CHARACTERIZING THE AQUIFER PROPERTIES OF THE CRETACEOUS NIOBRARA FORMATION IN EASTERN NEBRASKA

The Cretaceous Niobrara Formation (Turonian-Campanian) is a secondary aquifer in parts of eastern Nebraska. The Niobrara aquifer hosts some 230 active registered wells screened entirely in the formation, and another 320 wells partially screened in the formation. The aquifer is most productive in Cedar County in northeastern Nebraska, where nearly 200 of the 230 exclusively Niobrara wells are located. The median depth of these wells is 30 m (100 ft), and more than half of them are used for irrigation. The average yield for exclusively Niobrara irrigation wells in the county is about 1,630 lpm (430 gpm). Relatively high-yielding wells are mostly located around Bow Creek, Norwegian Bow Creek, and West Bow Creek, where overlying Cenozoic deposits are thin or absent. The groundwater gradient is generally north-northeast toward the Missouri River. Water-level contours “V”-upstream on the middle reaches of Bow Creek and West Bow Creek, suggesting that these creeks gain groundwater, at least in some reaches. Geologic cross sections and airborne geophysical surveys also indicate that these creeks could be hydrologically connected to the Niobrara aquifer, even though there may be thin remnants of the Pierre Shale atop the Niobrara Formation. In the vicinity of Norwegian Bow Creek, thicker Pierre Shale separates the alluvium from the Niobrara Formation, and the alluvium may not be hydrologically connected to the aquifer. Most of the Niobrara wells in Cedar County are unconfined, but 30 wells have static water levels at least 10 feet above the top of the Niobrara Formation. Confined conditions occur mostly in the southern part of the aquifer, but some confined wells are scattered among unconfined wells elsewhere in the county. Water quality in the aquifer is generally good, although pyrite weathering leads to localized elevated sulfate concentrations. The oldest active well in the aquifer was installed in 1959, and the management goal in Cedar County is perpetual sustainability.