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t y a In September 2010, AMEC Geomatrix (AMSC) submitted their Site Investigation and Air Sparging Pilot Test Report (Pilot Test Report) to the SJCEHD in which AMEC describes advancing four cone penetration test borings, installing one air-sparge(AS)well (AS-1) and three observation wells(OW-1, OW-2, OW-3) and performing an 8-hour AS pilot test. Soil samples collected during the installation of the four wells showed hydrocarbon-impacted soil within the saturated silty clay layer between 45 and 55 feet. AMEC also reported groundwater samples from the newly installed wells with concentrations of TPHg, benzene, ethylbenzene, total xylenes and EDB that were at least one order of magnitude greater than the previous historical high for the Site. The pilot test showed that AS would be a feasible remedial alternative to implement and that a soil vapor extraction (SVE) system would be necessary to capture the vapors stripped during the air sparging activities. The SJCEHD agreed with the y recommendations and directed the submittal of a RAW in their September 24, 2010, letter. SITE GEOLOGY AND HYDROGEOLOGY tmd The site geology was inferred from boring logs prepared during well installation activities at the Site and is generalized in the schematic cross-sections presented on Figures 3 and 4, Cross-Section A-A' and B-B', respectively. The surface of the Site is paved with 4 to 6 inches of asphalt concrete. Beneath the pavement, alluvial deposits consisting of interbedded silty clay, clayey silt, silty sand, sand and clayey sand were encountered to the total depth explored(approximately 80 feet). V Recent groundwater monitoring results show that the depth to first groundwater beneath the Site is typically between 42 and 50 feet below the top of the well casings (TOC). This water is unconfined ... and represents the piezometric surface beneath the Site. Historical groundwater flow beneath the Site is directed generally towards the east-southeast with average gradients of 0.001. A summary of the TOC elevations, groundwater depth measurements and mean sea level elevations is presented on Table 2. The flow direction and gradients from the most recent groundwater monitoring event are depicted on Figure 2. y REMEDIAL ACTION WORKPLAN The scope of services presented in this RAW has been developed to comply with the September 24, 2010, SJCEHD directives. It is proposed that seven new AS wells be installed and used in combination with previously installed well AS-1 as part of a full-scale AS system designed to address the groundwater and soil impacts remaining beneath the Site. Based upon recommendations made by AMEC in their Pilot Test Report, three additional SVE wells will be installed and used in `r combination with previously installed wells VW-1 and VW-2 as part of a full-scale SVE system designed to volatilize hydrocarbons in the vadose zone (VW-1 and VW-2) as wells,as to capture diffusive vapor volatilized into the vadose zone as a result of air sparging (three new SVE wells). The proposed remediation will include permitting and installing the AS and SVE wells, permitting and installing the AS/SVE system and performing compliance sampling and regular maintenance on the remediation system. Each of these activities is further described in the following sections. Pre-field Activities ..� • Prepare a site health and safety plan (HSP) as necessary to address this phase of the project. The HSP will provide guidelines on the use of personal protective equipment and the health and safety procedures to be implemented during the proposed field activities. • Mark each of the proposed boring and trenching locations with white paint and provide a minimum of 48-hour notice to the local public utilities via Underground Service Alert prior to r Stockton Maintenance Station,Task Order No.05 Caltrans Contract No.06A1141,EA 10-ON8300 Project No.S9200-06-05A -3 - December 28,2010 V.r