CHARACTERIZING THE CODELL AQUIFER ON THE NEBRASKA SIDE OF THE MISSOURI NATIONAL RECREATIONAL RIVER

Multiple aquifers exist in and around the boundaries of the Missouri National Recreational River in Nebraska and South Dakota. The Codell aquifer hosted by the Codell Sandstone Member of the Carlile Shale (Upper Cretaceous) is utilized in northeastern Nebraska and southeastern South Dakota, and elsewhere in the Western Interior. Herein, we present the results of the first comprehensive study of the aquifer In Nebraska, where it is a management concern.

The Codell aquifer underlies a geologically and geomorphically heterogeneous area of 1,190 km² in Boyd and Knox counties. Approximately 70 active registered wells averaging 87 m in depth are screened entirely within the aquifer in this area. These wells have an average pumping rate of about 0.95 L/s and are 10 cm in diameter or less. The Codell Sandstone varies in thickness from 2.4 to 24 m and is overlain by as much as 15 m of the upper Carlile Shale. The elevation of the contact between the Carlile Shale and the Niobrara Formation ranges from 354 m to 329 m near the southern edge of the park. The top of the Codell Sandstone is similar in configuration to that of the Carlile-Niobrara contact, but there are minor differences between the two surfaces. The elevation of the top of the Codell Sandstone ranges from about 351 m to 326 m. Groundwater in the Codell aquifer is under pressure everywhere in the study area, except possibly under the Missouri River trench. Groundwater in the aquifer flows northward towards the Missouri River, and potentiometric head values range from about 402 m to 366 m, which is between approximately 30.5 m and 122 m below ground surface. The steepest flow gradient is 0.002 (2 m/km). Hydraulic heads in both South Dakota and Nebraska indicate that water in the Codell aquifer flows towards the Missouri River, and possibly discharges into its alluvium. This configuration probably did not develop until the Missouri River become entrenched in the Late Pleistocene.

Analyses of 18 water samples from active wells in the study area collected during 2015 indicate variable water quality and overall elevated total dissolved solids, sodium, chloride, and in some places, sulfate. Chloride concentrations correlate positively with well depth. Sulfate concentrations may have some relation to zones of pyrite weathering in the Pierre and Carlile shales.