Investigating hydrologic and environmental controls on uranium isotopes in a natural mountainous environment

Alissa White1, Jennifer McIntosh2, Lin Ma2, Bryan Moravec3, Jon Chorover3

1Department of Hydrology and Atmospheric Sciences, The University of Arizona, Tucson, Arizona

2Department of Geological Sciences, University of Texas at El Paso, El Paso, Texas

3Department of Soil, Water, and Environmental Science, The University of Arizona, Tucson, Arizona

Hydrologic processes in snowmelt-dominated regions of the semi-arid mountainous west are not well understood, yet those regions rely most heavily on mountain recharge for water. In the Jemez River Basin Critical Zone Observatory (JRB-CZO) in a remote NM headwater catchment within the Valles Caldera National Preserve, water isotopes and solute chemistry have shown that snowmelt infiltrates and is stored before later discharging into springs and streams via subsurface flowpaths that change seasonally. Therefore, water transit times (WTT) and water-rock interactions are expected to also change seasonally as hydrologic flowpaths vary. Uranium-series isotopes have recently been shown to be a novel tracer of water-rock reactions and source water contributions; therefore, this study seeks to understand how uranium isotope signatures evolve along different water flowpaths. More specifically, this work examines the relationship between seasonality, WTT, and U-series isotopes in several catchments within the JRB-CZO.

In order to determine the effect of WTT on the U isotopic composition of natural waters within the Valles Caldera, samples from ten springs, for which WTT are already known from tritium analysis, were collected during the dry seasons of 2015 and 2016 and analyzed for U and strontium isotopes. Preliminary results do not suggest that WTT can fully explain the variability of U (1.60 to 3.09) and Sr (0.70704 to 0.70817) isotope composition in springs throughout the JRB-CZO. Water samples were also collected from streams within three catchments across three water years to establish how seasonality controls water’s isotopic composition. U (1.97 to 2.06) and Sr (0.70737 to 0.70844) isotope values vary through changing seasons; however, those changes are not constant between catchments suggesting that differences in the mineralogy and structure of the deep Critical Zone likely also control isotopic variability. Ongoing work investigating the distribution of U-series isotopes in solid phase core samples with depth beneath the surface will be vital for the characterization of hydrogeologic controls on isotopic composition in this complex lithologic terrain. Understanding the controlling factors of U-series isotope variations in natural waters largely devoid of human interaction in the JRB-CZO will provide an important natural baseline for future U isotope studies and geochemical modeling efforts in geologically complex mountainous regions.

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