Geochemical evolution of ground water in the Great Plains (Dakota) aquiferof Nebraska: Implications for the management of a regional aquifer system

The Great Plains (Dakota) aquifer system is one of the most extensive in No rth America extending from the Arctic Circle to New Mexico, and underlies a pproximately 94% of Nebraska, In Nebraska, we do not have the physical grou nd water monitoring data at the scale that is necessary to manage ground wa ter flow systems. However, first-order management strategies for this regio nal aquifer can be developed by understanding the geochemical evolution of the ground water. Using major-ion water chemistry data from 203 wells in 19 counties in eastern Nebraska, reconnaissance delta O-18, deltaD, and delta Sr-87 data, and two geochemical models, PHREEQC and SNORM, we determine th at modern meteoric water, NaCl brines from underlying formations, and cold glacial melt water are the primary sources for the water in the Dakota Aqui fer, Based on these three water sources and the geochemical evolution of th e various water types, the following first-order management strategies are suggested, In areas where CaSO4 and Ca-Na SO4 type water occur, Pleistocene -age glacial meltwater is the source. This water supply is not easily renew able. It is recommended that detailed water resource evaluation be conducte d before extensive development occurs, The source of Ca ( +/- Mg) HCO3 type water is from recharge by local precipitation and should be managed to mai ntain them as a renewable resource. In mixed ground water type areas, the g round water chemistry reflects the interaction of two distinct water types, one of which is meteoric water and the other is either CaSO4 and Ca-Na SO4 -type water or NaCl-type water. If the relatively fresh ground water is ext racted at a rate that changes the location of the interface between the end members, then monitoring changes in water chemistry in a well over time cou ld be used as an early warning system for the onset of potential problems r elated to overpumping.