Date of this Version
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 migra¬tion 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 (18O, 2H, 3H, 14C, 13C, 34S, 18O-SO4, 87Sr, 37Cl). Results show that groundwater beneath the eastern, un¬confined 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 δ18O values (−9.6‰ to −12.4‰) simi¬lar to local, modern precipitation (−7.4‰ to −10‰), and tritium values reflecting modern recharge. In the west, groundwater is calcium-sulfate type, having depleted δ18O values (−16‰ to −18‰) relative to local, modern precipitation, and 14C ages 32,000 to more than 47,000 years before present. Sulfate, δ18O, δ2H, δ34S, and δ18O-SO4 concentrations are similar to those found in Madison Aquifer groundwater in South Dakota. Thus, it is pro¬posed that Madison Aquifer source water is also present within the Dakota Aquifer beneath northeastern Ne¬braska. 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, discrep¬ancies between 14C and Darcy age estimates indicate that 14C ages may not accurately reflect aquifer residence time, due to mixtures of varying aged water.