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Near surface ground water in the Nucla area is partially recharged by irrigation return flow <br />through Quaternary aeolian and alluvial deposits. The lower Dakota and underlying <br />Morrison Formations are regional aquifers, although water from the Dakota Formation is not <br />heavily utilized due to its high salinity. The Momson Formation water is widely used for <br />stock and domestic wells. <br />Very little hydrologic documentation is available regarding the alluvium found along the <br />small tributaries draining the area. Peabody drilled one well in the alluvium of Calamity <br />Draw in mid-1986. The well appears to have been completed in a clay lens. The alluvial <br />water table fluctuates seasonally within 5 to 10 feet of the surface with highest water levels <br />in August and lowest water levels in January. Recharge is primarily derived from irrigation <br />return flow and secondarily from Flow in Calamity Draw. Transmissivity vanes from 6.6 to <br />10.1 ftZ/day. The hydraulic conductivity vanes from 0.4 to 0.6 ft/day. The water is <br />characterized as a saline, hard, neutral pH, calcium sulfate water with average TDS of <br />3291 mg/l. <br />The aquifer overlying the coals in the upper strata of the Dakota Sandstone is unconfined and <br />flows generally toward the southwest. However, in the New Horizon Mine 1 area it <br />dischazges into both Tuttle and Calamity Draws (north and south respectively), Water levels <br />fluctuate between 3 and 23 feet below the ground surface. Highest levels (closest to the <br />surface) of this water table occur between June and August and lowest levels between <br />December and March. Two wells south of Calamity Draw (GW-N31 and GW-N32 on the <br />monitoring stations map) have exhibited limited water level fluctuations since their <br />construction in mid-1986. <br />The Upper Dakota aquifer is predominantly rechazged by return flow from the Lower Second <br />Pazk Irrigation Ditch and the West Lateral Irrigation Ditch (at New Horizon Mine 1 and <br />New Horizon Mine 2 respectively). Some rechazge is probably derived from infiltration of <br />precipitation and subsurface ground water flow. The transmissivity of the overburden <br />(Upper Dakota aquifer) vanes from 3 to 53 ft~/day, averaging 13.3 ft'/day. The McWhorter <br />analysis of pit inflow conservatively estimated that the hydraulic conductivity of the <br />overburden was 0.61 ft/day. Hydraulic conductivity calculations from field tests of the <br />overburden showed a range of 0.39 to 1.05 ftlday, indicating moderate permeability. <br />The water found in the overburden is characterized as very hard, saline, calcium/magnesium <br />sulfate water with neutral pH. TDS levels vary from 1494-10074 mg/1 and average 4613 <br />mg/l. Sulfate concentrations are very high, varying from 875 mg/l to 6872 mg/1 and <br />averaging 3176 mg/1. In some of the overburden wells, sulfate, TDS, manganese and <br />fluoride exceed recommended standards for livestock drinking water. In addition, <br />concentrations of manganese and fluoride exceed the agricultural use standards in some <br />wells. <br />The Dakota coal has been described as an aquifer. Wells completed in the Dakota show <br />water level fluctuations of 1.3 to 14.6 feet. The water table exhibits seasonal fluctuations <br />with high water levels in late fall. The aquifer is recharged from the east northeast and <br />discharges to the west and along Tuttle and Calamity Draws. The transmissivity vanes from <br />7 <br />