2020-06-04_PERMIT FILE - C1981008A (13)

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at the New Horizon 1 area has had no discernible effects on the water levels of the overburden monitor wells. Dakota Coal Water Levels Ten monitor wells were installed to monitor water levels in the Dakota coal aquifer. Two of the monitor wells (GW -N4 and GW-N4R) were located at New Horizon 1 and the remaining eight monitor wells (GW -N 14, GW -N17, GW-N17P1, GW -N20, GW -N29, GW -N30, GW -N42 and GW -N45) were located at New Horizon 2 (Map 2.04.7-1A). Water level data for Well GW -N4 was collected since August, 1979 and collection of water level data for Wells GW -N14, GW -N17, GW-N17P1, and GW -N20 began in July 1986. GW-N4R, GW -N29 and GW -N30 were installed in September, 1987 and collection of water level information began immediately after they were completed and developed. Map 2.04.7-3 shows the Dakota coal potentiometric surface. Monthly water level data including hydrographs for New Horizon 2 monitor well sites are contained in Appendix D of Attachment 2.04.7-5. An additional well, GW -N37, was drilled in May, 1993 in the north-east corner of the permit area. Ground water in the Dakota coal is under artesian conditions. Mean artesian heads in the lower Dakota coal range from 18 feet at well GW -N14 and 42 feet at well GW -N29. The average artesian head for the lower Dakota coal is 34 feet. Water levels in the Dakota coal display seasonal variability that coincides with the irrigation season and operation of the ditch irrigation system. The maximum seasonal water level fluctuation observed in well GW -N4 is about 3.0 feet due to its location in the valley bottom. The New Horizon 2 wells display seasonal variability from 2.8 feet at well GW-N17P1 to 14.6 feet at well GW -N20. Shallowing occurs from April to September and there appears to be little lag between when the irrigation season starts and the shallowing of the static water levels occurs. There are no discernible water level responses to precipitation, nor can any affect from mining be observed at any of the Dakota coal monitor wells at the New Horizon 1 mining area. Underburden Water Levels Fourteen monitor wells have been installed to monitor water levels in the underburden aquifer system. Six of those wells (GW -N1, GW-N1P1, GW -N3, GW -N5, GW -N6 and GW -N11) were located in and proximate to the New Horizon 1 mine area. The remaining six monitor wells (GW -N13, GW -N16, GW-N16P1, GW -N19, GW -N33, GW -N34, GW -N43 and GW -N46) were located proximate to the New Horizon 2 mining area. Maps 2.04.7-1A and 2.04.7-4 show the underburden monitor well site locations and the potentiometric surface, respectively. Monthly water level data including hydrographs for New Horizon 2 Mine sites may be found in Appendix D of Attachment 2.04.7-5. In May, 1993, an additional well (GW -N36) was installed in the northeast corner of the permit area. Ground water in the underburden aquifer is under artesian conditions. Mean artesian heads in the underburden aquifer range from 9.7 feet at well GW -N11 to 86.7 feet at well GW -N33. The average artesian head for the underburden aquifer is 40.0 feet. A downward vertical head is generally observed in the underburden but an upward (recharging) vertical head is exhibited at well GW -N16. All of the underburden wells at New Horizon 1 and 2 display seasonal variability to some degree. The seasonal water level fluctuations at the New Horizon 1 and 2 mining areas range from only a few tenths of a foot at wells GW -N3 and GW -NI I to nearly 11.0 feet at well GW -N13. All of the underburden wells exhibit steady-state water levels over time with the exception of wells GW -N1, Page 2.04.7-5 March 2017 (TR -77)