Noble gases, stable isotopes, and radiocarbon as tracers of flow in the Dakota aquifer, Colorado and Kansas

A suite of chemical and isotope tracers (dissolved noble gases, stable isot opes of water, radiocarbon, and Cl) have been analyzed along a flow path in the Dakota aquifer system to determine likely recharge sources, ground wat er residence times, and the extent of mixing between local and intermediate flow systems, presumably caused by large well screens. Three water types w ere distinguished with the tracers, each having a very different history. T wo of the water types were found in south-eastern Colorado where the Dakota is poorly confined. The tracer data suggest that the first group recharged locally during the last few thousand years and the second group was compos ed of ground water that recharged earlier during a cooler climate, presumab ly during the last glacial period (LGP) and mixed aged water. The paleotemp erature record archived in this groundwater system indicates that south-eas tern Colorado was about 5 degrees C cooler during the LGP than during the l ate Holocene. Similar temperature changes derived from dissolved noble gase s in other aquifer systems have been reported earlier for the south-western United States. The third water type was located down gradient of the first two in the confined Dakota in western and central Kansas. Groundwater resi dence time of this water mass is on the order of 10(4)-10(5) yrs and its re charge location is near the Colorado and Kansas border down gradient of the other water types. The study shows the importance of using multiple tracer s when investigating ground water systems. (C) 1998 Elsevier Science B.V. A LI rights reserved.