USE OF ENVIRONMENTAL TRACER GEOCHEMISTRY TO DETERMINE SOURCES OF RECHARGE, DELINEATE FLOWPATHS, AND CHARACTERIZE THE GROUNDWATER RESOURCES OF AN INTERMOUNTAIN BASIN-FILL AQUIFER, RUSH VALLEY, TOOELE COUNTY, UTAH

Rush Valley is a rural valley located in west-central Utah about 50 mi. southwest of Salt Lake City. Development of groundwater resources in Rush Valley recently has been proposed to supply water to the growing populations in neighboring Tooele and Cedar Valleys, both of which are closed to new groundwater appropriations. Previous studies have shown the three valleys to be hydraulically connected, with groundwater movement out of Rush Valley and into Tooele and Cedar Valleys.

The water resources of Rush Valley were assessed during 2008-2010 with an emphasis on providing water managers with information useful in addressing future water resource management options. As part of this investigation, groundwater samples from 25 sites were analyzed for a suite of environmental tracers to determine sources of recharge, delineate groundwater flowpaths, and estimate residence times. For example, concentrations of ³H between 0.4 and 10 TU indicate the presence of modern (less than 60 years old) groundwater in 7 of 25 samples. Apparent ³H/³He ages of these samples range from 3 to 35 years. Adjusted minimum radiocarbon ages of pre-modern water samples range from 1.6 to 42 thousand years with most being more than 11 thousand years. These data help to characterize three areas of more-active groundwater movement that receive and circulate modern recharge on timescales of decades or less. Large parts of the groundwater flow system are much less active and receive little, if any, modern recharge. Much of the current groundwater withdrawal in Rush Valley occurs within or near the three areas with modern recharge. At the current rates of withdrawal, no long-term water-level declines have been observed in these areas. The surrounding parts of the groundwater flow system appear to store potentially large, but unknown quantities of water that are thousands to tens of thousands of years old. The age of groundwater in these areas indicates that aquifer conditions may need to be monitored as development continues because future water-level declines may represent irrecoverable groundwater storage depletion.