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6421 W. Capitol Ave. <br /> SCM/Closure Report Review <br /> May 2006 NCH <br /> Problem: Analyze and explain MTBE concentration trends in Chevron wells MW-2 and <br /> MW-8A. Main issue, do the concentration trends indicate a potential threat to the <br /> municipal wells(CSAI and CSA2)toward the southeast? <br /> Groundwater extraction system operated on the Flag City Chevron site from November <br /> 2000 to August 2004, and had intermittent(?) or partial operation from April 2000 to <br /> November 2000. During full operation of the GYVE system, groundwater elevation <br /> contours indicate a depression on the water table in the extraction area,presumably <br /> inducing radial inward flow from all directions. The inferred groundwater flow direction <br /> near MW-2 shifted from easterly to westerly during the GWE operation, and shifted <br /> toward the ESE following termination of active GWE. A cone of depression was mapped <br /> during operation of the GWE system. <br /> MW-8A, completed in the second water zone, was ND for MTBE from initial monitoring <br /> in May 2000 to December 2001, and has been MTBE-impacted from March 2002 to <br /> present(last event March 2006), concentrations usually in the hundreds. Flow direction at <br /> MW-8A during active GWE is problematical as the well was the up-gradient monitoring <br /> point in the second zone and had no other up-gradient or cross-gradient monitoring points <br /> within approximately 270 degrees. Groundwater flow at MW-8A appeared to have been <br /> generally toward the southeast at a gradient of approximately 0.001 to 0.002,but there is <br /> also some uncertainty regarding the inferred gradient. <br /> Cross sections show several significant sand units with great lateral extent and few <br /> laterally continuous clay intervals. Silt intervals are more common,but the West Haz Mat <br /> CPT logs indicate that the silt(clay)units generally do not have elevated pore pressures, <br /> hence there does not appear to be significant barriers to vertical migration of <br /> contaminants in the area. <br /> EHD-produced cross-sections yield slightly different water zone intervals compared to <br /> the Flag City Chevron and New West conventions as follows: <br /> Flag City Chevron New West Shell - EHD <br /> Vadose 0— 10 ft bsg Vadose:0— 14 ft bsg Vadose-variable <br /> 1"zone: 10 to 35 ft bsg A zone:5 to 25 ft bsg I"zone: 5-30 ft bsg <br /> 2'd zone:40 to 60 ft bsg B zone:40—50 ft bsg 2° zone:33—50 ft bsg <br /> Rocky's zone: 70—75 ft bsg C zone: 70—75 ft bsg <br /> Secondary aquifer:90-110 ft <br /> Primary aquifer: 120-150 ft <br />