Groundwater flow and its effect on salt dissolution in Gypsum Canyon watershed, Paradox Basin, southeast Utah

Groundwater flow is an important control on subsurface salt dissolution. Natural evaporites are salt deposits that fracture when dry or unloaded and flow ductilely when wet or loaded. These dynamics drive faulting and associated subsidence on the land surface. Dissolution also increases salinity in groundwater and river systems. This study aims to understand the groundwater flow system and its role in these processes. The study area is Gypsum Canyon watershed in the Paradox Basin of southeast Utah. The area comprises regional sedimentary formations underlain by salts of the Paradox Formation. Active faults in the region slip at a rate of ∼1-2 mm/year, probably due to salt flow and dissolution. This study characterizes the groundwater flow and solute transport systems of Gypsum Canyon watershed using a 3D finite element flow and transport model, SUTRA. Data collection included sampling and mapping groundwater springs, analyzing stable isotopes of water and total dissolved solids in spring samples, and conducting hydraulic conductivity tests, providing constraints for model parameters. Model results indicate that regional groundwater flow is northwest, towards the Colorado River and shallow flow systems are also present. Modeled solute output implies that groundwater-driven salt dissolution is small, the watershed contributes on the order of 10-3 tons of salt per meter of discharge to the Colorado River per year, and groundwater-driven dissolution cannot account for the volume changes implied by fault slip rates.