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Mr. Andrew Smith - 2 - 46 16 April 2002 <br /> at the site are likely due to the higher percentage of MTBE added to gasoline since late 1991 as <br /> opposed to the low percentage required when the USTs were active prior to October 1991. <br /> 3. The figures provided in the Response to Comments show water level elevations versus MTBE <br /> concentrations. H&H Marina presents these figures to show a"core of MTBE impacted <br /> groundwater"moving through the groundwater. Board staff believes that the concentrations of <br /> MTBE correlate to the groundwater elevations as opposed to MTBE being "flushed" from the <br /> UST area to below the AGT area, as proposed in the SCM. In addition, monitoring well MW-2, <br /> which is downgradient of the USTs, does not show the core of gasoline leading or trailing from <br /> the well. <br /> 4. The Response to Comments asks where the benzene is, if indeed, the pollution is from the AGTs. <br /> Based on the peat soils at the site, hydrocarbon adsorption by the soils may account for the <br /> reduced benzene levels, whereas MTBE does not readily adsorb to soils. Furthermore, benzene <br /> accounts for less than 10 percent of gasoline, and based on Table 2 of the Report, benzene is <br /> detected at about 10 percent of the gasoline concentrations. <br /> Therefore, Board staff will continue to oversee this site as an AGT site. The work plan, as requested in <br /> the Letter, is still due by 15 May 2002. If you have any questions, you may contact me at <br /> (916) 255-3119 or by email at lewisd(a,rb5s.swrcb.ca.gov. <br /> �jju� 4E lJ l 12 <br /> DEVRA LEWIS <br /> Environmental Scientist <br /> cc: Ms. Margaret Lagorio, San Joaquin County Environmental Health Department, Stockton <br /> Mr. Brian Millman, Advanced GeoEnvironmental, Inc., Stockton <br />