2018-05-18_PERMIT FILE - C2009087A (3)

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RULE 2 - PERMITS Williams and Clark (1994) reported the discharge from spoils springs at their Spring Creek site (near the proposed portal area of the Sage Creek mine) to be approximately 1.7 million cubic feet, or 24 gpm, in 1988. However, measurements taken by Seneca mine personnel indicate a spoil spring discharge high of 583 gpm occurred in 1988 in SSSPG5. Spoil spring SSSPG5 discharges generally range between 25 and 200 gpm. Data since 1988 show a strong seasonality to the discharges from SSSPG5, SSSPG3, SSSPG4, SSSPG6, and SSSPGIO. Discharges from the majority of these spoil springs ranged between less than 1 to 100 gpm, with most of the measurements being below 50 gpm No clear correlation between water levels in the spoils aquifer and the listed spoil spring discharges is apparent in the data. A section of undisturbed bedrock between the southern portion of the spoils aquifer and the PSCM workings will remain in place, so groundwater flow from the spoils to the mine workings will be limited by the low hydraulic conductivity of the undisturbed bedrock. As shown by calculations above, the long -term inflow from the spoils aquifer to the mine workings is expected to be less than four gpm. Consequently, little if any diminishment of the overall outflow from the spoils aquifer is anticipated from the mining activities proposed for the PSCM. The underburden unit exhibits low hydraulic conductivities, and over much of the mine area provides a slight upward vertical gradient to the Wadge Coal. Estimated flow into the mine from the underburden unit where the vertical gradient is upward is about 6.4 gpm. Towards the southern portion of the mine area, there appears to be a downward hydraulic gradient from the Wadge Coal unit into the underburden unit, and contributions to mine inflows in this area will be minimal. There are no known water supply wells completed specifically in the underburden unit immediately below the Wadge Coal within or adjacent to the PSCM mine area, and the unit is not considered to • be a regional aquifer of significance similar to the Trout Creek Formation. As mentioned previously, the Trout Creek sandstone lies some 235 feet below the Wadge Coal and is hydraulically separated from the coal by significant layers of low permeability shales and the hydrologically - significant, regionally- extensive bed of altered volcanic ash (kaolinitic claystone, or "tonstein ") referred to as the Yampa Bed. The Trout Creek sandstone aquifer could be affected if significant hydrologic communication is established between the underground workings and the Trout Creek sandstone. This is not likely to occur except possibly in the vicinity of major fault zones, due to the existence of thick, intervening bedrock sequences of very low permeability as described in the general description of geology and hydrology (sections 2.04.5, 2.04.6 and 2.04.7). No such fault zones are known to exist in the area proposed to be mined. Therefore, the potential impacts of mining on the underburden unit immediately below the Wadge Coal and the deeper Trout Creek sandstone will be minimal, and the PSCM does not propose to continue monitoring of the underburden unit. Upon approval by the CDRMS, PSCM plans to abandon and reclaim all three underburden monitoring wells in accordance with plans contained in Section 2.05.4(g). Following completion of mining, mine openings including the portal area will be sealed, the surface facilities will be regraded, and the mine workings underground will be allowed to refill with groundwater. The potentiometric surface will recover to a condition similar to pre- mining conditions but affected by the higher permeability in the mine area due to the mine voids. The groundwater flux is not expected to vary in direction in the permit area as a whole; however, small - scale flow direction variations and velocities will exist in and around mine openings. During the time that the mine is refilling, groundwater flow in the mine vicinity will continue to be toward the • mine workings, as it was during the time the mine was being dewatered. The time required for refilling of the mine and recovery of the potentiometric surface can be calculated from the average areal recharge rate (0.31 in/yr as described in Section 2.04.7, which equals 0.0258 ft /yr) the volume of void space in the mine, the pore volume of the dewatered area and remaining coal in the mined PSCM Permit App. 2.05 -78 Revision 03/05/10