|
Modeled mean water qualities of in- stream flows in Fish Creek (Exhibit 49, Tables E49 -12 to E49 -14) indicate
<br />during the year with maximum discharge, the in- stream water quality will increase from a baseline of 588
<br />µmhos /cm to 916 µmhos /cm. Therefore, the alluvial water will still be suitable for irrigation, as it will be less
<br />than the material damage level of 1,500 µmhos /cm. The Fish Creek alluvium is not used for domestic purposes,
<br />sites are found in the 1998 Annual Hydrologic Report, Figures 49, 51, and 52.
<br />Modeled mean water qualities for Trout Creek in- stream flows (Exhibit 49, Tables E49 -12 to E49 -14) indicate
<br />that, at maximum discharge, in- stream water quality will increase from a maximum baseline level of 527
<br />µmhos /cm to 665 µmhos /cm. This is a small increase. In addition, the alluvial water will still be suitable for
<br />irrigation, as it will be less than the material damage level of 1,500 µmhos /cm. The maximum modeled sulfate
<br />concentration for the mean flow conditions is less than the drinking water standard of 250 mg/l.
<br />The wells in the Foidel Creek alluvium, downstream of Site 109 are already affected by the spoil spring
<br />discharges from CYCC's surface mine. The conductivities in Foidel Creek alluvial wells downstream of Site
<br />109 already exceed 100 µmhos /cm. Site 109 discharged from 1984 to 1996. During this period, the spoil
<br />springs were also discharging. Based on plots of conductivity in Foidel Creek alluvial wells, impacts from Site
<br />109 discharge were not detectable. In addition, the effects of the reduced 1996 discharge, and the elimination of
<br />discharge in 1997 and 1998, were not observed (see 1998 Annual Hydrologic Report, Figure 38, 41, 45, and 47.)
<br />Potential infiltration from the 6MN Mine Water Storage Reservoir will be minimized by a designed compacted
<br />clay liner with a permeability on the order of 2.0 x 10 -7 cm/in. Mine water quality is generally compatible with
<br />applicable effluent limits (refer to information for Fish Creek Borehole discharge), and any minor infiltration
<br />will mix with shallow groundwater, and be further diluted before it infiltrates downward or laterally. While
<br />there is some potential for water infiltrating from the Reservoir to reach the Fish Creek alluvium, given the
<br />considerations noted, any potential impacts would be negligible.
<br />• Subsidence Impacts on Ground Water
<br />Longwall mining of coal seams causes collapse, fracturing, bed separation, and bedding plane slip in the roof
<br />strata above the seam. All of these impacts on the overlying strata can result in changes to surface and ground
<br />water, if a major water resource is within reach of the disturbance. The height of the disturbed area depends on
<br />the thickness of the mined coal, geometry of the mined panel, the rate of the mining face advance, and on the
<br />geological characteristics of the overburden. According to Singh (1986), the area of disturbance above a
<br />longwall panel is generally divided into the following five zones, based upon the extent of the fracturing:
<br />ZONE 1: Zone of primary caving where the caved rock is completely disintegrated
<br />ZONE 2: Zone of bed separation, where separation occurs primarily along pre- existing bedding planes
<br />ZONE 3: Zone of vertical relaxation where, a slight increase of permeability is experienced
<br />ZONES 4 & 5: Zone of horizontal extension. Zone of tensile strain at the surface where shallow
<br />fractures develop. Zone of horizontal compression.
<br />According to international experience, the total thickness of the first and second zones, where the changes of
<br />permeability are substantial, typically reaches 3 to 3.5 times (Ropski and Lama, 1973), and rarely more than 10
<br />times the height of the extracted seam (Wardell, 1976). The height of the third zone, or the total height where
<br />changes in permeability due to subsidence can occur, is described by various authors in a range from 30 t to 60 t
<br />(where t is the fully extracted seam thickness), 58 t (Gviroman, 1977), 33.7 t (Williamson, 1978), and 30 t
<br />(Wardell, 1976).
<br />A series of studies performed in the Appalachian bituminous coal region confirmed the experience from
<br />overseas. A study of subsidence in the Dunkard Basin (Owili -Eger, 1982) concluded that water levels in units,
<br />• located within 330 feet above the mined coal, recovered after mining, and that there was no lasting deterioration
<br />of ground water quality. Another study of subsidence in Western Pennsylvania (Hill and Price, 1983) concluded
<br />that with an average thickness of the overburden of 550 feet, the shallow unit system was isolated from major
<br />impact caused by mining. Authors also observed that after longwall mining underneath wells, the subsidence
<br />slows and the strata settle and ground water levels rebound as flow paths to the mine become less direct. Stoner
<br />PR09 -08 2.05 -146 04/27/09
<br />
|