2010-06-04_PERMIT FILE - C1982056A (10)

Home Browse Search

Ground Water Inflow to the Mine The underground mining activities at the Foidel Creek Mine have the potential to drain ground water from the mined coal seam, and overlying and immediately underlying rock units. Bedrock units overlying and underlying the Wadge Coal Seam, which contain ground water, would typically have formation pressures higher that the atmospheric pressure that would exist in any open mine workings. Consequently, if there is sufficient hydrologic communication, there will be movement of ground water from these units into the active mine - workings. The overburden unit will be directly affected by the removal of the Wadge Coal Seam. Good hydrologic communication with the unit may occur through natural fractures and faults, which are enhanced by stress - relief, and subsidence related to the mining activity. Partial dewatering of the immediately overlying overburden unit by gravity drainage is anticipated in active mine areas. Lateral movement of ground water in the overburden unit toward the active mine areas will cause some dewatering and more general lowering of potentiometric levels nearthe mine. Historic Inflows - The Foidel Creek Mine has been operated since 1983. Since that time, inflows and discharges have been monitored. In addition, future inflows have been estimated. Past inflow estimates have been made using theoretical models. At the time the most recent inflow estimates were made (using historic inflow rates to project future inflows), approximately 9,000 acres of area had been mined. The quantity of mine inflow and the resultant effects on potentiometric levels on the Wadge overburden unit were studied using a two - dimensional finite - difference, simulation model for the original Foidel Creek Mine permit application (Exhibit 13, Ground Water Model Documentation). The results and predictions of the model were used in the development of a Mine Water Control Plan included in Exhibit 33. Most of the estimated inflow was projected to occur as leakage from the Trout Creek Sandstone through faults intersecting the mine. This is based on the assumption that the faults would act as conduits and allow for flow of ground water into the mine. The mine has intercepted several faults, however, because the faults are typically in compression they are • relatively tight and inflows through the faults have been limited. Through June 2006, none of the faults identified in this report had yielded sustained inflows. In addition, no evidence has been found that there is any significant connection between the mine workings and either the underlying Trout Creek Sandstone or the overlying Twentymile Sandstone. Only two significant localized inflows were encountered; one in the 6 -Right Entry in late 1997, and one in the Northeast Mains fault crossing in August 1999. The discussion of the historic inflows are presented by mining area: 1) The North Mains and portal areas (NMS); 2) The 1 -South Panel plus the Southwest panels (Panels 1 -SW to 3 -SW) plus the South Panels (1 -Left to 6 -Left) which will be called the Southwest Mining District (SWMD); 3) The East Mining District (EMD) consisting of Panels 2Right to 9- Right; and 4) The North Mining District (NMD) consisting of Panels 12 -Right through 16- Right. Future mining areas include the remaining panels in the North Mining District (NMD) and the Western Mining District (WMD) containing the reserves to the north and west of the SWMD. Mining areas and designations are shown on the Mine Plan Map, Map 23. Inflows have historically been encountered in the NMS area. From 1993 to 2006, the inflows to the (NMS, water from overlying surface -mined spoils), as measured in the annual mine inflow surveys, remained constant at approximately 30 to 40 gpm. The conductivity of this water, which ranged from approximately 2,500 to 4,500 µmhos /cm, indicates that it is primarily derived from ground water that has been mixed with spoil water from the up -dip spoils. A typical spoil well analysis is shown in Exhibit 49, Table E49 -1. It has a mixed cation - sulfate/bicarbonate water with a conductivity of 3,610 µmhos /cm. In addition, most of this inflow has been observed from seeps that are within 2,000 feet of the portal. • No specific areas of high inflow occurred during development of the SWMD. Also, because the panels where sealed sequentially after mining, these areas could not be accessed for the more recent mine inflow surveys, so detailed information on mine inflows for this area could not be obtained. Estimates based on a composite of several prior inflow studies and dewatering rates indicate that flows on the order of approximately55 gpm PR09 -08 2.05 -135 04/22/09