Abstract

New analyses of the isotopic composition of water, 14-dating of dissolved inorganic carbon, and order-of-magnitude Darcy calculations suggest that a dilute body of water, trending north-south in the Cambrian-Ordovician aquifer of Iowa, was emplaced as vertical recharge of Pleistocene-age water from the base of the Des Moines lobe of late Wisconsin time. The recharge occurred through more than 300 m of overlying Silurian to Mississippian age rocks. The δ18O values range from -10‰ to -9‰ for the dilute water body and are consistent with a mixture of Des Moines lobe meltwater and precipitation having an isotopic content similar to or heavier than that of average precipitation found today in the north-central United States. These results suggest that (1) the climate at the end of the last glaciation was mild and (2) a ground-water stable isotope signature similar to that of modern precipitation in an aquifer's recharge area is not a priori evidence for relatively recent recharge.

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