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All of the designated mining areas (SMD, EMD, AIMD, and WMD) are located within the Twentymile Pazk <br />Basin which is formed by the locally significant Twentymile Park' Syncline. This syncline and associated <br />anticlinal structures form an enclosed ground water basin, which is rimmed by outcrops of the major water- <br />. bearing units described in Section 2.04.6, Geology Description. The limits of potential mining-related impacts <br />on the ground water hydrologic system aze shown on Map 4, Proposed Hydrologic Area Boundaries. <br />BEDROCK AOUIF'ER SYSTEM <br />' ' ,. <br />Twentvmile Sandstone Aquifer i <br />Aquifer characteristics of the Twentymile Sandstone have been measured within the permit area in six wells, <br />and the results for transmissivity are reported in a range of 0.25 to 32 ftZ/day (Applied Hydrology Associates, <br />1984). Water levels of the Twentymile Sandstone have been rrionitored in the mine plan azea in Wells 006- <br />BRDH-7, 006-82-48A, FBR-11-B, FBR-12-A, FBR-9 & 10, and FBR-2. Recharge, discharge, ground water <br />movement, and water quality in the Twentymile Sandstone are discussed later in this section. The <br />potentiometric surface map for the Twentymile Sandstone is presented as Figure 6R. <br />Trout Creek Sandstone Aquifer <br />Recharge to the Trout Creek Sandstone occurs along the unit outcrop, as shown on the Twentymile Sandstone <br />Potentiometric Surface Map (Figure 6R). There is very limited hydrologic connection between the Trout Creek <br />Sandstone and the Wadge Coal Seam, which could effect the proposed mining operations. Results of <br />monitoring in the current mining area indicated that vertical hydrologic communication between major aquifers <br />along the fault zones is very limited or non-existent. <br />Wadge Overburdeu Aquifer <br />. Field data from ground water monitoring wells was used to construct a potentiometric contour map for the <br />Wadge Overburden aquifer in the permit area. This was developed by using water level data from 1983, prior to <br />the start of underground mining in the eastern portion of the study, area, and recent data (1993) from the western <br />sector, as presented by Figure 6S. This figure was updated following completion of the installation and <br />monitoring of additional monitoring wells. The potentiometric map for the Wadge Overburden aquifer was <br />constructed using only water level data, which were deemed to be,reliable. Data from the new wells was used to <br />check and update the referenced figures. Additionally, TDS contours have been added to the figures; which <br />have been moved to Exhibit 11D. Figure 6S has been retained in the PAP to provide historic perspective. <br />Exhibit 1 ID indicates a TDS plume extending into the Eastern Mining District. The highest TDS concentration <br />is to the southwest, in the vicinity of the adjacent surface mine, and diminishes to the northeast. <br />• <br />PR06-07 2.04-41 1 I /07/06 <br />