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Soil Mineralogy

Salt Mineralogy of Las Vegas Wash, Nevada

Morphology and Subsurface Evaporation

^a Dep. of Geoscience, Univ. of Nevada Las Vegas, 4505 Maryland Pkwy., Las Vegas, NV 89154
^b Nicholas School of the Environment and Earth Sciences, Duke Univ., Durham, NC 27708
^c Natural Resources Conservation Service, 5820 S. Pecos Rd. Bldg. A. Suite 400, Las Vegas NV 89120
^d Dep. of Geological Sciences, New Mexico State Univ., Las Cruces, NM 88003

^* Corresponding author (buckb{at}unlv.nevada.edu)

Las Vegas Wash drains the Las Vegas Basin in Nevada by capturing a series of tributaries and ending in Lake Mead and is being developed into an urban wetland. The soils are part of the Land series and contain high concentrations of pedogenic salts because of local sulfate-rich parent materials and high evapotranspiration rates. These salts cause damage to property, affect plant communities in the wetlands, and contribute to salinity in the Colorado River System. To gain a better understanding of these salts, two soil profiles were described. Whole soil samples were analyzed for ammonium acetate–extractable and water-soluble elemental analyses, cation exchange capacity (CEC), pH, electrical conductivity, and particle size. Salt minerals were analyzed using scanning electron microscope (SEM)/energy dispersive spectrometer (EDS) and x-ray diffraction (XRD). Results of XRD analyses indicate hexahydrite, bloedite, mirabilite, gypsum, thenardite, halite, vivianite, and sepiolite. SEM/EDS analyses found bloedite, eugsterite, halite, hexahydrite, gypsum, thenardite, and possibly kainite. Gypsum occurs at the surface and in all subsurface horizons. The more soluble salts occur at the surface and in two subsurface horizons. This study documents the third occurrence of eugsterite in the USA and is the first study to document salts other than gypsum forming snowball morphology. We interpret the subsurface zones of soluble salts to represent relict water tables where capillary action, combined with subsurface evaporation, has concentrated Na-Mg sulfates and halite. These relict water tables may represent previous high-level water tables that were lowered because of increased urbanization resulting in flooding, erosion, and incision of the Las Vegas Wash.

Abbreviations: CEC, cation exchange capacity • EC, electrical conductivity • EDS, energy dispersive spectrometer • LVW, Las Vegas Wash • LVW1, Las Vegas Wash 1 • LVW2, Las Vegas Wash 2 • SEM, scanning electron microscope • XRD, x-ray diffraction