HYDROGEOLOGIC CHARACTERIZATION AND VULNERABILITY ASSESSMENT OF THE ASHLEY SPRING GROUNDWATER FLOW SYSTEM, UINTA MOUNTAINS, UTAH

Ashley Spring, in Uintah County, Utah, provides water for drinking and irrigation use for residents of Vernal, Utah, and surrounding communities. The spring originates from Mississippian-age carbonates that are confined by sandstones at a depth of approximately 1,000 feet. Enhanced fracturing of the sandstones in areas where anticlines have been incised by deep canyons allows water to move upward to the surface at Ashley and other springs. Activities related to a proposed open-pit phosphate mine have prompted concerns that the water quality and discharge of Ashley Spring could be affected.

Results of dye-tracer tests over the past 50 years show that surface runoff from the core of the Uinta Mountains abruptly loses through fluvioglacial channel deposits into the karstic Madison Limestone along the flank of the uplift and then discharges downgradient at Ashley and other large karst springs. Groundwater travel times from losing reaches of streams as much as 10 miles from Ashley Spring are as short as 3 days under low-flow conditions. Results of dye tracing also have documented movement of water between Ashley and adjacent groundwater basins as well as diverging groundwater flow paths to Ashley and other springs. Groundwater movement to Ashley Spring likely is influenced by structural dip, fault zones, and northwest-trending joint sets.

Discharge of Ashley Spring is highly variable, ranging from about 15 to 90 cubic feet per second and responds primarily to snowmelt runoff and rainfall events. Continuous monitoring of specific conductance and temperature from May 2013 to April 2015 indicates a generally inverse relation to discharge. During this period, specific conductance of the spring water varied from 89 microSiemens/cm at 25 ^oC during snowmelt to 167 at base-flow conditions when a greater proportion of discharge originates from dispersed infiltration and storage within the aquifer matrix. Analyses of water samples collected from Ashley Spring at varying discharge rates and from springs and wells in other aquifers in the vicinity of the spring indicate that concentrations of major ions, nutrients, and selected trace elements generally are very low or below laboratory reporting limits. These results will be used to establish baseline conditions to which the potential effects of mining can be compared.