George H. Swihart

Hydrochemistry and isotope composition of springs in the Tecopa basin, southeastern California, USA

Abstract Springs and seeps in the Tecopa basin were sampled and analyzed for their major ion chemistry and oxygen, hydrogen, and boron isotope compositions in order to assess the possible origins of the waters. The Tecopa basin is a groundwater discharge area along the Amargosa River drainage basin south of the Nevada Test Site and proposed high-level nuclear waste repository at Yucca Mountain, NV. The spring waters are categorized into three end-member hydrochemical facies: (1) pH-neutral, Na+Ca+Mg+SO4+HCO3 water with moderate to high total-dissolved solids; (2) mildly alkaline, Na+Cl+SO4+HCO3 water with high total dissolved solids; and (3) alkaline, Na+HCO3+CO3 water with high total dissolved solids. The temperatures of most of the springs range from 17°C to 32°C; however, waters in the Tecopa Hot Spring area range from 9°C to 47°C. The oxygen and hydrogen isotope compositions (normalized to SMOW) of the waters are as follows. The facies 1 springs have a mean δ18O of −12.77±0.14‰ (1σ) and δD of−94.9±0.5‰ (1σ). The facies 2 springs have a mean δ18O of −12.73±0.19‰ (1σ) and δD of −97.7±0.9‰ (1σ). The facies 3 spring has a mean δ18O of −13.09±0.17‰ (1σ) and δD of −103.5±0.7‰ (1σ). The mean boron concentrations and isotope compositions range from 2.4 mg/l and δ11B of +2.5‰ for facies 1 springs to 72.8 mg/l and δ11B of −3.8‰ for the facies 3 spring. The facies 1 waters have chemical and isotope compositions consistent with meteoric recharge in the Spring Mountains, NV, groundwater flow through the regional carbonate aquifer, and minor mixing with more concentrated water. The thermal, facies 2 waters are isotopically and chemically distinct from waters in the regional basin-fill and carbonate aquifer systems. These waters are interpreted to originate from fractured water-bearing strata beneath the regional carbonate aquifer that discharge only in the southernmost part of the Great Basin. The facies 3 spring is isotopically and chemically similar to basin-fill aquifer waters in the Amargosa Desert, but has higher sodium, alkalinity, and boron contents due to interaction with saline, alkaline-lake deposits of the Plio-Pleistocene Lake Tecopa beds. Geologic, geochemical, and isotope data suggest that these three distinct water types flow through hydrogeologic compartments in the Tecopa basin. The compartments are interpreted to have been created by the impermeable Lake Tecopa beds and geologic structures within the basin.

A BORON ISOTOPIC STUDY OF A MINERALOGICALLY ZONED LACUSTRINE BORATE DEPOSIT : THE KRAMER DEPOSIT, CALIFORNIA, U.S.A.

Abstract An investigation of the boron isotopic composition of hydrated borates in the Kramer lacustrine deposit of southern California was undertaken in an effort to better inderstand the origins of mineralogically zoned deposits of this type. Twenty-one samples from fifteen depths along a drill core through the deposit reveal that isotopic zoning accompanies mineral zoning. The range of δ 11 B for borax through the 24.9-m-thick Na-borate core facies is + 0.1 to + 1.7%. except for samples inand just below a clay-rich 6-m interval where δ 11 B varies from −5.1 to + 2.3%. The δ 11 B of three cottonball ulexite samples in a 3.0-m-thick NaCa-borate facies above the Na-borate facies ranges from −5.5 to −4.6%., whereas two samples from a 6.1-m-thick basal NaCa-borate facies both yield −2.1%. The δ 11 B of colemanite from near the top of the upper NaCa-borate facies is −8.6%. The small range of δ 11 B through much of the Na-borate facies indicates that the source waters of the borax-precipitating lake varied little in δ 11 B for a time interval of (5−7) · 10 4 yr. The δ 11 B variations within and among borax crystals in the 6-m clay-rich interval, some of which are estimated to have occurred over a period of weeks, were probably produced by a combination of pH change and Rayleigh effect during partial desiccation cycles. The distinctly different ranges of δ 11 B exhibited in the upper and lower NaCa-borate facies and the distinctive cottonball habit of the crystal aggregates indicate that ulexite originated through growth in the lake-margin muds.

A close look at silver oxide colloid for surface-enhanced Raman scattering.

A silver oxide hydrosol has been prepared by reacting silver nitrate with sodium hydroxide in an aqueous solution. The colloidal particle composition and particle size have been characterized by X-ray diffractometry and transmittance electron microscopy. We have demonstrated that the sol is surface-enhanced Raman scattering active. One major advantage of this sol is the absence of the spectral interference arising from reaction products of the sol formation process. The mechanism of surface-enhanced Raman scattering activity is essentially contributed by the silver particles generated from the photochemical process during the laser excitation.