REP27186 - Laserfiche WebLink

Home Browse Search

indicating that backfilled aeeas have become spoil aquifers in equilibrium with charge and dischazge cycles. As revegetation of reclaimed becomes more and more representative ofpre-mining vegetative cover, and as reclaimed surfaces are similar in topography to pre-mining conditions, surface runoff likely approaches pre-mining levels. The difference at this time is that surface runoff is collected and directed to sedimentation ponds, with point discharges. For most of the site, these point discharges are representative ofpre-mining drainage discharges, and the ponds have been permitted as permanent features. As the ponds aze generally full, inflow is equivalent to dischazge. Any infiltration of precipitation over and above pre-mining conditions is likely recovered in the spoil spring discharges. Disturbed areas of the Oak Creek drainage have been, for the most part, reclaimed, and sedimentation ponds have been removed. There is no post-mining condition that would deplete runoff from these areas from pre-mining conditions. Flows in Oak Creek aze not monitored. An impact to surface water quality was anticipated in Trout Creek. The amount was anticipated to vary significantly, and is greatly dependent upon flows (seasonal and irrigation calls) in Trout Creek as variations in spoil spring flows are more moderated. TDS values were expected to rise as high as 1,186 mg/1 at some time during the mining and postmining era at the Edna Mine (PAP, page 2.SA-7). However, an annual average of 196 mg/1 is anticipated on page 4.6-61(a} of the PAP. The operator is asked to review this section of the PAP. Sulfate concentrations were expected to reach an upper limit of approximately 842 mg/1 (PAP page 2.SB-3) in Trout Creek. Monitoring station TR-D is below mining activity on Trout Creek. TDS values reported For TR-D ranged from 170 to 950 mg/1 during the 2004 water year. Sulfate concentration ranged from 110 to 590 mg/1 at this surface water monitoring site. A summary statement that all values for TDS in Trout Creek are consistent with the probable hydrologic consequences projections is made on page 9 of the AHR. This review validates this statement, and also finds that sulfate concentrations were within the anticipated range. An impact to alluvial groundwater of Trout Creek was anticipated. TDS in alluvial materials was expected to rise to 2000 - 3000 mg/1(PAP page 2.5-97). Alluvial groundwater is monitored at three stations, TR-1.5 above mining, TR- 3 at the northern limit of mining (downstream), and TR-4 approximately a mile below mining. TDS values in alluvial monitoring well TR-1.5 ranged from 4210 to 4620 during the 2004 water year. High TDS values in TR-1.5 have been noted for a number of years, and they continue to rise. There is only speculation (in the PAP) for high and rising TDS values in TR-1.5. TDS values in TR-3 ranged from 430 to 860 mg/1, and in TR-4 from 460 to 960 mg/l. Impacts to alluvial groundwater of Trout Creek are within those predicted. No impact to alluvial groundwater in Oak Creek was anticipated (PAP page 2.5-95b), and no groundwater monitoring was conducted. No probable hydrologic consequence was predicted in the PAP for bedrock groundwater. Bedrock aquifers that abut spoil backfill probably are too impermeable to transmit any significant amount of leachate. Those bedrock units in close proximity to the floor of the pit aze exposed (or covered with a shallow mantle) below the pit and above Trout Creek. However, a monitoring station (an existing, privately-owned well) was established in the Trout Creek Sandstone (a unit some 150 feet or more below the pit floor) directly downdip of mining activity. Monitoring from 1995 indicates that TDS values have remained consistent in a range from 440 to 840 mg/1, with no appazent rising trend. Monitoring at this station was not (due to equipment problems - no power) accomplished during the 2004 water year (brought to the attention of the operator). CWQCCR 31 Basic standards and methodologies for surface water are defined in Colorado Water Quality Control Commission (CWQCC) Regulation (CWQCCR) 31. Surface runoff and spoil leachate from mined areas at the Edna mine Page 5