2013-10-29_GENERAL DOCUMENTS - C1981008

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infiltrating precipitation water and irrigation water, the pyrite breaks down at a faster rate but is buffered by the calcite, as described above. Therefore, the overall impacts to water quality are the following: i) 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 and recharge the spoil due to the increase in porosity. The TDS of the spoil water at the New Horizon #1 Mine will increase approximately 6% compared to that of the overburden water immediately after re- establishment of irrigation. This is approximately 3300 ppm of TDS. Since the flowpath through the New Horizon 92 spoil is greater than the New Horizon 41 spoil, the expected increase in TDS for this water is 10% over background TDS in the overburden water. This is approximately 3425 ppm of TDS. A potential maximum is 4000 ppm TDS. Water movement through the spoil will also be considerably faster than the movement in the overburden. ii) Spoil springs will be present at both reclaimed mine areas which discharge the majority of the spoil water to the surface. This is an impact since no spoil springs existed in the pre -mine condition. iii) Once the pyrite and easily dissolved salts are washed out of the spoil, the water in both spoil springs will gradually get lower in TDS until sometime many years from now, the spoil spring water quality will get better than the overburden quality. At a time much more distant in the future and much more difficult to calculate, the spoil water will approach the irrigation water quality. Time periods for this to occur are given later in this section. As can be seen from the table, the TDS of the NPDES 001 discharge at the New Horizon 91 Mine fluctuates inversely in response to flows during irrigation season. The overburden wells and Spoil Spring 41 fluctuate to a lesser extent. The August 1996 sample in the spoil spring appears to be an aberration and not a trend. Spoil Water Infiltration into Lowwall Figure 2.05.6(3) -1 shows how infiltration will build up in the spoil downgradient and begin to seep into the lowwall. This spoil water may enter one or more of the minor sandstone beds of the overburden (Dakota Sandstone). This annual infiltration is calculated below, assuming a 10' thick somewhat permeable bed in the lowwall strata: Seepage into Low Wall = (10' ft. thick permeable sandstone bed in low wall)(8500' ft. wide seepage area)(0.10 ft. per day seepage velocity)(1 /43,560 cu. ft. to ac. ft.)(365 days per year) = 72 ac.ft. per year.