2021-03-06_PERMIT FILE - C1981008A

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the New Horizon 2 mining area. Three wells (GW -N14, GW-N17P1 and GW -N20) monitored the Dakota coal aquifer water quality in the New Horizon 2 mining area. The individual water quality analyses for the Dakota coal aquifer wells in the New Horizon 2 mining area are presented in Attachment 2.04.7-7 (Peabody Appendix 7-4). The well locations are shown on Map 2.04.7-1A. The three Dakota coal monitoring wells exhibited two water types during periods of irrigation. Wells GW -N14 and GW -N30 showed magnesium calcium sulfate water types and well GW-N17P1 showed a sodium sulfate water type. During the non -irrigation periods, well GW -N20 changes to a calcium magnesium sulfate water type. The only significant TDS change when comparing irrigation versus non -irrigation periods was at well GW -N20. The TDS level decreases 672 mg/1 during periods of no irrigation. Slight TDS increases occured at wells GW -N14 and GW-N17PI during periods of no irrigation. Mean TDS levels for the three Dakota coal wells (GW -N14, GW-N17P and GW -N20) during periods of irrigation and no irrigation were 4,323 and 4,366 mg/l, 2,740 and 1,798 mg/1, and 2,198 and 2,251 mg/l, respectively. During periods of no irrigation, well GW -N14 showed a decrease in magnesium, but increases in calcium, bicarbonate and sulfate that accounted for the small increase in mean TDS. TDS actually increased when irrigation was discontinued. The seemingly large decreases in calcium and magnesium were more than offset by the increases in sodium, bicarbonate and chloride. The sodium increase was principally responsible for the increase in TDS because it occurred at milliequivalent levels that were a factor of 10 times greater than the calcium and magnesium levels. The high sodium values and very alkaline pH level in well GW -N 17P 1 strongly suggested that the well completion zone was partially open to high sodium overburden and interburden units as well as the coal. Well GW -N20 showed a significant decrease in TDS during non -irrigation periods. Calcium, magnesium, sodium and sulfate levels all decreased when irrigation was discontinued. Only bicarbonate increased slightly, but at very low milliequivalent levels. The pH level for wells GW -N 14 and GW -N20 ranged from 4.7 to 7.6 while well GW -N 17PI ranged from 8.2 to 9.8. As was previously mentioned, these very alkaline pH's were atypical of coal aquifer levels and were believed to be due to the well completion. Mean pH values for wells GW -N14, GW -N20 and GW-N17P1 were 4.95, 5.03 and 8.45, respectively. There was some suggestion of a limited amount of seasonal variability to the Dakota coal chemistry at the New Horizon 2 mining area. In comparing spring versus fall chemistry, magnesium, sulfate and TDS levels decreased and sodium and potassium levels increased in the fall. A comparison of summer versus fall chemistry suggested that bicarbonate, chloride and pH levels increased and magnesium levels decreased in the winter. Chemical trends overtime are also evident in the Dakota coal aquifer. Well GW -N14 showed a decreasing trend in calcium levels during irrigation periods. Well GW-N17P1 showed a decreasing trend in calcium, magnesium and bicarbonate levels and an increasing trend in sodium and chloride levels during periods of irrigation. Irrigation period trends for well GW -N20 included decreasing calcium, magnesium, sodium, sulfate and TDS levels and increasing chloride levels. Trace elements in the Dakota coal wells were measured. Well GW -N14 water exceeded the CWQCC agricultural standards for aluminum, iron, fluoride, manganese and pH; well GW -N 17P 1 water exceeded the pH standard; and well GW -N20 water exceeded the iron, manganese and pH standards. In comparison to Page 2.04.7-12 March 2017 (TR -77)