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02 May 2005 AGE-NC Project No. 01-0827 Page 2 of 6 AGE had proposed to augment the IAS system with a low-flow ozone generator in Ozone Sparging and Additional Monitoring Well Installation Work Plan, dated 07 October 2002. An EHL letter dated 03 January 2003,requested procedures for evaluating the effectiveness of the operation ofthe in-situ air sparing(IAS)system with a low-flow,ozone generator.Essentially what benchmarks for proposed remediation system would be acceptable. The acceptable performance of the ozone sparging system and the specific modification to the system; including the installation of a micro- diffuser at the former UST area.Operation ofthe ozone injection system has been ongoing since July 2003, resulting in a general downward concentration trend, with an increase in dissolved concentrations after an additional sparge-diffuser OZ-1 installation and operation. AGE prepared Site Conceptual Model,dated 25 February 2005 was prepared at the verbal request ofthe EHD during a meeting on 20 January 2005.The lateral extent of dissolved hydrocarbons north and northeast of the former UST release area may not be fully defined and requires additional shallow assessment of the impacted soil and impacted ground water limits. 2.1. REGIONAL GEOLOGIC/HYDROGEOLOGIC SETTING The site is situated within the Great Valley Geomorphic,Province of California, a large,elongate, northwest trending, 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 alluvial plain, extending from the Tehachapi Mountains in the south to the Klamath Mountains in the north.The western and eastern boundaries ofthis province are comprised ofthe California Coast Range and the Sierra Nevada,respectively.Rocks composing the basement complex ofthe province have not been completely defined but are believed to be metamorphic and igneous in origin. The northern and southern portions ofthe Great Valley Province have been designated the Sacramento and San Joaquin Valleys, respectively. Based on the General Soil Map from the San Joaquin County Soil Survey,published by the United States Department ofAgriculture Soil Conservation Service in 1992,the soil type within the site area is poorly drained Jacktone clay, in large areas covered by urban land. Jacktone soils are located within basins, and generally form in alluvium derived from mixed rock sources. The Modesto, Riverbank, 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. The closest surface water feature to the site is Mormon Slough located approximately 500 feet west of the site. A&wneedGeoEnvironrnental,Inc,