Salt Mineralogy of Las Vegas Wash, Nevada: Morphology and Subsurface Evaporation

Las Vegas Wash drains the Las Vegas Basin in Nevada by capturing a series of tributaries and ending in LakeMead 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 highlevel water tables that were lowered because of increased urbanization resulting in flooding, erosion, and incision of the Las Vegas Wash. ASYSTEM OF TRIBUTARIES within Las Vegas Valley, Nevada, drains water into Las Vegas Wash (LVW) and finally into Lake Mead and the Colorado River. The LVWencompasses a saline meadow ecosystem that is being developed as an urban wetland. Before 1940, LVW was an ephemeral stream characterized by desertscrub vegetation typified by honey mesquite, Prosopis glandulosa Torr. and fourwing saltbush, Atriplex canescens (Malmberg, 1965). Population growth in the Las Vegas Valley during the 1950s increased wastewater and industrial discharges until flow in the LVW became permanent (Jackson and Patten, 1988). Cattail and reed marshes dominated the LVW by 1970 (Jackson and Patten, 1988). Continued urbanization increased the flow velocity in the LVW through urban runoff and increased irrigation of lawns and golf courses, sewage treatment discharges, and ground-water seepage.Urbanization also increased the impermeable surface area of the LVW drainage, and after 1980 significant erosion and incision of the LVW began (Jackson and Patten, 1988). Rapid urban and suburban growth in Las Vegas Valley for about 30 yr has involved consistent disruption of natural drainage paths and associated salt flux processes in the native soils. For overland and open channel flow, the net effect has been to collect and channelize flow through urban portions, totally impede and block sheet flow, and reduce the discharge capacity of lesser tributaries. Incision into the underlying unconsolidated Neogene Muddy Creek Formation is greater than 6 m in places and has drained much of the lower LVW marshes and reduced the extent of the hydrophytic vegetation, such as cattail (Typha domingensis), common reed (Phragmites australis), and Baltic rush (Juncus balticus), and has lowered water tables (Jackson and Patten, 1988). The lowered water tables have caused spatial shifts in vegetation. Hydrophytic vegetation, such as Sedge (Carex spp.), Baltic rush (Juncus balticus), and cattail (Typha domingensis) occurs where water tables are within approximately 60 cm of the surface and are replaced by phreatophytes salt cedar (Tamarix ramosissima) and arrowweed (Pluchea sericea) when lowered (Burbey, 1993). Because the LVW drains into Lake Mead, the source for approximately two thirds of the city of LasVegas’drinking water (Burbey, 1993), there are many relevant environmental concerns. Some of these include managing water quality standards (primarily to reduce the contribution of salts and contaminants to the Colorado River), flood control, maintenance as an artificial wetland for recreational and educational activities and wildlife habitat, selenium levels in the LVW, andwater rights for return flow to Lake Mead. Critical to addressing these issues is an understanding of the soil properties in the LVW. The soils in the LVW are heavily concentrated with salts as a result of high evapotranspiration rates combinedwith throughflow and runoff from calcic and gypsic parent materials, including Neogene Muddy Creek and Horse Springs Formations, Triassic Moenkopi and Chinle Formations, and a complex assemblage of Paleozoic marine carbonates (Page et al., 2003). Las Vegas has an average annual precipitation of approximately 10 cm yr, with an estimated evaporation rate of over 200 cm yr. The sampled soils in the LVWweremapped as part of the Land series: fine-silty, mixed, superactive, thermic Typic Aquisalids, in the 1977 Soil Survey of Las Vegas Valley. The Land series also occurs along two other southern Nevada perennial riparian systems: theVirgin andMuddy rivers. The occurrence of a surface crust of salt minerals because of capillary fringe evaporation is a typical feature B.J. Buck, Dep. of Geoscience, Univ. of Nevada Las Vegas, 4505 Maryland Pkwy., Las Vegas, NV 89154; K. Wolff, Nicholas School of the Environment and Earth Sciences, Duke Univ., Durham, NC 27708; D.J.Merkler,NaturalResourcesConservation Service, 5820 S. PecosRd. Bldg. A. Suite 400, Las Vegas NV 89120; N.J. McMillan, Dep. of Geological Sciences, New Mexico State Univ., Las Cruces, NM 88003. Received 19 Aug. 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:1639–1651 (2006). Soil Mineralogy and Urban Soils doi:10.2136/sssaj2005.0276 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA 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. R e p ro d u ce d fr o m S o il S ci e n ce S o ci e ty o f A m e ri ca Jo u rn a l. P u b lis h e d b y S o il S ci e n ce S o ci e ty o f A m e ri ca . A ll co p yr ig h ts re se rv e d . 1639 Published online August 3, 2006