INTEGRATION OF GROUND-WATER FLOW PATHS AND DETERMINATION OF MEAN RESIDENCE TIMES, SIX MILE CREEK, TOOELE VALLEY, UTAH

Mean residence time of ground-water within an aquifer can potentially be estimated if water samples that represent a complete distribution of flow paths can be collected. The feasibility of collecting representative water samples from gaining stream reaches is being tested at Six Mile Creek, Tooele Valley, Utah. Six Mile Creek starts at a series of small springs and visibly gains ground water along a 700-meter reach directly downstream of the initial springs. Discharge is from basin-fill deposits, and average flow at the lower end of the reach is about 110 liters/second.

The method being tested is that the apparent age as determined using CFC-12 concentrations of stream water can be corrected for gas-exchange across the air-water interface. The corrected concentrations represent a flow-weighted integration of all ground-water flow paths that converge at the stream and can be used to determine mean residence time. Surface-water samples were collected from the gaining stream reach and analyzed for CFC-12. Two tracer experiments were conducted in the stream, one using CFC-12 to quantify gas-exchange rates, and another using sodium bromide to quantify ground-water gains. Ground-water samples were collected from a set of short-screened wells located at numerous points in the stream and also analyzed for CFC-12 concentration.

Preliminary simulations indicate that corrected CFC-12 concentrations of stream samples are similar to the flow-weighted average concentration determined from samples collected from wells. Water discharges as artesian flow from the wells, and the apparent age determined for water discharging from individual wells is flow weighted by using the artesian yield. These initial results suggest that gaining stream reaches integrate ground-water characteristics and may provide reasonable sampling locations for the determination of mean residence time.