2012-12-06_REVISION - C1981008 (4)

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been identified. All monitoring wells installed by the permittee and domestic wells within the surrounding area are shown on Map 2.04.7 -1 -A. No wells are within the release area. Of the 29 water rights within the surrounding are, 27 wells have intakes too deep to be affected by the pit pumping induced draw downs in the overburden aquifer. These wells are installed in the Burro Canyon Formation, which is below the Dakota coals to be mined. Significant shales separate the coals from the strata of the wells. Two righted wells W -009 and W041 have intakes close to the location of the bottom of the mine pit. The wells area located approximately 2,300 feet east of New Horizon #2. At the eastern boundary of New Horizon #2, mining extended only 10 feet below the water table. Therefore, drawdown impacts to these wells are expected to be small. The wells are righted for irrigation water use. This area has been mined and reclaimed over 10 years ago and there has not been any problem identified with these wells. 5.2 Impact of Spoil Material on Ground Water Flow and Recharge The Phase III release areas have been fully reclaimed for approximately 10 years. The potential long term impact on ground water flow and recharge is the change in permeability of the spoil material placed as backfill. The only vertical and horizontal permeability change that will have occurred in the backfill will be an increase in permeability due to the large amount of voids created in the spoil compared to the original sedimentary strata. The hydraulic conductivity (K) of the spoil, as measured in well GW -N27, 40 ft /day, which is far greater that any measurement made in the overburden. K values in the overburden generally range from 3.0 to 5.5 ft /day. GW -N27 is located in the old New Horizon #1 Mine northwest of the release area. Based on rainfall data for the site, soil information, and irrigation practices, recharge for the release area has been determined to be 2" from rain and snowmelt, 13" from irrigation, and 1" from underburden recharge and overburden recharge at the uphill spoil /overburden contact. Total long -term recharge is therefore 16" per year. These predications are approximate, but are accurate enough in predicting the behavior of the groundwater in the spoil. This recharge water will flow readily through the permeable spoil and percolate downward to the bottom of the original pit, where a shale layer will prevent significant further movement downward. Map R -2 shows the elevation of the bottom of the pit and the pit extent over the life of the mine. The elevations were determined from coal drill holes in the mine areas and then modeled in Carlson and AutoCAD. As is seen from the Map, spoil water should build up in the corner of the mined out area near wells GW- N16 -18 until the level reaches an approximate elevation of 5608', after which it will then flow to the west along the southern wall of the original strata to the southwest corner of the current pit. Some spoil water should be entering the strata at the south end of the pit as well, albeit at a lower rate due to the lower permeability. SL -14 03DEC12.wpd 55