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• 31 August 2005 AGE-NC Project No 96-0232 Page 22 of 30 7.0. RISK ASSESSMENT ANALYSIS MTBE in soil and ground water is identified as the chemical of concern(COC)at the site, with the potential to impact a down-gradient municipal well as well as indoor air (Figure 18) Although TPH-g, TPH-d, BTEX compounds and TBA have been identified at the site, they were detected at low concentrations only in one soil boring sample(P31-10) and two soil samples from fuel piping locations, and only sporadic occurrences of TPH-d and xylenes, at low concentrations, have been detected in site ground water since 2003 Based on the data collected to date from soil borings and ground water monitoring wells, AGE has developed a site conceptual model (Figure 19) for the release,migration and distribution of MTBE in the subsurface Release(s)of MTBE occurred in the area of the USTs and/or fuel dispensers Once released,the MTBE migrated vertically into the surrounding soil and shallow ground water MTBE- impacted ground water subsequently migrated westward towards to the area of the former Rocky's Restaurant domestic water well, at which point the plume appeared to converge with an MTBE ground water plume from the Three B's Truck Plaza site • After interim remediation via ground water extraction was initiated in November 2000 and abandonment of the Rocky's Restaurant well in March 2002, ground water flow was generally towards the east and southeast except for the water-table or First Water Zone interval,where ground water flowed inward towards a ground-water depression created by remedial ground-water extraction As a result of off-site First Zone ground water flowing towards the Flag City Chevron ground water depression,a third,distinct MTBE ground water plume(New West Petroleum ground water plume) was drawn off-site from the north-northeast and onto the Flag City Chevron site in First Water and Second Water Zones (Figures 16 and 19) Since secession of ground-water extraction to 2004, site ground-water flow has been towards the southeast The remediation efforts via ground water extraction have removed approximately 13 33 lbs of MTBE from ground water since November 2000 (Table 9),presently, MTBE concentrations above MCL are detected only in site wells MW-2, MW-8A, and EW-7 through EW-10 (Table 7) MTBE is listed under California and US EPA as a chemical with potential carcinogenic and hazard risks AGE utilized RISC Workbench 4(RISC),a commercial risk assessment and fate and transport software initially developed by British Petroleum Company in 1994,to determine potential impacts to down-gradient municipal well CSA No 2, as well as the multi-level sentinel well MW-10, and to assess human health risk, in addition, clean-up levels for all identified exposure routes were evaluated for the more conservative residential exposure at the site The RISC analyses follows standard procedures outlined in the U S EPA's Risk Assessment Guidance for Superfund, dated 2001, to calculate exposure assessment, toxicity assessment, and risk characterization, and ASTM • E-1739 for nsk-based corrective action(RBCA) Mathematic equations used in Fate and Transport Advanced GeoEnvsronmental,Inc