TRACKING SALINITY SOURCES IN THE UPPER COLORADO RIVER BASIN: STREAMS CROSSING MESOZOIC MARINE STRATA OF THE SAN RAFAEL SWELL, UTAH

The San Rafael River and Muddy Creek both originate in the Wasatch Plateau of central Utah and flow eastward across the San Rafael Swell before joining the Colorado River. These streams have high solute (salinity) loads, of which >50% by mass is the sulfate ion. Both streams, upon exiting the Wasatch Plateau, cross the full Mesozoic section of central Utah. Elemental and isotopic signatures from marine depositional/diagenetic systems in the Mesozoic can be used to fingerprint solute sources to specific geologic units. The hypothesized solute sources, based upon the presence of sulfate minerals, are the Mancos Shale, the San Rafael Group (including Carmel, Entrada, and Summerville Formations), and the Moenkopi Formation. Sampling of soils, salt crusts, and localized springs combined with baseflow synoptic sampling of the San Rafael River and Muddy Creek established that sulfur and strontium isotopic ratios were largely consistent with the hypothesized Mesozoic marine solute source. This work confirms the dominance of Mancos Shale derived solutes in the San Rafael River, but in Muddy Creek an important solute source in the San Rafael Group was identified in both baseflow and stormflow samples. Muddy Creek stormflow samples indicated a much greater contribution from the San Rafael Group than would be predicted by exposed surface area of geologic units. Time series monitoring of a stormflow event on the San Rafael River documents rapidly changing solute sources, implicating dynamic and reactive surface hydrology. Further development of this geologically based framework for solute tracking in the Upper Colorado River Basin will provide useful tools for a wide range of scientific questions as well as monitoring and management applications.