Late-quaternary recharge determined from chloride in shallow groundwater in the central Great Plains

An extensive suite of isotopic and geochemical tracers in groundwater has b een used to provide hydrologic assessments of the hierarchy of flow systems in aquifers underlying the central Great Plains (southeastern Colorado and western Kansas) of the United States and to determine the late Pleistocene and Holocene paleotemperature and paleorecharge record. Hydrogeologic and geochemical tracer data permit classification of the samples into late Holo cene, late Pleistocene-early Holocene, and much older Pleistocene groups. P aleorecharge rates calculated from the Cl concentration in the samples show that recharge rates were at least twice the late Holocene rate during late Pleistocene-early Holocene time, which is consistent with their relative d epletion in O-16 and D, Noble gas (Ne, Ar, Kr, Xe) temperature calculations confirm that these older samples represent a recharge environment approxim ately 5 degrees C cooler than late Holocene values, These results are consi stent with the global climate models that show a trend toward a warmer, mor e arid climate during the Holocene, (C) 2000 University of Washington.