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d 15 November 2000 • AGE-NC Project No 95-0142 Page 3 of 9 , T r , ground water due to a reduction of the thickness of the impacted vadose zone and an increased thickness (approximately 55 feet),and volume of impacted soil and ground water in the saturated zone The impacted soil is primarily fine grained, which may make the upward and lateral path of sparged air through the saturated zone unpredictable and stripped hydrocarbons potentially unrecoverable Sparging at a depth sufficient to address all impacted ground,water may actually cause lateral migration of the impacted ground`water plume The anticipated limited overall effectiveness and reliability of IAS technology to remediate impacted soil and ground water require a re-evaluation of the proposed method to remediate ground water and reduce the hydrocarbon mass AGE recommends evaluation of ground water extraction as an alternative ground water remediation technique at the site Ground water extraction should be evaluated for site-specific hydrogeologic conditions Factors controlling eftectiveness of ground water extraction are soil hydrogeologic properties, dissolved hydrocarbon mass distribution, and adsorption affinity of the hydrocarbons to the soil matrix The soil at the site has a high percentage of fine-grained matrix and overall, the soil at the site is homogenous, mostly fine-grained, fostering a high adsorption capacity for the soil and low permcabihty limiting migration of dissolved hydrocarbons On the other hand, the limited areal distribution of high concentrations of dissolved hydrocarbons near the source of the release favors • a high potential for capturing the major portion of the dissolved hydrocarbons on the site 24 REGIONAL GEOLOGICAL SETTING The site is situated within the Great Valley Geomorphic Province of California, a large, elongate northwest tending, asymmetric structural trough The Great Valley Province has been filled with thick sequences of sediment ranging in age from Jurassic to Recent, creating a nearly flat lying r alluvial plain extending from the Tehachapi Mountains in the south to the Klamath Mountains in the north The western and eastern boundaries of this province are comprised of the California Coast Range and the Sierra Nevada, respectively Rocks composing the basement complex of the province have not been completely defined but-are believed to be of metamorphic and igneous origin The Great Valley Province has been subdivided into two major divisions identified as the Sacramento and San Joaquin Valleys The Modesto, Riverbank and Turlock Lake Formations and overlying Recent alluvium are the principal sources of domestic ground water in the 13,500-square mile San Joaquin Valley Ground Water Basin (Basin 5-22) This basin is drained primarily by the San Joaquin River 25 GROUND WATER DEPTH AND FLOW DIRECTION Based on review of the Lines of Equal Depth to Groundwater Spring 1999 map published by the San Advanced CeoEnvironmental Inc r i