aerated on site is typically used to backfill the excavation <br /> This option has already been performed during the 1993 UST removal and subsequent excavations In <br /> addition, further excavation would not be a cost-effective viable alternative due to the non-detectable <br /> concentrations of benzene in soils and groundwater on site, and the relatively low (i e , up to 510 mg/kg) <br /> TPH-G concentrations in soil <br /> Furthermore, a comparison of measured hydrocarbon concentrations in soil obtained during the 1993 <br /> (Wallace-Kuhl) and 1995 (Groundwater Technology)site assessment activities with theoretical <br /> concentrations expected to result from natural attenuation (Section 5 0), shows that measured BTEX and <br />. TPH-G levels are lower than those predicted to result from natural degradation These results show that <br /> natural attenuation of hydrocarbons is occurring on site The modest concentrations of TPH-G in site soils <br /> does not justify the cost associated with excavation <br /> 42 Additional Assessment and Groundwater Monitoring and Sampling <br /> This option would involve additional site assessment and the continuation of a groundwater monitoring and <br /> sampling program on either a quarterly or bi-annual schedule Additional assessment would include the <br /> installation and sampling of an upgradient groundwater monitoring well to assess the possibility of <br /> petroleum hydrocarbons originating from an off-site source Because the extent of hydrocarbon-impacted <br /> soil and groundwater has already been determined from previous site investigations (Wallace-Kuhl, 1994, <br /> Groundwater Technology, 1995), continued groundwater monitoring and sampling would serve to monitor <br /> the effects of natural attenuation of petroleum hydrocarbons in groundwater and assure that the dissolved- <br /> phase plume does not migrate beyond the affected area <br /> This option is not viable for several reasons First samples obtained from borings B-1 through B-10 have <br /> shown non-detectable concentrations of TPH-G and benzene in soil and groundwater at the site perimeter <br /> (Figure 2 Tables 1-3) These boring locations are more remote from the former source area than are the <br /> monitoring wells, and samples collected from the borings provide information on distal site conditions In <br /> addition the extent of hydrocarbon-impacted soil and groundwater has already been determined from <br /> 1 previous site investigations Second, the existing monitoring wells provide sufficient data showing natural <br />. degradation of BTEX concentrations in groundwater, as demonstrated by a comparison with theoretical <br /> r' <br /> C191FEA RaT(M,.15) <br /> FLUOR DANIEL GTI <br />