2021-03-06_PERMIT FILE - C1981008A

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ridge top toward both Tuttle and Calamity Draws. The primary recharge area is from the northeast and from vertical leakage from the irrigation ditches and flood irrigation. The average water level gradient in the overburden is .0342 ft/ft with a range between .0333 ft/ft and .0375 ft/ft. Dakota Coal The potentiometric surface map of the Dakota coal is presented on Map 2.04.7-3. The water level contours were constructed based on the static water level measurements collected in September, 1987 from seven Dakota coal monitor wells and two multiple completion (overburden and Dakota coal) monitor wells. It was necessary to include the two multiple completion monitor wells (GW -N8 and GW -N9) in the Dakota coal potentiometric contour map because they are the only two wells in the northwest portion of the permit area open to the Dakota coal. Generally (in the Dakota coal) static water levels are slightly deeper than the overburden so the potentiometric surface in the vicinity of the multiple completion wells may have a slightly higher water surface elevation than the Dakota coal. Ground water in the Dakota coal is confined in all of the areas monitored. The flow in the Dakota coal aquifer is generally from the northeast to the southwest. The average water level gradient is .0297 ft/ft with a range between .0228 ft/ft and .0343 ft/ft. Recharge to the Dakota coal is generally from the agricultural crop areas situated to the northeast and from vertical leakage from the irrigation ditches and flood irrigation via overlying units. Discharge mainly occurs along the draws where the Dakota aquifer intersects the ground surface and by discharge to contiguous aquifers. Underburden The potentiometric surface map of the underburden is presented on Map 2.04.7-4. The water level contours were constructed based on September, 1987 water level data from eight underburden monitoring wells. Ground water in the underburden is under confined conditions. Flow in the underburden is from the northeast to the southwest. The average water level gradient is .0301 with a range between .0237 and .0368. The primary recharge area is located northeast of the mine via leakage from the overlying units. Discharge occurs in the draws where the underburden aquifer intersects the ground surface and to contiguous aquifers. The mining operation at the New Horizon 1 area appears to have had no effect on the underburden ground water potentiometric surface. Well GW -N11 was omitted from use in the construction of the underburden potentiometric contour map. This well had enigmatic static water level readings when compared to the rest of the underburden monitor wells. Because of the poor completion record and the anomalous static water level readings, it was decided to exclude this well from the potentiometric surface map. Effects of the Irrigationystem on Potentiometric Surfaces During the summer months when flood irrigation is in full swing, each of the aquifers monitored exhibits a rise in static water level. This ground water shallowing creates a uniform rise in the potentiometric surface. Perusal of the water level data indicates that the rise in the potentiometric surface during the summer months has a negligible effect on the ground water gradients. A predominate source of recharge to the shallow bedrock aquifer system at the New Horizon 1 and 2 mining areas is leakage from the irrigation ditch system and infiltration of excess flood irrigation waters. Recharge also results from infiltration of precipitation and subsurface ground water inflow. The Dakota coal and the underburden are generally confined but there is enough leakage between the adjacent units to afford some recharge. Page 2.04.7-7 March 2017 (TR -77)