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An Assay Method for Determining Extra-Cellular Lipases from Pseudomonas aeruginosa

Description: The applicability of an isotopically labelled assay system to determine the lipase production in Pseudomonas aeruginosa was evaluated. Supernatant from cultures of Pseudomonas aeruginosa grown in a medium containing olive oil was incubated with a substrate containing labelled trioleate. Fatty acids were isolated by means of a liquid-liquid partition system. Enzyme activity was determined by measuring the amounts of free fatty acid by liquid scintillation counting. Findings indicate that the iso… more
Date: May 1978
Creator: Christensen, John N.
Item Type: Refine your search to only Thesis or Dissertation
Partner: UNT Libraries
open access

U-Sr isotopic speedometer: Fluid flow and chemical weatheringrates inaquifers

Description: Both chemical weathering rates and fluid flow are difficultto measure in natural systems. However, these parameters are critical forunderstanding the hydrochemical evolution of aquifers, predicting thefate and transport of contaminants, and for water resources/water qualityconsiderations. 87Sr/86Sr and (234U/238U) activity ratios are sensitiveindicators of water-rock interaction, and thus provide a means ofquantifying both flow and reactivity. The 87Sr/86Sr values in groundwaters are controlled… more
Date: December 27, 2005
Creator: Maher, Kate; DePaolo, Donald J. & Christensen, John N.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
open access

Uranium-series comminution ages of continental sediments: Case study of a Pleistocene alluvial fan

Description: Obtaining quantitative information about the timescales associated with sediment transport, storage, and deposition in continental settings is important but challenging. The uranium-series comminution age method potentially provides a universal approach for direct dating of Quaternary detrital sediments, and can also provide estimates of the sediment transport and storage timescales. (The word"comminution" means"to reduce to powder," reflecting the start of the comminution a… more
Date: April 30, 2010
Creator: Lee, Victoria E.; DePaolo, Donald J. & Christensen, John N.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
open access

Sediment transport time measured with U-Series isotopes: Resultsfrom ODP North Atlantic Drill Site 984

Description: High precision uranium isotope measurements of marineclastic sediments are used to measure the transport and storage time ofsediment from source to site of deposition. The approach is demonstratedon fine-grained, late Pleistocene deep-sea sediments from Ocean DrillingProgram Site 984A on the Bjorn Drift in the North Atlantic. The sedimentsare siliciclastic with up to 30 percent carbonate, and dated by sigma 18Oof benthic foraminifera. Nd and Sr isotopes indicate that provenance hasoscillated be… more
Date: June 5, 2006
Creator: DePaolo, Donald J.; Maher, Kate; Christensen, John N. & McManus,Jerry
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department

Isotopic Tracers for Biogeochemical Processes and Contaminant Transport: Hanford, Washington

Description: Our goal is to use isotopic measurements to understand how contaminants are introduced to and stored in the vadose zone, and what processes control migration from the vadose zone to groundwater and then to surface water. We have been using the Hanford Site in south-central Washington as our field laboratory, and our investigations are often stimulated by observations made as part of the groundwater monitoring program and vadose zone characterization activities. Understanding the transport of co… more
Date: April 19, 2007
Creator: DePaolo, Donald J.; Christensen, John N.; Conrad, Mark E. & Dresel, and P. Evan
Item Type: Refine your search to only Poster
Partner: UNT Libraries Government Documents Department
open access

Isotopic mass-dependence of metal cation diffusion coefficients in liquid water

Description: Isotope distributions in natural systems can be highly sensitive to the mass (m) dependence of solute diffusion coefficients (D) in liquid water. Isotope geochemistry studies routinely have assumed that this mass dependence either is negligible (as predicted by hydrodynamic theories) or follows a kinetic-theory-like inverse square root relationship (D {proportional_to} m{sup -0.5}). However, our recent experimental results and molecular dynamics (MD) simulations showed that the mass dependence … more
Date: January 11, 2009
Creator: Bourg, Ian C.; Richter, Frank M.; Christensen, John N. & Sposito, Garrison
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
open access

Laboratory Experiments Bearing on the Origin and Evolution of Olivine-rich Chondrules

Description: Evaporation rates of K2O, Na2O, and FeO from chondrule-like liquids and the associated potassium isotopic fractionation of the evaporation residues were measured to help understand the processes and conditions that affected the chemical and isotopic compositions of olivine-rich Type IA and Type IIA chondrules from Semarkona. Both types of chondrules show evidence of having been significantly or totally molten. However, these chondrules do not have large or systematic potassium isotopic fraction… more
Date: June 24, 2011
Creator: Richter, Frank M.; Mendybaev, Ruslan A.; Christensen, John N.; Ebel, Denton & Gaffney, Amy
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
open access

Isotopic Tracking of Hanford 300 Area Derived Uranium in the Columbia River

Description: 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, a… more
Date: October 31, 2010
Creator: Christensen, John N.; Dresel, P. Evan; Conrad, Mark E.; Patton, Gregory W. & DePaolo, Donald J.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
open access

Pb Isotopes as an Indicator of the Asian Contribution to Particulate Air Pollution in Urban California

Description: During the last two decades, expanding industrial activity in east Asia has led to increased production of airborne pollutants that can be transported to North America. Previous efforts to detect this trans-Pacific pollution have relied upon remote sensing and remote sample locations. We tested whether Pb isotope ratios in airborne particles can be used to directly evaluate the Asian contribution to airborne particles of anthropogenic origin in western North America, using a time series of samp… more
Date: October 25, 2010
Creator: Ewing, Stephanie A.; Christensen, John N.; Brown, Shaun T.; Vancuren, Richard A.; Cliff, Steven S. & DePaolo, Donald J.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
open access

Multi-Scale Mass Transfer Processes Controlling Natural Attenuation and Engineered Remediation: An IFRC Focused on Hanford’s 300 Area Uranium Plume January 2010 to January 2011

Description: The Integrated Field Research Challenge (IFRC) at the Hanford Site 300 Area uranium (U) plume addresses multi-scale mass transfer processes in a complex subsurface hydrogeologic setting where groundwater and riverwater interact. A series of forefront science questions on reactive mass transfer focus research. These questions relate to the effect of spatial heterogeneities; the importance of scale; coupled interactions between biogeochemical, hydrologic, and mass transfer processes; and measurem… more
Date: February 1, 2011
Creator: Zachara, John M.; Bjornstad, Bruce N.; Christensen, John N.; Conrad, Mark S.; Fredrickson, Jim K.; Freshley, Mark D. et al.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department
open access

Multi-Scale Mass Transfer Processes Controlling Natural Attenuation and Engineered Remediation: An IFRC Focused on Hanford’s 300 Area Uranium Plume January 2011 to January 2012

Description: The Integrated Field Research Challenge (IFRC) at the Hanford Site 300 Area uranium (U) plume addresses multi-scale mass transfer processes in a complex subsurface biogeochemical setting where groundwater and riverwater interact. A series of forefront science questions on reactive mass transfer motivates research. These questions relate to the effect of spatial heterogeneities; the importance of scale; coupled interactions between biogeochemical, hydrologic, and mass transfer processes; and mea… more
Date: March 5, 2012
Creator: Zachara, John M.; Bjornstad, Bruce N.; Christensen, John N.; Conrad, Mark S.; Fredrickson, Jim K.; Freshley, Mark D. et al.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department
open access

Multi-Scale Mass Transfer Processes Controlling Natural Attenuation and Engineered Remediation: An IFRC Focused on Hanford’s 300 Area Uranium Plume

Description: The Integrated Field-Scale Subsurface Research Challenge (IFRC) at the Hanford Site 300 Area uranium (U) plume addresses multi-scale mass transfer processes in a complex hydrogeologic setting where groundwater and riverwater interact. A series of forefront science questions on mass transfer are posed for research which relate to the effect of spatial heterogeneities; the importance of scale; coupled interactions between biogeochemical, hydrologic, and mass transfer processes; and measurements a… more
Date: February 1, 2010
Creator: Zachara, John M.; Bjornstad, Bruce N.; Christensen, John N.; Conrad, Mark E.; Fredrickson, Jim K.; Freshley, Mark D. et al.
Item Type: Refine your search to only Report
Partner: UNT Libraries Government Documents Department
open access

Identifying the sources of subsurface contamination at the Hanford site in Washington using high-precision uranium isotopic measurements

Description: In the mid-1990s, a groundwater plume of uranium (U) was detected in monitoring wells in the B-BX-BY Waste Management Area (WMA) at the Hanford Site in Washington. This area has been used since the late 1940s to store high-level radioactive waste and other products of U fuel-rod processing. Using multiple collector ICP source magnetic sector mass spectrometry (MC ICPMS) high precision uranium isotopic analyses were conducted of samples of vadose zone contamination and of groundwater. The ratios… more
Date: March 30, 2004
Creator: Christensen, John N.; Dresel, P. Evan; Conrad, Mark E.; Maher, Kate & DePaolo, Donald J.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department
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