Quantifying Shallow Groundwater-River Recharge and Flow Variability to Support Habitat Assessment Near the Moab UMTRA Site

Maria Castro, University of Arizona
mariaandrade@arizona.edu
Pieter Hazenberg, Florida International University
phazenbe@fiu.edu
Daniel Parras, Florida International University
dparr058@fiu.edu
Peter Troch, University of Arizona
patroch@arizona.edu

Abstract

The U.S. Department of Energy’s Moab Uranium Mill Tailings Site spans a Colorado River corridor that contains nursery backwaters for the endangered Colorado Pikeminnow (Ptychocheilus lucius). After milling ceased in 1984, 16 million tons of tailings remained; since then, ammonia and uranium have leached into the shallow alluvial aquifer, migrated toward the river, and threatened sensitive aquatic habitats. In response, the U.S. Department of Energy’s Office of Environmental Management (DOE-EM) is implementing remediation to reduce impacts on groundwater and surface water. In this reach, seasonal discharge variability, shallow groundwater–river exchange, and legacy tailings seepage together determine hydraulic and water-quality conditions that regulate nursery habitat. Recruitment depends on shallow, low-velocity backwaters created by spring floods and sustained by adequate summer baseflows, which provide warm, sheltered environments for larval and juvenile stages. However, altered flow regimes and potential contaminant exposure may compromise the persistence and quality of these features. Understanding the hydrologic drivers that sustain them is therefore key to effective management and recovery planning. This study is the first phase of a broader multidisciplinary effort to quantify how hydrologic variability and river hydraulics govern the availability and persistence of backwater nursery habitats adjacent to the Moab site. We integrated more than two decades of hydrologic and ecological monitoring, deriving seasonal discharge metrics—means, maxima, minima, and one-year lags for spring and summer—from daily flows at the Colorado River near Cisco (USGS 09180500) and relating them to long-term records of young-of-year (YOY) Colorado Pikeminnow abundance expressed as catch per unit effort (CPUE). Spearman rank correlations identified previous-year spring variability and spring maximum discharge as the strongest predictors of YOY abundance (|ρ| ≈ 0.37), indicating that antecedent hydrologic fluctuations may strongly influence spawning success and early survival. By contrast, Pearson correlations were generally weak (|r| < 0.20), with only the same spring-variability and spring-maximum metrics exhibiting modest linear associations (|r| ≈ 0.25), reinforcing that the flow–recruitment relationship is predominantly monotonic but nonlinear.  Complementing the hydrologic analysis, a two-dimensional unsteady flow model was developed in HEC-RAS to simulate floodplain connectivity and hydraulic conditions along the Moab reach. Simulation results delineate the spatiotemporal dynamics of depth, velocity, flood extent, and Froude number across the years. Going forward, we will evaluate this hydrology–hydraulics framework under climate-change scenarios to quantify shifts in backwater connectivity and persistence. These scenario runs will then be coupled with species-specific habitat suitability (HSI) and contaminant transport analyses to assess how uranium and ammonia exposures interact with changing flow conditions to influence long-term habitat resilience.