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CONESTOGA-ROVERS &ASSOCIATES April 5,2011 -3- Reference No.060223 events,TPHc concentrations in well MW-5B were 740µg/L after the first event,and less than the TPHc method reporting limit of 250 µg/L after the second event. Similar results were observed between boring B-66 and well MW-5C where up to 74,000 µg/L TPHc was detected at 171.5 ft bgs in the depth discrete groundwater sample collected from B-66, and no TPHc has been detected in well MW-5C at the same depth. As a result,the depth discrete groundwater analytical data do not agree with the groundwater analytical data collected from wells screened at the same depths. This further confirms that cross contamination during drilling and sampling operations is likely the reason for the elevated TPHc detections in the depth discrete samples. Based on the above discussion,we believe collecting groundwater samples from a properly constructed monitoring well will provide more reliable (and repeatable) data than collecting depth discrete samples during drilling activities. Installing wells will also allow for evaluating groundwater flow direction and gradient in the deeper water-bearing zones beneath the site. This approach is also consistent with comment number 2 in CVWB's 5 November 2010 letter,which states: "The laboratory analytical result of ND for the groundwater sample colleted from MW-SC preliminarily indicates that the vertical extent of the TPH release from the pipelines in the vicinity of Trench 3 has been delineated. However, the groundwater data obtained from B-77 and B-78 in October 2008 shows that the lateral extent of the pipeline release in the Trench 3 area has not been delineated. Figure 3b in the Work Plan shows that TPHc has been detected at 570µg/L about 250 east of Trench 3 at a depth of 146 feet bgs. This pollution could have migrated with the flow of deep zone groundwater because no contaminants of concern were detected above 100 feet bgs in borings B-77 and B-78. The flow direction of groundwater in aquifers underlying the shallow zone has not been determined. Additional investigation and monitoring wells are needed in this area to delineate the lateral extent of COCs below the shallow zone." The proposed deep well cluster in the vicinity of borings B-77 and B-78 may not define the plume's extent in the northeasterly direction as CVWB staff suggest. Therefore,the well cluster will be moved approximately 80 feet further northeast of boring B-78,which should adequately define the plume's extent in this direction(Figure 1). The screened intervals of the two deep wells will be constructed to coincide with the water-bearing zones monitored by wells MW-5B and MW-5C. Worldwide Engineering, Environmental, Construction, and IT Services