A Black Hills-Madison Aquifer Origin for Dakota Aquifer Groundwater in Northeastern Nebraska

Previous studies of the Dakota Aquifer in South Dakota attributed elevated groundwater sulfate concentrations to Madison Aquifer recharge in the Black Hills with subsequent chemical evolution prior to upward migration into the Dakota Aquifer. This study examines the plausibility of a Madison Aquifer origin for groundwater in northeastern Nebraska. Dakota Aquifer water samples were collected for major ion chemistry and isotopic analysis ( 18 O, 2 H, 3 H, 14 C, 13 C, 34 S, 18 O-SO 4 , 87 Sr, 37 Cl). Results show that groundwater beneath the eastern, unconfined portion of the study area is distinctly different from groundwater sampled beneath the western, con fined portion. In the east, groundwater is calcium-bicarbonate type, with δ 18 O values (−9.6‰ to −12.4‰) similar to local, modern precipitation (−7.4‰ to −10‰), and tritium values reflecting modern recharge. In the west, groundwater is calcium-sulfate type, having depleted δ 18 O values (−16‰ to −18‰) relative to local, modern precipitation, and 14 C ages 32,000 to more than 47,000 years before present. Sulfate, δ 18 O, δ 2 H, δ 34 S, and δ 18 OSO 4 concentrations are similar to those found in Madison Aquifer groundwater in South Dakota. Thus, it is proposed that Madison Aquifer source water is also present within the Dakota Aquifer beneath northeastern Nebraska. A simple Darcy equation estimate of groundwater velocities and travel times using reported physical parameters from the Madison and Dakota Aquifers suggests such a migration is plausible. However, discrepancies between 14 C and Darcy age estimates indicate that 14 C ages may not accurately reflect aquifer residence time, due to mixtures of varying aged water.