Laserfiche WebLink
Effects on Ground Water Quality of Aquifers <br /> • No significant water quality effects in The Trout Creek or Twentymile Sandstone or alluvial units are <br /> anticipated to occur during active mining operations from discharges from the mine, as ground water flows will <br /> tend to be from these units to the workings. Mine inflows will be collected in sump areas within the mine and <br /> pumped to the surface, where they will be treated if necessary, and discharged to surface streams. The effects of <br /> this discharge on surface water flows and quality are discussed under Probable Hydrologic Consequences for <br /> Surface Water Systems. <br /> After mining operations have been completed and the mine workings are allowed to flood, there is the potential <br /> for water quality effects on units that will receive water that has been in communication with the workings or <br /> related zones disturbed by subsidence. The increased surface effect increases the potential for leaching by <br /> ground water in the flooded workings. This will lead to increased solute concentrations and changes in water <br /> quality characteristics compared with premining conditions. <br /> The Trout Creek Sandstone aquifer water quality is unlikely to be affected by the operations, as the potential for <br /> flow, under both premining and postmining conditions, tends to be toward the proposed mining zone. The <br /> Twentymile Sandstone, as previously indicated, is effectively isolated from the proposed mine zone by 700 feet <br /> of marine shale. Mining - related subsidence is not expected to generate appreciable permanent fracturing over <br /> about 350 feet above the mine zone. Consequently, the potential for movement of significant quantities of <br /> ground water from the mine workings to the Twentymile Sandstone is limited to major natural fault zones, <br /> which may provide sufficient communication. Fault zones penetrated to -date, however, have not yielded any <br /> significant long -term inflows from the Trout Creek Sandstone or Twentymile Sandstone. Fault zones, which <br /> have the potential to provide this communication through the intervening marine shale, have not been identified <br /> within the proposed mining area. No movement of this type can occur until the potentiometric levels in the <br /> mine workings recover to a level higher than that of the overlying Twentymile Sandstone. It has been calculated <br /> in an earlier section that this may take about 310 years. <br /> The Wadge overburden unit is the only bedrock unit which is likely to be impacted with respect to water quality <br /> effects. Subsidence effects will probably result in rubblization of most of the existing unit above mined areas. <br /> This zone is up to 100 feet thick. Above the rubblized zone there will probably be considerable subsidence - <br /> induced fracturing in the overlying marine shale. The thickness of this fractured zone is likely to be on the order <br /> of 200 feet. <br /> The postmining water quality that will result from flooding of the mine workings and associated overlying <br /> rubblized and fractured zones may undergo two phases of development. Initially, ground water filling the mine <br /> workings will be under oxidized conditions and the quality of water that results from leaching the overburden <br /> material and remaining coal zones will reflect these conditions. After the rubblized zone is filled completely, <br /> continued solute leaching will occur under reduced conditions. Due to the structure of the basin, flooding of the <br /> workings will tend to occur first in the lowest sections of the proposed mine near the center of the basin and <br /> spread out to the margins, even though most of the postmining recharge will occur at the margins. It can be seen <br /> that oxidized and reduced conditions will exist in different areas of the mine at the same time. <br /> The pool of water at the Fish Creek Borehole built -up, un- pumped for approximately two years. When pumping <br /> restarted in August 1998, the conductivity had risen from approximately 4,000 µmhos /cm, to 7,500 µmhos /cm <br /> (Exhibit 49 Table E49 -5). This appears to be due to leaching from the flooded gob. Active mining areas that <br /> are pumped regularly have never developed conductivities of greater than approximately 4,500 µmhos /cm. <br /> In areas where caving of the overburden will occur soon after mining, solute leaching under oxidizing <br /> conditions during mine flooding is expected to yield water quality similar to that seen in backfilled areas of the <br /> • adjacent surface mines. The primary source of inflow to the underground workings to date (June 1999) appears <br /> to be the spoils deposited up -dip in the reclaimed surface mine pits. These spoils parallel most of the southeast <br /> side of the underground mine, and extend up -dip from it for over 6,000 feet.. The water quality of the main <br /> inflows to the mine resembles that of the spoil water. Mine inflow studies have shown the seeps generally have <br /> a conductivity of 3000 µmhos /cm and dominated by calcium, magnesium, and sulfate ions. <br /> PRO9 -08 2.05 -142 04/27/09 <br />