| Description |
Moab City and Grand County rely on groundwater for public water supply. Recent development and an increase in water right applications prompted area water managers to call for an updated evaluation of local groundwater resources. The purpose of this study is to (1) prepare a conceptual groundwater flow model for lower Moab-Spanish Valley by delineating flow paths and identifying sources of recharge to the valley-fill aquifer, in order to (2) quantify groundwater outflow to the Colorado River to improve estimates of groundwater available for public use. Samples were collected from 30 wells to analyze major ions, tritium, noble gases, CFCs, SF6, and deuterium and oxygen-18 stable isotopes. The groundwater budget was evaluated by estimating discharge to the Colorado River and loss to Mill Creek. Groundwater discharge was estimated first by performing a Darcy Flux calculation. Twelve new observation wells were drilled and installed in a transect across the Scott M. Matheson Wetlands Preserve, ranging in depth from 25 to 60 feet below ground surface. Eight single-well tests and two dual-well tests were performed to determine transmissivity, which ranged from 90 to 5,400 ft2/day, with a median of approximately 1000 ft2/day. The hydraulic gradient was determined by creating a potentiometric surface map using water levels from both new observation wells and previously existing private wells. Discharge was estimated to be 300 acre-feet per year. A second, independent estimate of groundwater discharge was made using environmental tracer data to determine change in age across some distance along a flow path. 3H/3He ages in the valley-fill aquifer range from 0 to 57 years. Average discharge had a value of 1,000 acre-feet per year. A bromide tracer test was performed to evaluate whether some groundwater was lost to Mill Creek before discharging into the Colorado River. Gain in Mill Creek was found to be negligible. Geochemical properties of valley wells indicate that the valley-fill aquifer is not recharged by water from Glen Canyon Group Aquifer (GCGA), as previously hypothesized by Sumsion (1971); rather, it is more likely recharged by loss from Mill and Pack Creeks. |
