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Open Access Publications from the University of California

Scholarly Works (38 results)

  • Article
  • Peer Reviewed
Lawrence Berkeley National Laboratory (2012)
Our objectives in this study are to quantify the discharge rate of uranium (U) to the Columbia River from the Hanford Site's 300 Area, and to follow that U down river to constrain its fate. Uranium from the Hanford Site has variable isotopic composition due to nuclear industrial processes carried out at the site. This characteristic makes it possible to use high-precision isotopic measurements of U in environmental samples to identify even trace levels of contaminant U, determine its sources, and estimate discharge rates. Our data on river water samples indicate that as much as 3.2 kg/day can enter the Columbia River from the 300 Area, which is only a small fraction of the total load of dissolved natural background U carried by the Columbia River. This very low?level of Hanford derived U can be discerned, despite dilution to <1percent of natural background U, 350 km downstream from the Hanford Site. These results indicate that isotopic methods can allow the amounts of U from the 300 Area of the Hanford Site entering the Columbia River to be measured accurately to ascertain whether they are an environmental concern, or are insignificant relative to natural uranium background in the Columbia River.
    Cover page: Isotopic Tracking of Hanford 300 Area Derived Uranium in the Columbia River
    • Article
    LBL Publications (2003)
      Cover page: Measuring the timescales of sediment production, transport, and deposition-u-234 sediment comminution ages
      • Article
      LBL Publications (2003)
        Cover page: In-Situ Reaction Rates In The Vadose Zone At The Hanford Site Inferred From U Series Disequilibrium
        • Article
        • Peer Reviewed
        Lawrence Berkeley National Laboratory (2006)

        High precision uranium isotope measurements of marine clastic sediments are used to measure the transport and storage time of sediment from source to site of deposition. The approach is demonstrated on fine-grained, late Pleistocene deep-sea sediments from Ocean Drilling Program Site 984A on the Bjorn Drift in the North Atlantic. The sediments are siliciclastic with up to 30 percent carbonate, and dated by sigma 18O of benthic foraminifera. Nd and Sr isotopes indicate that provenance has oscillated between a proximal source during the last three interglacial periods volcanic rocks from Iceland and a distal continental source during glacial periods. An unexpected finding is that the 234U/238U ratios of the silicate portion of the sediment, isolated by leaching with hydrochloric acid, are significantly less than the secular equilibrium value and show large and systematic variations that are correlated with glacial cycles and sediment provenance. The 234U depletions are inferred to be due to alpha-recoil loss of 234Th, and are used to calculate "comminution ages" of the sediment -- the time elapsed between the generation of the small (

          Cover page: Sediment transport time measured with U-Series isotopes: Results from ODP North Atlantic Drill Site 984
          • Article
          LBL Publications (2004)
          Uranium and Th isotope measurements for some of the youngest basaltic- to rhyolitic lava flows associated with the Long Valley caldera and Mono-Inyo Craters indicate that the lavas have pronounced U and Th isotopic disequilibrium. The (238U/232Th) ratios range from 0.69 to 1.23, and the (232Th/230Th) ratios range from 0.80 to 1.03. The majority of the lavas plot to the right of the equiline, which corresponds with a subduction zone influence on the U and Th isotope compositions. A basaltic inclusion from the Mono Inyo dacite (age = 18 Ka, epsilon-Nd = +2) is one of the samples farthest to the right of the equiline, which indicates that the basaltic magmatism associated with the formation of the silicic magma system is subduction-like. In contrast, an older (ca. 100 Ka) trachybasalt lava from the Long Valley north moat (epsilon-Nd=-3) plots to left of the equiline. We interpret these differences as reflecting lithospheric sources for the small-volume alkalic lavas and asthenospheric (subduction-affected) sources for the larger volume silicic system. The preservation of strong U-Th isotopic disequilibrium in high-silica lavas clearly indicates that the rhyolites do not have long magma chamber residence times as has been inferred for other silicic systems in western North America. This observation is in accord with the young age of the Mono Craters (less than about 18 Ka) and suggests that this system has not yet stored up a large amount of silicic magma that could feed an eruption of Bishop Tuff size.
            Cover page: Short timescales of high-silica rhyolite generation in the Mono-Inyo Craters indicated by U-Th isotopic disequilibrium.
            • Article
            LBL Publications (2003)
              Cover page: Vadose zone weathering rates inferred from U-series disequilibrium
              • Article
              LBL Publications (2005)
              Long-term vadose zone infiltration rates at the Hanford Site, Washington, are critical for assessing the transport of radionuclides to the groundwater and eventually the Columbia River. Sr isotopes (87Sr/86Sr) in the unsaturated zone pore water are controlled by the ratio of the dissolution rate of the solids to the infiltration flux, and therefore provide either a long-term estimate of the infiltration flux if the weathering rate is constrained, or vise versa. The 234U/238U of the pore water responds to chemical weathering, infiltration, and the a-recoil flux of 234Th to the pore fluid. Previous estimates of the ratio of the dissolution rate to the infiltration flux from Sr isotopes for a 70 meter vadose zone core are combined with a model describing the evolution of 234U/238U values in the pore water and solids to constrain the infiltration flux and dissolution rate. The weathering rates calculated from the two isotopes systems behave differently in response to the assumed value for the infiltration rate weathering rates from the Sr method increase as a function of increasing infiltration, whereas U rates decrease as a function of increasing infiltration. This opposing behavior is due to the difference between the isotopic composition of the solids relative to the fluid at the top of the profile. The coupled model for both the 234U/238U and the 87Sr/86Sr data converges on a long-term infiltration flux of 3-5 mm/yr, and bulk silicate dissolution rates between 10-15.7 and 10-16.8 mol/m2/sec. This suggests that the coupled application of U and Sr isotopes in natural systems may be useful for constraining two widely important variables, fluid flow and reaction rates. In addition, methods to accurately determine the a-recoil loss fraction in heterogeneous sediments are also compared, including geometric arguments and measurements of the 234U/238U in fine-grained size fractions and strong acid leaches. In general, strong acid leaches provide a means of quantifying both the a-recoil loss fraction, and the isotopic composition of the dissolving solids. The U-series isotopic method may be especially useful in determining solid-fluid reaction rates where precise age constraints for the system are not available because the process of a-recoil loss, when accurately quantified, provides a chronometer for the dissolution rate.
                Cover page: A Coupled Sr/U isotopic method for measuring rates of silicate dissolution and infiltration rates at the Hanford Site, Wa.
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2019)
                We present the results of experiments involving the isotopic fractionation by diffusion of K+ cations in methanol at 298 K along with supporting molecular dynamic simulations. The experiments, using glass Rayleigh fractionators filled with a methanol solution of KCl, constrain the ratio of the individual diffusivities of 41K and 39K (D 41K/D 39K) in methanol, and so the isotopic fractionation of K+ due to diffusion in methanol, to 0.9995 ± 0.0001. This isotopic fractionation is 25% of the fractionation resulting from K+ diffusion in water. This is consistent with published molecular dynamics simulations indicating greater hydrodynamic coupling between K+ ions and solvating methanol molecules compared to K+ ions and solvating water molecules.
                  Cover page: Isotopic Fractionation of Potassium by Diffusion in Methanol
                  • Article
                  LBL Publications (2004)
                    Cover page: Strategy and Techniques for Monitoring Bioremediation of Metals and Radionuclides in the Vadose Zone and Groundwater at Contaminated Sites
                    • Article
                    • Peer Reviewed
                    Lawrence Berkeley National Laboratory (2005)

                    Both chemical weathering rates and fluid flow are difficult to measure in natural systems. However, these parameters are critical for understanding the hydrochemical evolution of aquifers, predicting the fate and transport of contaminants, and for water resources/water quality considerations. 87Sr/86Sr and (234U/238U) activity ratios are sensitive indicators of water-rock interaction, and thus provide a means of quantifying both flow and reactivity. The 87Sr/86Sr values in ground waters are controlled by the ratio of the dissolution rate to the flow rate. Similarly, the (234U/238U) ratio of natural ground waters is a balance between the flow rate and the dissolution of solids, and alpha-recoil loss of 234U from the solids. By coupling these two isotope systems it is possible to constrain both the long-term (ca. 100's to 1000's of years) flow rate and bulk dissolution rate along the flow path. Previous estimates of the ratio of the dissolution rate to the infiltration flux from Sr isotopes (87Sr/86Sr) are combined with a model for (234U/238U) to constrain the infiltration flux and dissolution rate for a 70-m deep vadose zone core from Hanford, Washington. The coupled model for both (234U/238U) ratios and the 87Sr/86Sr data suggests an infiltration flux of 5+-2 mm/yr, and bulk silicate dissolution rates between 10-15.7 and 10-16.5 mol/m2/s. The process of alpha-recoil enrichment, while primarily responsible for the observed variation in (234U/238U) of natural systems, is difficult to quantify. However, the rate of this process in natural systems affects the interpretation of most U-series data. Models for quantifying the alpha-recoil loss fraction based on geometric predictions, surface area constraints, and chemical methods are also presented. The agreement between the chemical and theoretical methods, such as direct measurement of (234U/238U) of the small grain size fraction and geometric calculations for that size fraction, is quite good.

                      Cover page: U-Sr isotopic speedometer: Fluid flow and chemical weathering rates inaquifers
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