RETHINKING MOUNTAIN BLOCK RECHARGE TO A DEVELOPING DESERT COMMUNITY USING A MULTIPLE-TRACER GEOCHEMICAL APPROACH, MOAB-SPANISH VALLEY, UTAH

The Glen Canyon Group Aquifer (GCGA) is the sole source of public water supply for the city of Moab, Utah where rapid population and tourism growth is increasing the demand for groundwater resources. Water from the deep GCGA is the likely target for future water resources, but our analysis indicates that additional withdrawals would likely come from storage and would not be sustained by recharge. This study illustrates the utility of testing existing conceptual models (and associated quantitative water budgets) with additional data such as environmental tracers.

A direct quantitative estimate of groundwater discharge from the GCGA in this area is problematic because the downstream boundary is the Colorado River and a small discharge into a large river is difficult to measure. A water budget based on a conceptual model of GCGA discharging into the adjacent Valley-Fill Aquifer (VFA) was reported by Sumsion (1971) and numerous subsequent studies have utilized this water budget. However, new environmental tracer and geochemistry data collected by USGS (Masbruch and others, in review) and the University of Utah (Nelson 2017) are inconsistent with the Sumsion (1971) conceptual model. GCGA groundwater is characterized by depleted stable isotopes of water, low tritium, low ³He/⁴He ratios, low dissolved solids, and low sulfate concentrations. The geochemistry of the VFA groundwater is distinct from GCGA groundwater, suggesting that the VFA is recharged by local streams (Mill and Pack Creeks) rather than from the GCGA. Estimated discharge from the VFA using groundwater age gradients is less than previously thought. Pore waters extracted from vadose-zone cores and analyzed for tritium and chloride indicate only modest amounts (14 to 35 mm/yr) of GCGA recharge occurs at altitudes below 2,450 m. The revised GCGA conceptual model has primary recharge confined to an area above 2,750 m in altitude. Using a lumped parameter model approximating the revised conceptual model and groundwater travel times based on ¹⁴C, we estimate recharge to be 4.1X10⁶± 2.3X10⁶m³/yr which agrees with measured GCGA discharge from wells and springs.