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IHistorical Background & Site Conceptual Model AGE-NC Project No 01-0827 Page 4of9 need to reduce overall project time(cost),a sparge-diffuser was installed to a depth of approximately ' 30 feet bsg, in the zone of porous media at the lower portion of the upper aquifer under the former UST area ' AGE proposed to augment the IAS system with a low-flow ozone generator in the Ozone Sparging and Additional Monitoring Well Installation Work Plan, dated 07 October 2002 An EHD letter I dated 03 January 2003 requested procedures for evaluating the effectiveness of the operation of the in-situ air sparing(IAS)system with a low-flow,ozone generator,essentially,what benchmarks for proposed remediation system would be acceptable AGE prepared the Ozone Sparging Work Plan I Addendum dated 28 January 2003, outlining 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 The work plan addendum stated "If a 75% reduction of the dissolved ' contaminants had not occurred after six months of continual ozone delivery to the current air- sparging well network, the air-sparging/ozone distribution network should be modified by adding one ozone diffuser to the ozone delivery network " 3.0. SOIL PROPERTIES-HYDROSTRATIGRAPHY/SITE HYDROGEOLOGIC ' CONDITIONS Based on vertical soil profiles In soil-boring logs acquired during site assessment, the subsurface ' stratigraphy consists of a fine-grained,sandy silt and clayey silt,identified generally between surface grade to varied depths of approximately 10 feet bsg at the former UST area to 30 feet bsg at ' monitoring well MW-2, located 100 feet east of the former UST area The upper silt is in contact with a silty to well graded sand, locally with poorly graded gravel cut and fills Generally,the sand was identified to a depth of 30 or 35 feet bsg in the former UST area,while fine to medium sand and ' silty sand was identified generally at depths between 35 and 55 feet bsg in monitoring well MW-2, east of the former UST area An additional fine-grained sandy silt and clayey silt, were identified generally between 30 feet and 60 feet bsg at the former UST area A fine sand and silty sand was ' identified at depths between 60 and 65 feet bsg in former UST area Based on the subsurface soil distribution,AGE developed a preliminary subsurface model limited ' to the area near the former UST excavation, with the hydrostratigraphy consisting of four informal hydrogeologic zones Hydrogeologic units may be defined as significant vertical intervals of soil having significant lateral extent and having similar hydrologic characteristics (i e hydraulic ' conductivity, transmissivlty, etc ) over its vertical and horizontal extent AGE has informally identified the four informal hydrogeologic zones in geologic cross sections A-A 'and B-B'as having significant intervals of apparently porous to semi-porous grained soil separated by significant ' intervals of finer-grained,less permeable soil ,the four hydrogeologic zones identified are as follows . (depths are approximate and can vary by several feet across the site) ' Advanced GeoEnvironmental,Inc 1