Laserfiche WebLink
Some depletion of the stream/alluvial aquifer system could occur through the <br />coal seams themselves which subcrop beneath the Williams Creek and Ward Creek <br />valleys. Ground water movement in the permit area is updip, discharging to <br />the stream/alluvial aquifer system. Mining will dewater adjacent portions of <br />the coal seam aquifer and will intercept some of the ground water currently <br />discharging to the stream/alluvial aquifer system. In addition, the presence <br />of the underground workings beneath the valley could reverse the pre-existing <br />ground water flow direction between the valley and the mine workings, and <br />water from the stream/alluvial aquifer system could enter the mine workings <br />through the coal seams. This reversal of ground water flow direction would <br />occur only in areas where the coal seams are fractured. <br />The applicant has provided a prediction of inflow to the mine for the D and E <br />coal seams on a yearly basis. The maximum predicted inflow, 78.67 gpm (0.17 <br />cfs) in 1983, provides an estimate of worst case streamflow depletion. Upon <br />cessation of mining, mine inflows will not be pumped out of the mine and will <br />fill the workings. Mine inflows potentially represent stream depletion, since <br />inflow water, in the undisturbed condition, would eventually discharge to the <br />surficial aquifer via the subcrop and potentially enter Ward Creek. The <br />effect will be temporary, since once the workings become flooded, discharge <br />will once again be to the surficiai aquifer via the subcrop. <br />Since some of the ground water inflow comes from other sources, such as <br />depletion of ground water storage, the actual stream flow depletion would <br />probably be less than predicted. The applicant's modeling estimates that two <br />years after ground water removal begins, the total volume of water intercepted <br />by the mine would be taken directly from the stream. The addendum reports <br />that the minimum streamflow observed in Ward Creek by the ditch rider was 2.5 <br />cfs in 1977. Therefore, the total mine inflow (and worst case streamflow <br />depletion) represents only 7 percent of the lowest streamflow observed in Ward <br />Creek. The predicted mine inflow is only about 2 percent of the average flow <br />in Ward Creek. <br />During operations, most of the mine water is returned to the Ward Creek <br />drainage system. Mine water is pumped to a sediment pond, where water not <br />lost to evaporation is discharged to the Ward Creek system via Carbon ditch. <br />Mine water is expected to have a degraded water quality (see example analysis, <br />Figure 2.04.7(2)(b)-2, Sample 2> and would be expected to affect water quality <br />in Ward Creek. The average water quality observed in the sediment pond has a <br />total dissolved solids concentration of 1617 mg/1 and an SAR of 18.96. When <br />water of this quality 1s added at the rate of 0.17 cfs to irrigation flows in <br />Carbon Ditch, the resulting flow has a TDS of 340 mg/1 and an SAR of 2.79. <br />Such water is suitable for irrigation with a medium salinity hazard and a low <br />sodium hazard. Similar calculations for Ward Creek at low flow result in a <br />predicated TDS of 303 mg/1, and an SAR of 0.85. This water is also suitable <br />for irrigation, with a medium salinity hazard and a low sodium hazard. Both <br />the Carbon Ditch and Ward Creek currently have waters with low sodium hazard <br />and low salinity hazards. <br />-31- <br />