ENVIRONMENTAL TRACER CHARACTERIZATION OF A TRANSIENT, MULTI-DENSITY GROUNDWATER FLOW SYSTEM; RESULTS FROM THE SCOTT M. MATHESON WETLAND PRESERVE IN MOAB, UTAH

Dissolved gas concentrations and stable isotopes have been used to develop a conceptual model of a complex desert-wetland groundwater flow system adjacent to the Colorado River near Moab, Utah. The Scott Matheson Wetland Preserve lies directly across the Colorado River from a large uranium tailings pile at the topographic low of the Moab salt-collapse valley. Extreme salinity gradients and transients in hydraulic heads make the use of Darcy’s law problematic with regards to delineating groundwater flow, but a reasonably consistent model emerges when dissolved gas and stable isotope data are considered. Dissolved ⁴He concentrations are more than two orders of magnitude larger than atmospheric solubility as little as 2 m below the Colorado River. Helium transport modeling suggests that periodic upward groundwater flow is required to maintain such a steep He gradient. Dissolved ⁴He values further delineate the lower boundary of effective groundwater flow and mixing between fresh water and brine. Concentrations of Ne, Kr, Ar, and nitrogen were used to calculate recharge temperatures of between 5 and 15 °C for shallow groundwater beneath the wetland. The combination of dissolved gas with stable isotope measurements allowed for clear determination of source waters and indicate that groundwater underflows the Colorado River in a thick deposit of river gravels that is overlain with fine-grained overbank deposits. High concentrations of ammonia (up to 2100 mg/L as NH₃-N) from the mill tailings appears to be converted to nitrogen gas creating supersaturated waters and dissolved gas stripping in parts of the system. This study illustrates the utility of environmental tracers when conventional techniques fail to reveal the relevant intricacies of a hydrologic system.