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JAN-05-1996 08:39 P.05i07 <br /> Mr. Andrew Smith • -4- • <br /> from the current ASTs. The high concentrations of MTBE remain downgradient of the current <br /> ASTs as a result of a flatter groundwater gradient and much slower moving groundwater in the <br /> area downgradient from the current ASTs. <br /> Response: I respectfully disagree for several reasons. The velocity of the groundwater flow is <br /> based on groundwater gradient and hydraulic conductivity. To calculate groundwater gradient, <br /> all the monitoring wells used in the calculation must be screened in the same hydrogeologic <br /> units. The only two wells upgradient of the former USTs are MW-13 and MW-12 <br /> (replaced MW-1). MW-13 is screened entirely in the vadose zone unit while the wells <br /> downgradient of the current ASTs (MW-2, MW-3, and EW-1) are screened in Unit 1 (peat) and <br /> Unit 2 (silty sand). MW-12 is predominantly screened in the vadose zone with the bottom two <br /> feet of screen in Unit 1. It is my opinion that this well is predominantly reflecting the water level <br /> in the vadose zone unit because Unit I consists of peat with a reported hydraulic conductivity of <br /> 10-7 cm/sec. In addition, the close proximity of this well to the river suggests that it is reflecting <br /> the water level in the river rather than groundwater,making it inappropriate for use to estimate <br /> groundwater gradient and velocity in the vicinity of the former USTs, even if it was screened in <br /> the same hydrogeologic unit. Finally, the reported.hydraulic conductivity of 10-7 cm/sec for both <br /> Unit 1 (peat) and Unit 2 (silty sand) indicates that groundwater would move very slowly through <br /> these units regardless of the hydraulic gradient. <br /> AGE Comment No. 3. The highest concentration of MTBE detected in soil samples collected <br /> from the site was found in MW-6 at five feet and B 16 at one and four feet bgs. These samples <br /> are upgradient from the ASTs and downgradient from the USTs. The heavy concentration of <br /> MTBE in groundwater near the ASTs has migrated from the higher UST location. <br /> Response: As for groundwater, refer to the previous response. Whether the soil samples are <br /> downgradient or upgradient is not particularly relevant. Petroleum contamination released in soil <br /> will not migrate laterally in the unsaturated vadose zone. It will migrate vertically down to the <br /> water table and then flow with the groundwater. The three soil samples cited were collected at <br /> locations ranging from 20 to 35 feet away from the nearest former UST. Conversely,these soil <br /> samples were collected within five to eight feet from the current ASTs and the fuel lines running <br /> from the current ASTs to the fueling dock. <br /> AGE Comment No. 4. There is a large amount of hydrocarbon-contaminated soil in the levee at <br /> the UST area (soil samples collected from B13,B16, and MW-6). <br /> Response: Please see previous response regarding B 16 and MW-6. In addition, my review of <br /> the soil boring log for B 13 found that the soil samples did not contain MTBE, and the only <br /> significant contamination found was low levels of TPH-d. The corrective action at the site is <br /> being driven by MTBE in the groundwater at the site and not low levels of TPH-d in soil. <br /> AGE Comment No. 5. Concentrations of TPH and BTEX in grab groundwater samples collected <br /> from B15, B 16, and B 17 and soil samples collected from MW-6, B 15, and B1 6 indicate that <br /> there is not a vapor-only MTBE release at this site. <br /> CaMorms.Environmental Protection Agency <br /> Reeyded Pop,, <br />