Sources and Ages of Ground Water in Unconfined and Confined Aquifers Beneath the U.S. High Plains

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1994

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Groundwater is a valuable resource in unconfined and confined aquifers beneath the U.S. High Plains, but little is known about its age or hydraulic history. Thirty-two samples of groundwater were collected for measurement of δ^18O, δD, Δ^14C, tritium, and ^36Cl/^Cl, as well as ionic constituents from the unconfined High Plains aquifer in the Ogallala Formation and from confined aquifers in various hydrostratigraphic units beneath the southern, central, and northern High Plains. All samples of unconfined and confined groundwaters plot along the meteoric water line. δ^18O and δD from the unconfined aquifer are consistent with the modern distribution of meteoric water across North America. δD and δ^18O from the confined aquifer likewise vary from south to north across the High Plains, but beneath the southern and central High Plains, mean compositions are lighter in the confined aquifer than in the unconfined aquifer. Beneath the northern High Plains, δD and δ^18O are the same or heavier in the confined aquifer than in the overlying unconfined aquifer. In combination, these trends show that the north-to-south range in stable isotopic compositions within the various confined aquifers is smaller than observed in the unconfined aquifer. Although age dates are uncertain, preliminary calculations suggest that unconfined groundwaters are probably less than 1,000 years old and locally as young as 25 years. Ages of groundwaters in the confined aquifers are between 20 and 32 ka and similar in the different study areas. Age of groundwater appears to increase along the inferred intrastratal flow path in the confined aquifers but is determined by leakage rate as well as lateral velocity. The range in isotopic composition in confined aquifers supports earlier findings that seawater temperature and patterns of atmospheric circulation during the Late Pleistocene were different from conditions affecting modern recharge across the High Plains.

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