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Getty Task 2 Page 6 March 1, 1983 <br />3.0 DATA HASE <br />Although much of the data presented in this section were presented is the <br />~ Task. 1 report, those data that were used is the groundwater model development <br />• are presented again in the interest of continuity. Additional data, <br />~ naavailable at the time the Task 1 report was prepared, are also provided. As <br />detailed in the Task 1 report, the aquifer of interest has been denoted as the <br />overburden aquifer. Therefore, all of the information presented is this <br />section pertains to this aquifer. <br />i 3.1 Geologic Data <br />i <br />As noted in the previous section, elevations of the upper and lower <br />confining layers of the aquifer are required by the model. The program uses <br />this information to calculate the saturated thickness of the aquifer. To <br />accomplish this, the piezometric head is compared to the elevation of the <br />• upper confining layer. If the head is greater than that elevation, the <br />aquifer is confined sad the saturated thickness is calculated by subtracting <br />the lower confining layer elevation from the upper confining layer elevation. <br />Otherwise, the aquifer is unconfined and the saturated thickness is calculated <br />by subtracting the lower confining layer elevation from the piezometric head <br />elevation. A similar test is utilized to determine whether confined aquifer <br />storage coefficient or apparent specific yield should be need in the <br />calculations. <br />' The elevation of the lower confining layer was obtained from a structure <br />- co atonr map of the Wadge coal seam. This map was overlain with the finite <br />difference grid sad average values for each grid were estimated. Because the <br />bott® of the aquifer is also the bott® of the coal seam to be mined, a <br />special mathematical problem arises. To simulate the removal of water from <br />' mined grids, the piezometric head in the mined grid is set equal to the <br />elevation of the bottom of the coal seam. If this elevation also represents <br />the lower confining layer elevation, a discontinuity occurs because the <br />saturated thickness of the mine grid is zero and flow cannot occur from other <br />' grids to the mine grid. To overcome this difficulty, tea feet were subtracted <br />from the coal bottom elevations for each grid. This procedure insures that <br />• grids that become mined during the inflow simulation maintain some flow <br />• capacity. <br />