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Thrifty Oil Company <br /> 1230 N.Wilson Way <br /> page 2of3 <br /> exceeding 10,000 <br /> In any case, as an MTBE-Impacted site with concentrations <br /> Istaed through laboratory <br /> parts per billion in groundwater, vertical assessment demo <br /> les collected from more <br /> ''°sitbe strata, is required to <br /> analysis of groundwater samp <br /> demonstrate vertical extent of impacted groundwater on the <br /> The location of the proposed three CPT borings is agreeable to EHD, but target depths <br /> should be validated through a fourth CPT (installed first) in the source area to a <br /> recommended 120' bgs total depth. A grab groundwater sample should be collected <br /> from all permeable zones deeper than 70' bgs, or at predetermined depths only if no <br /> permeable intervals are identified. The target total depths for subsequent CPTs should <br /> then be set based on data from the source-area CPTJi <br /> rn Extent: Appears adequately addressed for this phase <br /> 2. Latera!Delineation —Western pp q <br /> of investigation. EHD has no additional recommendations st this time. <br /> 3, 4, & 5. Groundwater Concentration, Gradient Rate`s, Mass Removal. EHD's point for <br /> noting removal of 62,900+ pounds of material (TPHg)iand the occurrence of high MtBE <br /> concentrations in MW-5 is that, intuitively, a high mass of MtBE can be inferred to have <br /> impacted groundwater. Soil Vapor Extraction (SVE) will not significantly remediate <br /> groundwater already impacted by MtBE. The lack of`significant MtBE impact in other <br /> existing monitoring wells is not well explained by remediation by SVE. With high <br /> groundwater gradients on the site reported and high MtBE concentrations in MW-5, <br /> EHD is still concerned that a migration pathway has been missed, which makes a <br /> properly designed site characterization study, in this case the CPT investigation, critical <br /> to properly characterizing and managing the plume of impacted groundwater related to <br /> this site. <br /> a <br /> 6. Stagnation Point Calculation. EHD believes there'is some confusion between EHD <br /> and TOC regarding the terms `stagnation point', 'radius of influence' and `capture zone'. <br /> EHD inferred that TOC's use of the term `stagnation point' was implying capture zone, <br /> but apparently TOC was thinking more along the lines of radius of influence; the two <br /> concepts are quite distinct and could have a profound, influence on remediation system <br /> design. While a radius of influence of 223 feet indicates a measurable loss of head 223 <br /> feet from the pumping well, it does not indicate that impacted groundwater under <br /> dynamic conditions 223 feet from the pumping well will be captured. <br /> s+ <br /> Using data provided by TOC and a simple two-dimensional'capture zone analysis <br /> method described by Todd (1980, other methods are,available and acceptable), EHD <br /> calculates that the down-gradient extent of the capture zone for a single well pumping at <br /> 1.6 gallons per minute would range from 44.9 feet (ata gradient of .003) to 6.7 feet (at a <br /> gradient of 0.02). The cross-gradient and up-gradient capture zone extents would of <br /> course, be much greater. <br /> { <br />