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1 <br /> 16 SITE CONCEPTUAL MODEL AND REMEDIAL ACTION PLAN 5 <br /> 7-Eleven Store No 14117,2725 Country Club Blvd , Stockton, CA June 20, 2002 <br /> Three conceptually-feasible remediation alternatives were considered for oxygenate/hydrocarbon cleanup <br /> at the site with relation to time and cost, to evaluate which method or combination of methods would be <br /> most cost effective The methods considered were excavation, air sparging/soil vapor extraction <br /> (ASISVE), and dual-phase extractiontintrinsic bioremediation (DPEIIB) <br /> 41 Excavation <br /> ' Excavation of gasoline-containing soil in the UST pits as the primary source area would likely remove a <br /> majority of the source of the hydrocarbon/oxygenate plume, and is therefore technically feasible <br /> 1 <br /> This remediation method Is less feasible economically and logistically, due to the disruption of store <br /> ' business, UST removal and replacement, need for shoring, dewatering, and protection of subsurface and <br /> above-ground structures, and waste disposal costs The primary source area in the immediate tank <br /> vicinity could not be removed without replacement of the USTs, which would be costly and unneccessary <br /> from a maintenance standpoint(the existing tanks are In compliance and in assumed good condition) <br /> 42 Air Sparging/Soil Vapor Extraction <br /> SVE Involves thea application of negative pressure to the vadose zone to remove organic vapors from the <br /> PP 9 p 9 <br /> soil AS is the Injection of gases below the water table to either physically desorblvolatilize organic <br /> constituents from the saturated zone, and/or introduce gases (such as oxygen) to enhance intrinsic <br /> biodegradation or to chemically destroy contaminants (I e , ozone injection) These two processes are <br /> ' usually done in conjunction, so that SVE will capture the vapors generated by the AS process Low-flow <br /> AS can be implemented without concurrent generation of vapors AS point spacing Is inversely <br /> proportional to permeability (i e , for sands, the points should be spaced about 10 feet apart, and about 15 <br /> 1 feet for silts, however, interbedding of low- and high-permeability soils may significantly Increase the <br /> optimum spacing) SVE point spacing is directly proportional to permeability (I e , spacing should be 40 <br /> feet apart for sands, and 20 to 30 feet apart for silts) <br /> r <br /> Equipment required for SVE/AS Includes a blower, air compressor, orspecialty g as generator in addition <br /> to vapor extraction and destruction equipment The efficiency of AS as a remediation process is variable, <br /> with up to 90—95% reductions in dissolved levels (from Initial concentrations) reported using this form of <br /> remediation There is as yet no well-established track record of this treatment method for MTBE, but pilot <br /> tests (ACS Convention, April 2000) demonstrate some success in mass transfer of MTBE from the liquid <br /> to the vapor phase by conventional sparging There is limited available data as of the date of this report <br /> regarding mass transfer rates for TBA or other oxygenate species Heated air sparging assists in <br /> overcoming the high solubility of MTBE, but safety issues regarding treatment of mobile flammable vapors <br /> such as gasoline would need to be addressed prior to application <br /> ' 14117Scm2 doc <br />