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31 March 2006 <br /> AGE-NC Project No. 98-0471 <br /> Page 9 of 13 <br /> The concentration of TPH-d and benzene in well MW-3 have declined below detection limits since <br /> the initiation of ground water monitoring in June 2000; however, the concentration of MTBE <br /> increased slightly in samples collected in May 2005. <br /> Trend analyses for TPH-g, TPH-d, MTBE, and BTEX compounds in wells MW-1 through MW-4 <br /> show exponentially declining concentrations since initiation of ground water monitoring <br /> (Appendix M). <br /> 8.0. RISC WORKBENCH ANALYSIS <br /> Analytes detected in soil include TPH-g, TPH-d, TPH-mo, BTEX, and MTBE. Over-excavation <br /> remediation efforts have removed shallow subsurface (as deep as 11 feet bsg) soil contamination <br /> from the former 1,000-gallon AST and dispenser area (area of concern). The remnant zone of <br /> contaminated soil is generally located between 1 foot and 16 feet bsg around the periphery of, and <br /> below, the excavation. Based on the concentrations of petroleum hydrocarbon contamination <br /> detected in soil samples collected at 15 feet bsg in the area of concern, AGE estimates that soil <br /> contamination is unlikely to extend below 16 feet bsg. Localized areas of high contaminant <br /> concentrations tend to be found between surface grade and 4 feet bsg,in an interval of silty peat near <br /> the southwestern edges of the excavation. MTBE is the only analyte detected in ground water, <br /> limited to the immediate vicinity of monitoring well MW-3. MTBE is listed under California and <br /> U.S. Environmental Protection Agency(EPA) standards as a chemical with potential carcinogenic <br /> and hazard risks. <br /> AGE utilized RISC Workbench 4 (RISC), a commercial risk assessment and fate and transport <br /> software initially developed by British Petroleum Company in 1994,to determine potential impacts <br /> to any shallow down-gradient well located 130 feet from the source area(equivalent to the distance <br /> that the up-gradient, on-site domestic well is located from the source area). The assessment was <br /> conducted for benzene,toluene,ethylbenzene,xylenes,and MTBE in soil and for MTBE in ground <br /> water. <br /> 8.1. FATE AND TRANSPORT <br /> Utilizing RISC,AGE conducted a 100-year fate and transport study to analyze the potential impact <br /> of contaminated soil and ground water to any shallow down-gradient well located within 130 feet <br /> of the source area.Since the potentiometric surface of first-encountered ground water only fluctuates <br /> from an average high of 4.09 feet bsg between April and June to an average low of 4.89 feet bsg <br /> between September and December, AGE analyzed the fate of contaminant concentrations in soil <br /> beneath the former AST location utilizing a Saturated Soil To Ground Water contaminant transport <br /> Advanced Geo Environmental,Inc <br />