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The Foidel and Middle Creeks' alluvium is composed mostly of fine-grained sediments, silts, silty clays, and clays. <br />Only the area near the conFluence of Foidel and Middle Creeks, designated as AVF, contains more consistent <br />• gravel strata. The thickness and composition of the alluvial sediments was verified by the drilling of numerous <br />alluvial wells and excavation test pits within the study area. The alluvial sediments are saturated and the water <br />table, as measured in the existing monitoring wells along the Foidel Creek drainage area upstream and downstream <br />of the study area, Fluctuated during the annual seasons from 3 to 9 feet from the surface. <br />The geologic composition of the study area is shown on Figure 7, Cross-Section of Alluvial Valley Lower <br />Foidel/Middle Creeks Hay Meadows. This cross-section is located downstream of the proposed mining. <br />The longwall operation in the Eastem Mining District is 500 to 1,200 feet below the ground surface and the total <br />thickness of the coal removed is 8.5 to 9.5 feet. !n the Northem Mining District, the proposed longwall operations <br />will be approximately 700 - 1,600 feet below [he ground surface; the total thickness of the coal removed will be <br />8.0 to 8.5 feet. According to experience from the Appalachian and other coalfields, the extent of a zone with <br />increased permeability above the longwall panel will reach approximately 30 to 40 times the thickness of the <br />extracted coal seam. This conclusion is supported by information from the Foidel Creek Mine, where eleven <br />ground water monitoring points were installed in the area of the Alluvial Valley Floor to monitor effects of <br />subsidence. The monitoring wells and piezometers installed in the alluvium indicated no significant changes in <br />water level or water quality. However, one monitoring well installed in a shallow bedrock sandstone had to be <br />replaced after casing failure and a drop in water level. This was attributed to the casing being pinched off at the <br />interface between sandstone and shale after undermining. This well failure is attributed to near-surface cracks in <br />Subsidence Zones 4 and 5. <br />The geologic conditions of the Eastern and Northem Mining Districts are very favorable for limiting the impacts of <br />land subsidence on hydrology. The presence of 600 feet of marine shale above [he mined coal and the limited <br />extent and fine character of the unconsolidated sediments in the local valleys would reduce any potential <br />• subsidence impacts. The flow interruption in Foidel Creek was discussed above. The same appears to be the near <br />surface fracturing in Subsidence Zone 5 and it appears the additional pore space was filled with water which then <br />allowed the creek to return to "base' flow condition. Also, it is thought that some sealing of the cracks occurred <br />from infilling with fine grained sediments. <br />The mining in the Eastem Mining District will result in subsiding areas where springs exist. TCC's experience is <br />that there will be a temporary disruption in Flow and then recovery of the springs. The reader is referred to TCC's <br />1996 AHR for a further discussion of the recovery of springs SW I and SW2. No long-term impacts are anticipated <br />to the springs in this area. <br />The present interpretation of the impacts of subsidence on the water-bearing strata in the study area is supported by <br />numerous studies in various coalfields worldwide. <br />Subsidence Impacts on Ground Water Quality <br />The potential impacts of subsidence on ground water quality could develop in the following ways: <br />• Caved overburden materials replace part of the coal seam unit; and <br />• Fracturing develops a new hydraulic connection between units with different water quality. <br />The first potential impact is applicable only in cases where the coal seam acts as a unit, which is not the case in the <br />study area. The second example is more common in cases where several units with different water quality are <br />present. In the Foidel, Middle and Fish Creek alluvium areas, the potential interconnection could be, for example, <br />.between the Twentymile Sandstone and the alluvial unit. One apparent impact [o the Twen[ymile Sandstone from <br />subsidence is the drop in water level in monitoring well 006-82-48A. This drop in water level is attributed to a <br />PR 99-OS 2.05-149 AppR~vED JUN 2 R 2000 03/28/00 <br />