DETERMINING A PROBABLE RANGE OF RECHARGE RATES FOR RED BUTTE DRAINAGE BASIN, UTAH

Stream water that originates from an underlying ground-water system (a gaining stream) is a summation of flow paths that exist within a drainage basin. If the ground-water signature is not lost, the age of stream water is a reasonable estimate of the average ground-water residence time. Average residence time in an unconfined aquifer is directly proportional the ground-water recharge rate. On the basis of chlorofluorocarbons concentration in 8 samples collected during base-flow conditions along a 4.9-kilometer reach of Red Butte Creek, the stream-water age ranges from 12 to 17 years old. Chlorofluorocarbons in stream water exchange with the atmosphere and do not conserve the ground-water chlorofluorocarbon concentrations. If the flux of ground water-derived chlorofluorocarbons does not offset the rate of exchange with the atmosphere, ages will systematically appear younger the farther the stream water moves from areas of ground-water inflow. This does not occur in Red Butte Creek, which is in agreement with results from an ionic-salt tracer that indicate dispersed ground-water inflow along the entire reach of Red Butte Creek. Ground-water age will be determined for samples collected at springs and piezometers during the next field session. Spring water is assumed to be an integration of localized flowpaths and the piezometers were designed to sample individual flow paths. The age of water from the stream, springs, and piezometers will be used in combination with ionic-salt tracer dilution, long-term streamflow records, and drainage-basin characteristics to quantify the probable range of basin-wide recharge rates.