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

Home Browse Search

Effects on Ground Water Quality of Aquifers • No significant water quality effects in The Trout Creek or Twentymile Sandstone or alluvial units are anticipated to occur during active mining operations from discharges from the mine, as ground water flows will tend to be from these units to the workings. Mine inflows will be collected in sump areas within the mine and pumped to the surface, where they will be treated if necessary, and discharged to surface streams. The effects of this discharge on surface water flows and quality are discussed under Probable Hydrologic Consequences for Surface Water Systems. After mining operations have been completed and the mine workings are allowed to flood, there is the potential for water quality effects on units that will receive water that has been in communication with the workings or related zones disturbed by subsidence. The increased surface effect increases the potential for leaching by ground water in the flooded workings. This will lead to increased solute concentrations and changes in water quality characteristics compared with premining conditions. The Trout Creek Sandstone aquifer water quality is unlikely to be affected by the operations, as the potential for flow, under both premining and postmining conditions, tends to be toward the proposed mining zone. The Twentymile Sandstone, as previously indicated, is effectively isolated from the proposed mine zone by 700 feet of marine shale. Mining - related subsidence is not expected to generate appreciable permanent fracturing over about 350 feet above the mine zone. Consequently, the potential for movement of significant quantities of ground water from the mine workings to the Twentymile Sandstone is limited to major natural fault zones, which may provide sufficient communication. Fault zones penetrated to -date, however, have not yielded any significant long -term inflows from the Trout Creek Sandstone or Twentymile Sandstone. Fault zones, which have the potential to provide this communication through the intervening marine shale, have not been identified within the proposed mining area. No movement of this type can occur until the potentiometric levels in the mine workings recover to a level higher than that of the overlying Twentymile Sandstone. It has been calculated in an earlier section that this may take about 310 years. The Wadge overburden unit is the only bedrock unit which is likely to be impacted with respect to water quality effects. Subsidence effects will probably result in rubblization of most of the existing unit above mined areas. This zone is up to 100 feet thick. Above the rubblized zone there will probably be considerable subsidence - induced fracturing in the overlying marine shale. The thickness of this fractured zone is likely to be on the order of 200 feet. The postmining water quality that will result from flooding of the mine workings and associated overlying rubblized and fractured zones may undergo two phases of development. Initially, ground water filling the mine workings will be under oxidized conditions and the quality of water that results from leaching the overburden material and remaining coal zones will reflect these conditions. After the rubblized zone is filled completely, continued solute leaching will occur under reduced conditions. Due to the structure of the basin, flooding of the workings will tend to occur first in the lowest sections of the proposed mine near the center of the basin and spread out to the margins, even though most of the postmining recharge will occur at the margins. It can be seen that oxidized and reduced conditions will exist in different areas of the mine at the same time. The pool of water at the Fish Creek Borehole built -up, un- pumped for approximately two years. When pumping restarted in August 1998, the conductivity had risen from approximately 4,000 µmhos /cm, to 7,500 µmhos /cm (Exhibit 49 Table E49 -5). This appears to be due to leaching from the flooded gob. Active mining areas that are pumped regularly have never developed conductivities of greater than approximately 4,500 µmhos /cm. In areas where caving of the overburden will occur soon after mining, solute leaching under oxidizing conditions during mine flooding is expected to yield water quality similar to that seen in backfilled areas of the • adjacent surface mines. The primary source of inflow to the underground workings to date (June 1999) appears to be the spoils deposited up -dip in the reclaimed surface mine pits. These spoils parallel most of the southeast side of the underground mine, and extend up -dip from it for over 6,000 feet.. The water quality of the main inflows to the mine resembles that of the spoil water. Mine inflow studies have shown the seeps generally have a conductivity of 3000 µmhos /cm and dominated by calcium, magnesium, and sulfate ions. PR09 -08 2.05 -142 04/27/09