Laserfiche WebLink
Streamflow - Ground W ater Relationships <br />Underground mining operations may also affect streamflow conditions by affecting ground water discharge or <br />is by induced recharge. The two streams that could be affected are Foidel Creek and Fish Creek. Although <br />underground mining activities are below or downgradient from Foidel Creek, it is conceivable that underground <br />mining and reclamation activities could influence ground water discharge from surface mine spoils upgradient <br />(south) of Foidel Creek. Underground mining by increasing the effective permeability and storativity of the <br />Wadge overburden unit could increase the rate of recharge and thereby reduce ground water discharge to Foidel <br />Creek from upgradient mine spoils. This does not appear to have occurred to date. Springs from backfill spoils <br />have appeared since the onset of underground mining. The clay barrier constructed to cut off spoil water <br />inflows in the vicinity of the portal may have reduced the effective permeability of the Wadge overburden unit <br />to levels comparable or lower than premining. Still, some spoil water has by- passed the barrier as indicated in <br />the discussion on this issue in the ground water baseline section. <br />Drawdown in the Wadge overburden unit during mining could also result in induced recharge directly from <br />Foidel Creek. Induced recharge would be expected to show up with the onset of mining beneath the Foidel <br />Creek Valley, Stream depletions would be more than offset by mine discharge. After completion of mining, <br />induced recharge would decline back to premining conditions as potentiometric conditions equilibrate. If <br />induced recharge from Foidel Creek or its alluvium was to occur, a response should have been observed with the <br />onset of mining under the Foidel Creek Valley in 1984. Alluvial Wells S -5 and S -6, completed near the mining <br />operations beneath the Foidel Creek Valley, have shown no response because of initiation of mining beneath the <br />valley. Average inflow during the first year of mining beneath Foidel Creek was 83.2 gpm. Induced recharge <br />from Foidel Creek is a relatively small fraction of this total. <br />Underground mining could also affect streamflow conditions in Fish Creek by changing ground water discharge <br />to the stream or by induced recharge directly from the stream. The baseline hydrologic analysis indicates that <br />very little groundwater discharges to Fish Creek from the deeper unit units along the reach where the Wadge <br />overburden unit would be affected by underground mining. Thus, it does not appear that groundwater discharge <br />from these units to Fish Creek will be affected. Mining - related subsidence may result in minor shifts in <br />recharge /storage /discharge relationships between Fish Creek and the associated alluvium. Relative elevations <br />between alluvial deposits and base or high flow levels may change, and location and extent of streamside <br />alluvial deposits may be altered as minor changes in stream alignment along with accompanying erosion and <br />deposition occur. These changes will occur along a very short segment of Fish Creek. This is located in the <br />area above and immediately downstream of the 1 -Left pond. Both magnitude and significance of change will be <br />limited by the small area extent of alluvial deposits and the natural response of the dynamic stream system over <br />time. Induced recharge is not likely to occur because of the aquitards located between the Wadge Coal and Fish <br />Creek. <br />The placement of underground mine waste in the adjacent surface mine pit is an activity that has the potential to <br />affect surface water flows or water quality characteristics. The placement of wastes in the pit decreases the <br />storage capacity. While this may result in less attenuation of pit overflows, the overall water yield due to <br />displacement of fluids by solids is a temporary phenomena and relatively small when compared to surface <br />runoff contributions to the pit. About 5 ft' of water will be produced for every yard of waste placed in the pit, <br />provided the pit is at the overflow level. The estimated volume of waste rock disposed in the pit for the 32 -year <br />mine life is 231,000 CY. This would equate to a displacement of discharge of about 0.001 cfs. It is possible <br />that, as the pit is partially filled, pit water evaporation could decrease and, therefore, discharge may increase <br />slightly. Total salt load would remain unchanged, and any increase in flow would be associated with a <br />proportional decrease in dissolved solids. <br />The leaching characteristics of the waste rock indicate that it will produce lower concentrations of soluble salts <br />• and trace metals than the backfill in which it is being placed. Therefore, we do not anticipate any incremental <br />adverse water quality deterioration due to this activity, other than perhaps a slight shift in ionic composition of <br />water discharged. The ionic composition of leach water from waste rock differs slightly from the ionic <br />composition of backfill spoil water. The leach tests suggest higher levels of sodium and lower levels of calcium <br />in waste rock leachates than in backfill spoils. Thus, we could expect a slight shift in the chemical composition <br />PR09 -08 2.05 -159 04/27/09 <br />