2014-05-12_PERMIT FILE - C2010089 (5)

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A) In the pre -mine condition, a large portion of the irrigation water runs off the surface and picks up some TDS in the fields and is gathered in return ditches. In the post -mine condition, a large portion of the irrigation water will infiltrate the backfill due to the increase in porosity. The TDS and pH of the backfill water at the New Horizon North Mine is expected to approximate that of the overburden water quality, resulting in minimal impact to the ground water quality. Water movement through the spoil will be considerably faster than the movement in the overburden. B) Spoil springs will be present at the SS #1 location and will discharge the majority of the backfill water to the surface. This is an impact since it will decrease the recharge to the ground water zones and increase flow at the existing spring; however, it will also quickly return water to the San Miguel River by way of Tuttle Draw. The quality of water in Tuttle Draw is not suitable for drinking water or irrigation water but is suitable for livestock and the spring water flow will not change that. Q Once the pyrite is oxidized and easily dissolved salts are washed out of the spoil, the water in the spoil springs will gradually get lower in TDS until some time in the distant future, the spoil spring water quality will get better than the overburden quality. As dissolution of the backfill continues, the backfill water will approach the irrigation water quality. Time periods for this to occur are given later in this section. Spoil Water Infiltration Into Low -wall Figure 2.05.6(3) -1 shows how infiltrating water may build up in the spoil north of the Meehan Draw "protection pillar" ( Meehan Draw will not be disturbed by mining) and begin to seep into the low wall. It should be noted that the mining area north of Meehan Draw will be reclaimed as "dry land pasture" and will not be irrigated during reclamation. Any accumulated spoil water may enter one or more of the minor sandstone beds of the overburden (Dakota Sandstone) in the low wall. This annual infiltration is calculated below assuming a 10' average low wall height the low wall strata, a hydraulic conductivity of 2.1 (from GW -N9) and a hydraulic gradient of 0.053 (based on the water level gradients along the old Peabody Highwall).. Seepage into Low Wall is equal to: (10' ft. average low wall height) (1900' ft. wide seepage area)(hydraulic conductivity of 2.1 ft. /day)(hydraulic gradient of 0.053) (1/43,560 cu. ft. per ac- ft.)(365 days per year) is equal to 17.7 ac -ft per year, or an average of about 11 gpm. It is strongly believed that spoil water seepage into the low wall will not have any significant impact on water quality, flow rates, well usage etc. due to four reasons: 1) As described in this section, the water quality of the spoil water will be at a maximum 6% to 10% higher in TDS than the existing overburden water quality, which is relatively poor. Therefore, regardless of the seepage rates into the sandstone zones, the water quality will be very similar and consistent with the overburden, with a TDS from 2800 to 3500 ppm. 2) Due to the low seepage rates, the movement is very slow (400 feet every 10 years). ection 2.05.6(3) Page 23 November 2011