SITE INFORMATION AND CORRESPONDENCE

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Mr. Jim Barton • • 6421 W. Capitol Avenue Closure Consideration Page 2 of 3 March 2002 to present(last event March 2006), concentrations usually in the hundreds of parts per billion. Flow direction at MW-8A during active GWE is problematical as the well was the up-gradient monitoring point in the second zone and had no other up- gradient or cross-gradient monitoring points within approximately 270 degrees. Groundwater flow at MW-8A appeared to have been generally toward the southeast at a gradient of approximately 0.001 to 0.002,but there is also some uncertainty regarding the inferred gradient. Cross sections show several significant sand units with great lateral extent and few laterally continuous clay intervals. Silt intervals are more common, but the West Haz Mat CPT logs indicate that the silt units generally do not have elevated pore pressures,hence there does not appear to be significant barriers to vertical migration of contaminants in the area. EHD-produced cross-sections yield slightly different water zone intervals compared to the Flag City Chevron and New West conventions as follows: Flag City Chevron New West Shell E1ID Vadose 0 10 ft bsg Vadose: 0— 14 it bsg Vadose-variable 1'zone: 10 to 35 ft bsg A zone: 5 to 25 ft bsg 1' zone:5-30 ft bsg 2° zone:40 to 60 ft bsg B zone:40—50 ft bsg 2° zone:33—50 ft bsg Rocky's zone:70—75 ft bsg C zone:70—75 ft bsg Secondary aquifer: 90-110 ft Primary aquifer: 120-150 ft Intervals between sand zones are generally silt, locally clay,but give little indication of forming a laterally continuous barrier to vertical migration of impacted groundwater as shown on the West Haz Mat CPT logs. EHD did note some mention of positive and negative vertical gradients,which would imply some sort of barriers,but EHD has not reviewed this data. The nearly instantaneous cleanup of MW-2 following initiation of the GWE system and the rapid rebound in MW-2 following cessation of GWE suggest that impacted groundwater did not migrate very far past MW-2 prior to initiation of GWE; active extraction from the GWE system quickly cleaned up the area with inferred induced westward plume migration. With MW-2 on the margin of the GWE area, residual contamination didn't have far to go to `rebound'. The GWE wells have been sampled only once since the GWE operation ceased, providing minimal data to analyze any potential relationship to the rebound in MW-2.