A BLACK HILLS ORIGIN FOR GROUNDWATER IN THE DAKOTA AQUIFER OF NORTHEASTERN NEBRASKA?

Recent Dakota Aquifer water quality surveys located a zone of Ca-SO₄ groundwater in northeastern Nebraska with elevated SO₄ concentrations (503-738 ppm), and depleted δ¹⁸O (-16.0 to -18.6 ‰) signatures (relative to local weighted mean annual precipitation; -7.4 ‰). Initially, it was believed that these waters originated locally as recharge under cooler climates across the region during the late Wisconsinan glacial maximum, or possibly from infiltration of ice margin glacial meltwater. To further investigate this possibility, groundwaters were age dated using ³H and ¹⁴C. In addition, the waters' δ³⁴S and δ¹⁸O_sulfate compositions were determined to ascertain whether dissolved sulfate was from shallow dissolution of terrestrial sulfides found locally within the overlying glacial and sedimentary deposits, or from more regional scale evaporite dissolution within deeper, underlying sediments. Tritium was not detected in the high SO₄ waters, and corrected ¹⁴C ages range from 7,494 to 47,463 ypb, with the age of most waters being in excess of 17,000 ybp. The δ³⁴S (3.2 to 15.4 ‰) and δ¹⁸O_sulfate (8.0 to 11.5 ‰) results suggest that the majority of the waters derived their dissolved SO₄ from the dissolution of evaporites, which are not present locally within the Dakota Sandstone, rather than terrestrial sulfides. Combined, these results suggest that another origin for the waters might be more plausible. Within the Dakota Aquifer in southeastern South Dakota, there is a zone of high sulfate water that has been shown to have originated as recharge within the Black Hills region some 725 km to the west. This water flowed eastward within the underlying Madison Aquifer and subsequently migrated upward into the Dakota Aquifer where it is present today. Comparison of Nebraska’s Dakota Ca-SO₄ groundwaters with those in South Dakota suggests that they may share a common origin and transport history. Black Hills Madison Limestone groundwaters have δ¹⁸O (-17 ‰), and δ³⁴S (+12 ‰) compositions that are similar to those in Nebraska, and ¹⁴C ages are consistent with calculated regional transport times across the aquifer. Thus, another explanation might be that the anomalous groundwaters in the Dakota Aquifer of northeastern Nebraska result from regional cross-formational groundwater migration, and not from local Wisconsinan age recharge.