2009-10-29_REVISION - C1982056 (4)

Home Browse Search

Monitoring results from the Fish Creek alluvial wells and associated surface sites (AVF-14, SW-14, 008-AU3, and 305) were consistent with surface observations and surveys, indicating no significant subsidence-related changes in flows or water quality. Subsidence Cracks The potential occurrence of subsidence cracks is another feature that could disrupt the surface and shallow ground water regime. The current experience from Pennsylvania and West Virginia, supported by other experience gained from mines in the Somerset, Paonia areas in Colorado, indicates that subsidence cracks may develop in areas of low overburden and on bedrock outcrops. The subsidence monitoring conducted at the Fish Creek AVF study area, immediately downstream of the RCR27 culvert in Fish Creek in the WMD, did not indicate any visible cracks in the Fish Creek channel. Subsidence cracks occurring across a stream channel could cause the disruption of the stream. However, because of the presence of thick, fine-grained sediments in the stream bed of the study area, the cracks will naturally seal quickly, and no significant impacts on flows are expected. The occurrence of the subsidence cracks on the surface may not necessarily result in direct inflow into the mine when a surface water body was disrupted. Studies by various authors such as Orchard (1969, 1975) in Great Britain, Williamson (1978) in Australia, and Hydro-Geo Consultants, Inc. in West Virginia (1991) and Ohio (1988), indicated that the subsidence cracks are limited in depth by the development of compressive stresses replacing the tensile stresses that cause surface fracturing. The depth of surface subsidence cracks due to tensile stress does not typically exceed 50 feet. Development of this type of crack would be limited in the unconsolidated sediments of the Foidel and Middle Creek alluviums, which are to 30 feet thick near the confluence of these two streams. The presence of up to 600 feet of marine shales between the ground surface and the mined coal will also limit the impacts of subsidence. TCC has done considerable subsidence monitoring work at the Fish Creek AVF study area, area immediately downstream of RCR27 culvert in Fish Creek in the WMD, to determine if subsidence would impact the creek and its Alluvial Valley Floor. One component of the work focused on making visual observations of Fish Creek to determine if any cracking was occurring in the creek channel. The channel included sections of both sediment and exposed bedrock. The visual observations, which included those made by representatives of the DMG, indicated that no cracking of the channel occurred during longwall mining. Subsidence Effects on Springs and Seeps Mining in the NMD and WMD will result in subsiding areas where springs and seeps exist. TCC's experience is that there will be a temporary disruption in flow, and then recovery of the springs. Refer to TCC's 1996 AHR for further discussion of the recovery of Springs SW1 and SW2. No springs or seeps fall within the projected subsidence limits for the WMD. There are identified springs and seeps to the west and south, however, the springs to the south lie upgradient of the SWMD mine workings and would have been affected by this prior mining, if effects were to occur. Several of the springs to the west overlie units that are stratigraphically below the Trout Creek Sandstone. Prior geologic/hydrologic characterization and actual mining experience indicate no hydrologic connection between the Wadge Coal Seam and the underlying Trout Creek Sandstone, so the potential for mining and subsidence-related effects on these springs is negligible. In addition, extensive geologic characterization (drilling and seismic) in the WMD indicate that there are no faults within the planned mining area that could create a connection. Two springs to the west of the WMD overlie units of the Holderness Member. Worst case drawdown calculations for these springs indicate minimal potential for drawdown due to mining, as documented by the discussion of drawdown calculations and summary table presented in Exhibit 25Z. Available information and analyses indicate that the potential for impacts to springs and seeps due to mining and subsidence is minimal (worst-case projections) to negligible. GENERAL SUMMARY AND CONCLUSION The described streamflow and water quality impacts for expansion of underground mining into the EMD at the Foidel Creek Mine are within the limits of impacts discussed by Kaman Tempo (1982) and by Parker and Norris (1983) relative to the study area. It is apparent that such hydrologic impacts are relatively greater in stream reaches near mine discharges. Mine discharges may be treated, as necessary, to reduce levels of total suspended solids, and possibly to settle out oxidized trace metals prior to discharge to a stream. For the most part, dissolved solids and major ionic solute concentrations associated with mine flows will be unaffected by PR09-08 2.05-164 10/29/09