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25 February 2005 . AGE-NC Project No 01-0827 Page 4 of 10 ' concentrations of hydrocarbons in the SVE influent samples to below detection levels Subsequently, AGE had 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 and Ozone Sparging Work Plan Addendum, dated 28 January 2003 ' An ozone air sparging system was Installed 01 July 2003 on-site, utilizing the existing IAS system wells IAS-1 through IAS-3 The system has operated continuously since Installation Operation of the ozone infection system has been ongoing since July 2003, resulting in a general downward ' concentration trend However, based on the proposed contaminant reduction profile, the use of a stand alone ozone generator, without using a diffuser, has not meet the reduction goal At the direction of the City of Stockton and based on the residual contamination and need to reduce overall ' project time(cost)a sparge-diffuser was installed to a depth of approximately 30 feet bsg,the zone of porous media at the lower portion of the upper aquifer under the former UST area ' 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 EHD letter 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 Ozone Sparging Work Plan 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 I 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 a varied depth of 10 feet bsg at the former UST area to a depth of 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 While a fine sand and silty sand was IAdvanced GeoEnvnronmental,Inc