2018-11-19_PERMIT FILE - C2010089A (6)

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of the natural water means that there is sufficient acid -neutralizing ions such as hydroxyl (OH-) or bicarbonate (HCO3") in solution to absorb the acidic hydrogen (H+) ions produced by the pyrite oxidation. Soluble sulfate (SO4-2) ions are also produced by the pyrite oxidation, and they are quickly taken up by the calcite to produce calcium sulfate or gypsum (CaSO4), which is not very soluble and also precipitates out of solution, especially when the pH of the solution is near normal. The (CO3 -Z) anion in the calcite goes in solution to replace the sulfate. Calcium stays as a solid in the new gypsum produced. Overall, the net change to the water quality is not significant as compared to overburden water, but some pyrite has been converted to other solid compounds: gypsum and iron oxides such as limonite. Strong support for this occurring is seen in the water quality comparison of overburden ground water to spoil water. From historic data, (see New Horizon 1 Area permit) ground water monitoring hole GW -N9 is north of the mined areas of New Horizon #1 and water quality has not been affected by the mine since the flow gradient is from the northeast to the southwest and is best to use in the comparison. Spoil Spring 1 (SS#1), which developed near the southwestern end of the reclaimed mine (old Peabody Nucla Mine) best represents the spoil water quality. Table 2.05.6(3)-1 shows the chemistry of these waters sampled at the same time. It is important to understand what is truly different from the pre -mine condition to the backfill condition. In the pre -mine condition, good quality water from the irrigation ditches infiltrates through the soil and through the more permeable strata and picks up dissolved solids. The water quality becomes poor (approx. 3000 ppm TDS), but this process takes a very long time since the water moves very slowly through the tight strata with low hydraulic conductivity, (K value of 2.1 ft/day at GW -N9). In the case of the old Peabody backfill (New Horizon #1 Area permit), the hydraulic conductivity is 40 ft/day (at GW -N27) and the potential is high that much more irrigation water could be recharged rapidly through the more permeable, broken backfill material. The old Peabody backfill gets its recharge from both seepage from the bedrock zones (mostly overburden), and from irrigation water surface runoff. The old backfill is not directly recharged from irrigation itself as the old backfill is not irrigated. The distinction being that, pristine ditch water is not entering the old Peabody backfill but that runoff and bedrock water is. The average TDS of the irrigation ditch water is only about 200 mg/1 and pH of 8.3, (see Table 2.04.7-6, SW -N209). Runoff water is 2 to 3 times higher in TDS and in streams the TDS is 5 to 6 times higher, (see Table 2.04.7-7, SW -N202). The better quality water infiltrating the backfill picks up dissolved solids as in the case of the overburden; however, it simply does so much more rapidly. Water infiltrating into the spoil at the upper end of the New Horizon #1 spoil may only spend 15-45 days in the spoil before the water is discharged at the spring. Yet, during this short time, it has managed to become approximately the same quality as the overburden water. Due to the increased porosity and higher level of oxygen in the rapidly infiltrating water, the pyrite breaks down at a faster rate but this reaction is buffered by both the higher pH of the water and the Section 2.05.6(3) Page 21 March 2018 (TR -16)