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David Wang, P.E. , Chief <br /> Page 3 <br /> March 6, 1991 <br /> 0 Aquifers (water bearing units) were identified as: A; 30 feet, <br /> AB; 49 - 56 feet, B; 60 feet, C; 140 feet, CD 135 - 198 feet, <br /> D; 270 feet <br /> o water bearing units comprise approximately 26 percent of the <br /> sediments beneath DDRW Sharpe. These units consist primarily <br /> of cleaner sands. They represent the most likely pathway for <br /> ground water flow. <br /> 0 Due to the ratio of water bearing units to non-water bearing <br /> units, the estimate of potentially contaminated ground water <br /> is less than previously approximated. <br /> 0 The aquifer system can be described as a nonsteady-state, <br /> anisotropic, multiple, discontinuous, leaky artesian system. <br /> o Data obtained from aquifer tests indicates that some areas of <br /> DDRW Sharpe exhibit relatively high values of vertical <br /> permeability. <br /> o Seven distinct plumes of VOC contamination have been <br /> identified at DDRW Sharpe. <br /> o Vertical and horizontal migration of contaminants have been <br /> observed. Contamination is predominant in the upper three <br /> water-bearing zones. Downward migration is impeded by <br /> unfavorable hydraulic gradients. However, off-site pumpage of <br /> the ground water overcomes this impedance on a seasonal basis. <br /> 0 Ground water contamination has migrated off-site. <br /> o Fluctuations noticed in the contamination located in the North <br /> Balloon are probably due to the presence of a small highly <br /> concentrated slug of contamination moving back and forth under <br /> pumping influences. <br /> o Trichloroethene (TCE) concentrations in the South Balloon, <br /> generally near former disposal sites, have been observed to <br /> increase as result of rainfall infiltration. This suggests <br /> that sources may still be active. <br /> o VOC concentrations in the South Balloon are generally <br /> declining due to the operation of the area ground water <br /> treatment plant. <br /> 0 VOC concentrations downgradient and off-site for plume 4 and <br /> 5 are higher in the deeper aquifer zones. This is probably <br /> due to off-site pumping influences. <br />