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Gary A. Reents -2- 24 February 1989 <br /> 3. Hydrogeologic Interpretation <br /> A. One aquifer underlies the facility from water table to at least 450 feet <br /> BGS. <br /> B. Consists of discontinuous lenses of sand, gravelly sand, silty sand, <br /> clayey silt, silty clay, and clay. <br /> C. Logs indicate sandiest layer between 80 to 200 feet BGS. <br /> D. H&A divided the aquifer into four (4) hydrostratigraphic subunits based on <br /> lithologic and hydraulic properties. <br /> E. The four (4) hydrostratigraphic subunits were depicited on three (3) <br /> lithological and three (3) geophysical cross-sections. Two (2) cross- <br /> sections were oriented parallel to the plume, the remaining cross-section <br /> was oriented perpendicular to the plume. <br /> F. The uppermost subunit, the "shallow zone", extends form the water table to <br /> about 80 feet BGS. This submit consists primarily of electrically <br /> conductive sediments (silts and clays). <br /> G. Underlying the shallow zone is the "100-foot sand", occurring between <br /> approximately 80 to 120 feet BGS. The thickness of this unit varies from <br /> 5 feet at the south yard to 30 feet approximately 3500 feet downgradient. <br /> This unit consists of electrically resistive sediments (sands) . <br /> H. Underlying the 100-foot sand layer is the "intermediate zone", occurring <br /> between 110 to 150 feet BGS. This zone consists of highly resistive <br /> sediments (sand and gravel ) . Thickness varies from 30 to 50 feet. This <br /> unit and the overlying 100-foot sand layer are separated by approximately <br /> 60 feet of electrically conductive sediments (silts and clays) at the <br /> south yard. However, this fine-grained layer thins downgradient (to the <br /> south) and pinches out about 2500 feet downgradient. <br /> I. Underlying the intermediate zone is the "deep zone" , occurring between 170 <br /> to 200 feet BGS. This zone extends to at least 450 feet BGS and consists <br /> of alternating layers of resistive and conductive sediments (interbedded <br /> sands, silts and clays) . <br /> 4. Flow Patterns and Hydraulic Heads (December 1988) <br /> A. A ground water mound, within the shallow zone, has formed under the north <br /> yard. <br /> B. Shallow zone ground water migration is south to southwest with a hydraulic <br /> gradient of 0.0006 (contour map provided) . <br />