Academic Appointments

Administrative Appointments

  • Mendenhall Postdoctoral Fellow, U.S. Geological Survey (2005 - 2007)
  • Visiting Professor, Hydrogeology, Colorado College (2007 - 2007)
  • Assistant Professor, Stanford University (2007 - 2015)

Honors & Awards

  • Fellow, American Geophysical Union (2015)
  • James B. Macelwane Medal, American Geophysical Union (2015)
  • NSF CAREER Award, National Science Foundation (2013)
  • Allen V. Cox Award for Mentoring of Undergraduate Research, Stanford University (2012)
  • Distinguished Lecturer, Global Climate and Energy Project (GCEP ) (2012)
  • Terman Fellowship, Stanford University (2008-2011)
  • SEGRF Scholar, Lawrence Livermore National Laboratory (2002 - 2005)
  • ARCS Foundation Scholar, U.C. Berkeley (2000 - 2001)

Boards, Advisory Committees, Professional Organizations

  • Keynote Speaker: Goldschmidt Conference, Yokohama, Japan, Goldschmidt Conference (2016 - 2016)
  • Member, Policy and Planning Board (PPB), Stanford University (2015 - Present)
  • Steering Committee, National Science Foundation Critical Zone Observatory (CZO) Program, National Science Foundation (2015 - Present)
  • Member, U.S. Geological Survey Hiring Panel, U.S. Geological Survey (2015 - 2015)
  • Member, search committee for land-water systems position, EESS, Stanford University (2015 - 2015)
  • Participant, DOE-BER, Basic Research Needs for Environmental Management Workshop, Bethesda, MD, Department of Energy (2015 - 2015)
  • Participant, DOE-BES, Roundtable on Foundational Research Relevant to SubTER, Germantown, MD, Department of Energy (2015 - 2015)
  • Participant, SIno-U.S. Critical Zone Observatory Workshop, Guiyang, China, National Science Foundation (2015 - 2015)
  • Co-organizer of NSF workshop on “Research Infrastructure in Support of NSF-SEP Grand Challenges”, National Science Foundation (2014 - Present)
  • Director of Undergraduate Studies, GES Department, Stanford University (2014 - Present)
  • Co-organizer of NSF workshop on "The Role of Reactive Transport Models in Biogeochemical Sciences", National Science Foundation (2014 - 2015)
  • Organizer and instructor, Stanford Reactive Transport (StaRT) Summer School, Stanford University (2014 - 2015)
  • Member, School of Earth Sciences teaching task force, Stanford University (2013 - 2014)
  • Co-instructor, 2-day short course, "Reactive transport modeling using The Geochemist’s Workbench® ", Goldschmidt Conference (2013 - 2013)
  • Keynote Speaker, Goldschmidt Conference, Florence, Italy, Goldschmidt Conference (2013 - 2013)
  • Panelist, Hydrologic Sciences, National Sciences Foundation (2013 - 2013)
  • Theme Organizer (Climate, Weathering and Tectonics) and Session Chair, Goldschmidt Conference, Florence, Italy, Goldschmidt Conference (2013 - 2013)
  • Member, Stanford Center for Carbon Storage, Stanford University (2012 - Present)
  • Organizer, School of Earth Sciences Distinguished Lecture Program Committee, Stanford University (2012 - Present)
  • Member, Selection Committee for the Stanford Interdisciplinary Graduate Fellowships (SIGF), Stanford University (2012 - 2014)
  • Invited Abstracts: American Geophysical Union Fall Meeting, San Francisco, CA, American Geophysical Union (2012 - 2012)
  • Invited Abstracts: Goldschmidt Conference, Montreal, Canada;, Goldschmidt Conference (2012 - 2012)
  • Invited Lecturer, LCLS/SSRL Users' Meeting and Workshop, “Opportunities with Synchrotron Radiation at the Mesoscale", Stanford University (2012 - 2012)
  • Invited Lecturer, Symposium, “Opportunities with Synchrotron Radiation at the Mesoscale", Exxon-Mobil, DuPont, Schlumberger and General Electric, University of Oregon (2012 - 2012)
  • Member, Advisory Committee for Molecular Environmental and Interface Science (MEIS), SSRL/SLAC, Molecular Environmental and Interface Science (MEIS), SSRL/SLAC (2012 - 2012)
  • Invited Abstract, American Geophysical Union Fall Meeting, San Francisco, CA, American Geophysical Union (2011 - 2011)
  • Invited Abstracts: Goldschmidt Conference, Prague, Czech Republic, Goldschmidt Conference (2011 - 2011)
  • Invited Lecturer: Yale University, UC Berkeley, GCEP Research Symposium, Yale, UC Berkeley, and Global Climate and Energy Project (2011 - 2011)
  • Invited Participant: "Design of Global Environmental Gradient Experiments using International CZO (Critical Zone Observatory) Networks", University of Delaware (2011 - 2011)
  • Invited Participant: ICDP/Oman Drilling Workshop, International Continental Scientific Drilling Program (2011 - 2011)
  • Advisor for the GES Undergraduate Major and Minor, GES Department, Stanford University (2010 - Present)
  • Member, Geochronology Steering Committee, School of Earth Sciences, Stanford University (2010 - Present)
  • Member, SHRIMP-RG Advisory Committee, School of Earth Sciences, Stanford University (2010 - Present)
  • Co-instructor, 2-day short course, "Reactive transport modeling using The Geochemist’s Workbench® " (with Craig Bethke), Stanford University (2010 - 2010)
  • Invited Abstracts: American Geophysical Union Fall Meeting, San Francisco, CA, American Geophysical Union (2010 - 2010)
  • Invited Abstracts: Geological Society of America Conference, Denver, CO, Geological Society of America (2010 - 2010)
  • Invited Lecturer, California Institute of Technology, UCLA, Boston University, Rice University (2010 - 2010)
  • Keynote Speaker, Goldschmidt Conference, Knoxville, TN, Goldschmidt Conference (2010 - 2010)
  • Associate Editor, American Journal of Science (appointed through 2015), American Journal of Science (2009 - Present)
  • Instructor/Presenter, Bay Area Geoscapes Teacher Education Program, Stanford University (2009 - Present)
  • Member, Jasper Ridge Biological Preserve Advisory Board, Stanford University (2009 - Present)
  • Member, STREAM (Stanford Training, Research & Mentoring) Advisory Board, School of Earth Sciences, Stanford University (2009 - 2012)
  • Member, Undergraduate Field Program Committee, GES Department, Stanford University (2009 - 2012)
  • Organizer, GES Department Seminar Program, Stanford University (2009 - 2012)
  • Co-editor, (Special Volume): “Combined ecological and geologic perspectives in ecosystem studies”, Chemical Geology (2009 - 2009)
  • Invited Abstracts: American Geophysical Union (AGU) Fall Meeting, San Francisco, CA, American Geophysical Union (2009 - 2009)
  • Invited Lecturer, Duke University, UC Davis, University of Delaware (2009 - 2009)
  • Invited Participant: “Critical Zone II: Biological Aspects of Weathering”, Washington, DC, Washington, DC (2009 - 2009)
  • Member, DUSEL Experimental Design Team (THMCB), DUSEL (2009 - 2009)
  • Symposium Chair, Goldschmidt Conference: “Bridging the gap between theory and the field in critical zone processes”, Goldschmidt Conference (2009 - 2009)
  • ticipant, Experimental Coordination Workshop, Lead, SD, DUSEL (Deep Underground Science and Engineering Laboratory) (2009 - 2009)
  • Director, Stanford ICPMS/TIMS Facility, School of Earth Sciences, Stanford University (2008 - Present)
  • Member, Undergraduate Curriculum Committee, GES Department, Stanford University (2008 - Present)
  • Invited Abstracts: Geological Society of America Fall Meeting, Houston, TX, Geological Society of America (2008 - 2008)
  • Invited Lecturer, Lawrence Berkeley National Laboratory (2008 - 2008)
  • Member, Search Committee, Geochronology, Petrology, Geodynamics position, GES Department, Stanford University (2008 - 2008)
  • Symposium Chair, Goldschmidt Conference: “Chemical and isotopic tracers of sediment-pore fluid interactions”, Cologne, Germany, Goldschmidt Conference (2008 - 2008)
  • Symposium Chair, Goldschmidt Conference: “Isotopic and geochemical insights into the rates and mechanisms of erosion and weathering”, Cologne, Germany, Goldschmidt Conference (2008 - 2008)
  • Invited Lecturer, Yale University, ETH Zurich, Geological Society of Washington D.C., U.S. Geological Survey, Reston VA (2007 - 2007)
  • Symposium Chair, AGU Fall Meeting: “Controls on geochemical and biogeochemical processes in the critical zone”, San Francisco, CA, American Geophysical Union (2007 - 2007)

Professional Education

  • Ph.D., U.C. Berkeley, Earth and Planetary Sciences (2005)
  • M.S., U.C. Berkeley, Civil and Environmental Engineering (Fluid Mechanics/Hydrology) (2001)
  • B.S., Environmental Earth Sciences, Dartmouth College (1999)

Current Research and Scholarly Interests

Chemical reactions between fluids and minerals create the environments that are uniquely characteristic of Earth’s surface. For example, chemical weathering reactions support the growth of soils and organisms and regulate the flow of elements to the oceans. The rates of these reactions also control the release and storage of natural and human-derived contaminants. Over geologic timescales, mineral-fluid reactions have helped to maintain a mostly habitable planet. Over human timescales, these reactions will regulate our ability to use Earth’s resources, such as soils, waters, and minerals.

My research focuses on the rates of reactions in different environments using a combination of geochemical tools, including isotope geochemistry, geochemical and hydrologic modeling, and geochronology in order to address the following themes: (1) defining the controls on mineral-fluid reactions rates in the environment (2) finding new approaches to use mineral-fluid reactions to safely store carbon dioxide in the subsurface; and (3) development of isotopic approaches to study mineral-fluid reactions in the environments of Earth’s past. To support these research themes, I have constructed a new mass spectrometer and clean lab facility capable of high precision geochemical and isotopic measurements, and teach a number of classes and short courses on reactive transport.

My teaching focuses on introducing students to the questions and major challenges in low-temperature and environmental geochemistry, and the application of isotope geochemistry to environmental and geologic problems. In order to introduce incoming students to Earth surface processes, materials and geochemistry, I am also teaching a freshman seminar on forensic geoscience. At the graduate level, I offer classes on isotope geochemistry and modeling of environmental transformations and mass transfer processes (i.e., subsurface reactive transport).

2017-18 Courses

Stanford Advisees

All Publications

  • Relationships between CO2, thermodynamic limits on silicate weathering, and the strength of the silicate weathering feedback EARTH AND PLANETARY SCIENCE LETTERS Winnick, M. J., Maher, K. 2018; 485: 111–20
  • Global perturbation of the marine calcium cycle during the Permian-Triassic transition Geological Society of America Bulletin Silva-Tamayo, J., Payne, J. L., Wignall, P. B., Newton, R. J., Eisenhauer, A., DePaolo, D. J., Brown, S., Lau, K. V., Maher, K., Lehrmann, D. J., Altiner, D., Yu, M., Richoz, S., Paytan, A., Kump, L. R. 2018

    View details for DOI 10.1130/B31818.1

  • Growing new generations of critical zone scientists EARTH SURFACE PROCESSES AND LANDFORMS Wymore, A. S., West, N. R., Maher, K., Sullivan, P. L., Harpold, A., Karwan, D., Marshall, J. A., Perdrial, J., Rempe, D. M., Ma, L. 2017; 42 (14): 2498–2502

    View details for DOI 10.1002/esp.4196

    View details for Web of Science ID 000414348200022

  • Impact of Organics and Carbonates on the Oxidation and Precipitation of Iron during Hydraulic Fracturing of Shale ENERGY & FUELS Jew, A. D., Dustin, M. K., Harrison, A. L., Joe-Wong, C. M., Thomas, D. L., Maher, K., Brown, G. E., Bargar, J. R. 2017; 31 (4): 3643-3658
  • Snowmelt controls on concentration-discharge relationships and the balance of oxidative and acid-base weathering fluxes in an alpine catchment, East River, Colorado WATER RESOURCES RESEARCH Winnick, M. J., Carroll, R. W., Williams, K. H., Maxwell, R. M., Dong, W., Maher, K. 2017; 53 (3): 2507-2523
  • Expanding the role of reactive transport models in critical zone processes EARTH-SCIENCE REVIEWS Li, L., Maher, K., Navarre-Sitchler, A., Druhan, J., Meile, C., Lawrence, C., Moore, J., Perdrial, J., Sullivan, P., Thompson, A., Jin, L., Bolton, E. W., Brantley, S. L., Dietrich, W. E., Mayer, K. U., Steefel, C. I., Valocchi, A., Zachara, J., Kocar, B., McIntosh, J., Tutolo, B. M., Kumar, M., Sonnenthal, E., Bao, C., Beisman, J. 2017; 165: 280-301
  • The influence of mixing on stable isotope ratios in porous media: A revised Rayleigh model WATER RESOURCES RESEARCH Druhan, J. L., Maher, K. 2017; 53 (2): 1101-1124
  • An evaluation of paired delta O-18 and (U-234/U-238)(0) in opal as a tool for paleoclimate reconstruction in semi-arid environments CHEMICAL GEOLOGY Oster, J. L., Kitajima, K., Valley, J. W., Rogers, B., Maher, K. 2017; 449: 236-252
  • Uranium isotope evidence for temporary ocean oxygenation in the aftermath of the Sturtian Snowball Earth EARTH AND PLANETARY SCIENCE LETTERS Lau, K. V., Macdonald, F. A., Maher, K., Payne, J. L. 2017; 458: 282-292
  • Quantifying Cr(VI) Production and Export from Serpentine Soil of the California Coast Range ENVIRONMENTAL SCIENCE & TECHNOLOGY McClain, C. N., Fendorf, S., Webb, S. M., Maher, K. 2017; 51 (1): 141-149


    Hexavalent chromium (Cr(VI)) is generated in serpentine soils and exported to surface and groundwaters at levels above health-based drinking water standards. Although Cr(VI) concentrations are elevated in serpentine soil pore water, few studies have reported field evidence documenting Cr(VI) production rates and fluxes that govern Cr(VI) transport from soil to water sources. We report Cr speciation (i) in four serpentine soil depth profiles derived from the California Coast Range serpentinite belt and (ii) in local surface waters. Within soils, we detected Cr(VI) in the same horizons where Cr(III)-minerals are colocated with biogenic Mn(III/IV)-oxides, suggesting Cr(VI) generation through oxidation by Mn-oxides. Water-extractable Cr(VI) concentrations increase with depth constituting a 7.8 to 12 kg/km(2) reservoir of Cr(VI) in soil. Here, Cr(VI) is produced at a rate of 0.3 to 4.8 kg Cr(VI)/km(2)/yr and subsequently flushed from soil during water infiltration, exporting 0.01 to 3.9 kg Cr(VI)/km(2)/yr at concentrations ranging from 25 to 172 μg/L. Although soil-derived Cr(VI) is leached from soil at concentrations exceeding 10 μg/L, due to reduction and dilution during transport to streams, Cr(VI) levels measured in local surface waters largely remain below California's drinking water limit.

    View details for DOI 10.1021/acs.est.6b03484

    View details for Web of Science ID 000391346900016

    View details for PubMedID 27935688

  • Critical zone structure controls concentration-discharge relationships and solute generation in forested tropical montane watersheds Water Resources Research Wymore, A. S., Brereton, R. L., Ibarra, D. E., Maher, K., McDowell, W. H. 2017; 53 (7): 6279-6295

    View details for DOI 10.1002/2016WR020016

  • Effects of surface structural disorder and surface coverage on isotopic fractionation during Zn(II) adsorption onto quartz and amorphous silica surfaces Geochimica et Cosmochimica Acta Nelson, J., Wasylenki, L., Bargar, J. R., Brown Jr., G. E., Maher, K. 2017; 215: 354-376
  • Concentration–discharge patterns of weathering products from global rivers Acta Geochimica Ibarra, D. E., Moon, S., Caves, J. K., Chamberlain, C., Maher, K. 2017
  • Surface ages and weathering rates from Be-10 (meteoric) and Be-10/Be-9: Insights from differential mass balance and reactive transport modeling CHEMICAL GEOLOGY Maher, K., von Blanckenburg, F. 2016; 446: 70-86
  • Geochemistry of CO2-rich waters in Iceland CHEMICAL GEOLOGY Thomas, D. L., Bird, D. K., Arnorsson, S., Maher, K. 2016; 444: 158-179
  • Clumped-isotope thermometry of magnesium carbonates in ultramafic rocks GEOCHIMICA ET COSMOCHIMICA ACTA del Real, P. G., Maher, K., Kluge, T., Bird, D. K., Brown, G. E., John, C. M. 2016; 193: 222-250
  • Aluminous gneiss derived by weathering of basaltic source rocks in the Neoarchean Storo Supracrustal Belt, southern West Greenland CHEMICAL GEOLOGY Szilas, K., Maher, K., Bird, D. K. 2016; 441: 63-80
  • Differential weathering of basaltic and granitic catchments from concentration-discharge relationships GEOCHIMICA ET COSMOCHIMICA ACTA Ibarra, D. E., Caves, J. K., Moon, S., Thomas, D. L., Hartmann, J., Chamberlain, C. P., Maher, K. 2016; 190: 265-293
  • Cenozoic carbon cycle imbalances and a variable weathering feedback EARTH AND PLANETARY SCIENCE LETTERS Caves, J. K., Jost, A. B., Lau, K. V., Maher, K. 2016; 450: 152-163
  • Isotopic Evidence for Reductive Immobilization of Uranium Across a Roll-Front Mineral Deposit ENVIRONMENTAL SCIENCE & TECHNOLOGY Brown, S. T., Basu, A., Christensen, J. N., Reimus, P., Heikoop, J., Simmons, A., WoldeGabriel, G., Maher, K., Weaver, K., Clay, J., DePaolo, D. J. 2016; 50 (12): 6189-6198


    We use uranium (U) isotope ratios to detect and quantify the extent of natural U reduction in groundwater across a roll front redox gradient. Our study was conducted at the Smith Ranch-Highland in situ recovery (ISR) U mine in eastern Wyoming, USA, where economic U deposits occur in the Paleocene Fort Union formation. To evaluate the fate of aqueous U in and adjacent to the ore body, we investigated the chemical composition and isotope ratios of groundwater samples from the roll-front type ore body and surrounding monitoring wells of a previously mined area. The (238)U/(235)U of groundwater varies by approximately 3‰ and is correlated with U concentrations. Fluid samples down-gradient of the ore zone are the most depleted in (238)U and have the lowest U concentrations. Activity ratios of (234)U/(238)U are ∼5.5 up-gradient of the ore zone, ∼1.0 in the ore zone, and between 2.3 and 3.7 in the down-gradient monitoring wells. High-precision measurements of (234)U/(238)U and (238)U/(235)U allow for development of a conceptual model that evaluates both the migration of U from the ore body and the extent of natural attenuation due to reduction. We find that the premining migration of U down-gradient of the delineated ore body is minimal along eight transects due to reduction in or adjacent to the ore body, whereas two other transects show little or no sign of reduction in the down-gradient region. These results suggest that characterization of U isotopic ratios at the mine planning stage, in conjunction with routine geochemical analyses, can be used to identify where more or less postmining remediation will be necessary.

    View details for DOI 10.1021/acs.est.6b00626

    View details for Web of Science ID 000378469900010

    View details for PubMedID 27203292

  • Chromium fluxes and speciation in ultramafic catchments and global rivers CHEMICAL GEOLOGY McClain, C. N., Maher, K. 2016; 426: 135-157
  • Marine anoxia and delayed Earth system recovery after the end-Permian extinction. Proceedings of the National Academy of Sciences of the United States of America Lau, K. V., Maher, K., Altiner, D., Kelley, B. M., Kump, L. R., Lehrmann, D. J., Silva-Tamayo, J. C., Weaver, K. L., Yu, M., Payne, J. L. 2016; 113 (9): 2360-2365


    Delayed Earth system recovery following the end-Permian mass extinction is often attributed to severe ocean anoxia. However, the extent and duration of Early Triassic anoxia remains poorly constrained. Here we use paired records of uranium concentrations ([U]) and (238)U/(235)U isotopic compositions (δ(238)U) of Upper Permian-Upper Triassic marine limestones from China and Turkey to quantify variations in global seafloor redox conditions. We observe abrupt decreases in [U] and δ(238)U across the end-Permian extinction horizon, from ∼3 ppm and -0.15‰ to ∼0.3 ppm and -0.77‰, followed by a gradual return to preextinction values over the subsequent 5 million years. These trends imply a factor of 100 increase in the extent of seafloor anoxia and suggest the presence of a shallow oxygen minimum zone (OMZ) that inhibited the recovery of benthic animal diversity and marine ecosystem function. We hypothesize that in the Early Triassic oceans-characterized by prolonged shallow anoxia that may have impinged onto continental shelves-global biogeochemical cycles and marine ecosystem structure became more sensitive to variation in the position of the OMZ. Under this hypothesis, the Middle Triassic decline in bottom water anoxia, stabilization of biogeochemical cycles, and diversification of marine animals together reflect the development of a deeper and less extensive OMZ, which regulated Earth system recovery following the end-Permian catastrophe.

    View details for DOI 10.1073/pnas.1515080113

    View details for PubMedID 26884155

  • The imprint of climate and geology on the residence times of groundwater GEOPHYSICAL RESEARCH LETTERS Maxwell, R. M., Condon, L. E., Kollet, S. J., Maher, K., Haggerty, R., Forrester, M. M. 2016; 43 (2): 701-708
  • Physico-Chemical Heterogeneity of Organic-Rich Sediments in the Rifle Aquifer, CO: Impact on Uranium Biogeochemistry ENVIRONMENTAL SCIENCE & TECHNOLOGY Janot, N., Pacheco, J. S., Pham, D. Q., O'Brien, T. M., Hausladen, D., Noel, V., Lallier, F., Maher, K., Fendorf, S., Williams, K. H., Long, P. E., Bargar, J. R. 2016; 50 (1): 46-53
  • Multi-phase flow simulation of CO2 leakage through a fractured caprock in response to mitigation strategies INTERNATIONAL JOURNAL OF GREENHOUSE GAS CONTROL Vialle, S., Druhan, J. L., Maher, K. 2016; 44: 11-25
  • Stable runoff and weathering fluxes into the oceans over Quaternary climate cycles NATURE GEOSCIENCE von Blanckenburg, F., Bouchez, J., Ibarra, D. E., Maher, K. 2015; 8 (7): 538-U146

    View details for DOI 10.1038/NGEO2452

    View details for Web of Science ID 000357404200016

  • Isotopic and Geochemical Tracers for U(VI) Reduction and U Mobility at an in Situ Recovery U Mine. Environmental science & technology Basu, A., Brown, S. T., Christensen, J. N., DePaolo, D. J., Reimus, P. W., Heikoop, J. M., WoldeGabriel, G., Simmons, A. M., House, B. M., Hartmann, M., Maher, K. 2015; 49 (10): 5939-5947


    In situ recovery (ISR) uranium (U) mining mobilizes U in its oxidized hexavalent form (U(VI)) by oxidative dissolution of U from the roll-front U deposits. Postmining natural attenuation of residual U(VI) at ISR mines is a potential remediation strategy. Detection and monitoring of naturally occurring reducing subsurface environments are important for successful implementation of this remediation scheme. We used the isotopic tracers (238)U/(235)U (δ(238)U), (234)U/(238)U activity ratio, and (34)S/(32)S (δ(34)S), and geochemical measurements of U ore and groundwater collected from 32 wells located within, upgradient, and downgradient of a roll-front U deposit to detect U(VI) reduction and U mobility at an ISR mining site at Rosita, TX, USA. The δ(238)U in Rosita groundwater varies from +0.61‰ to -2.49‰, with a trend toward lower δ(238)U in downgradient wells. The concurrent decrease in U(VI) concentration and δ(238)U with an ε of 0.48‰ ± 0.08‰ is indicative of naturally occurring reducing environments conducive to U(VI) reduction. Additionally, characteristic (234)U/(238)U activity ratio and δ(34)S values may also be used to trace the mobility of the ore zone groundwater after mining has ended. These results support the use of U isotope-based detection of natural attenuation of U(VI) at Rosita and other similar ISR mining sites.

    View details for DOI 10.1021/acs.est.5b00701

    View details for PubMedID 25909757

  • Sedimentary reservoir oxidation during geologic CO2 sequestration GEOCHIMICA ET COSMOCHIMICA ACTA Lammers, L. N., Brown, G. E., Bird, D. K., Thomas, R. B., Johnson, N. C., Rosenbauer, R. J., Maher, K. 2015; 155: 30-46
  • Steering of westerly storms over western North America at the Last Glacial Maximum NATURE GEOSCIENCE Oster, J. L., Ibarra, D. E., Winnick, M. J., Maher, K. 2015; 8 (3): 201-205

    View details for DOI 10.1038/NGEO2365

    View details for Web of Science ID 000350770900017

  • Rise and fall of late Pleistocene pluvial lakes in response to reduced evaporation and precipitation: Evidence from Lake Surprise, California GEOLOGICAL SOCIETY OF AMERICA BULLETIN Ibarra, D. E., Egger, A. E., Weaver, K. L., Harris, C. R., Maher, K. 2014; 126 (11-12): 1387-1415

    View details for DOI 10.1130/B31014.1

    View details for Web of Science ID 000343759600001

  • The impact of neogene grassland expansion and aridification on the isotopic composition of continental precipitation GLOBAL BIOGEOCHEMICAL CYCLES Chamberlain, C. P., Winnick, M. J., Mix, H. T., Chamberlain, S. D., Maher, K. 2014; 28 (9): 992-1004
  • Modeling the influence of organic acids on soil weathering GEOCHIMICA ET COSMOCHIMICA ACTA Lawrence, C., Harden, J., Maher, K. 2014; 139: 487-507
  • Uranium Incorporation into Amorphous Silica ENVIRONMENTAL SCIENCE & TECHNOLOGY Massey, M. S., Lezama-Pacheco, J. S., Nelson, J. M., Fendor, S., Maher, K. 2014; 48 (15): 8636-8644


    High concentrations of uranium are commonly observed in naturally occurring amorphous silica (including opal) deposits, suggesting that incorporation of U into amorphous silica may represent a natural attenuation mechanism and promising strategy for U remediation. However, the stability of uranium in opaline silicates, determined in part by the binding mechanism for U, is an important factor in its long-term fate. U may bind directly to the opaline silicate matrix, or to materials such as iron (hydr)oxides that are subsequently occluded within the opal. Here, we examine the coordination environment of U within opaline silica to elucidate incorporation mechanisms. Precipitates (with and without ferrihydrite inclusions) were synthesized from U-bearing sodium metasilicate solutions, buffered at pH ∼ 5.6. Natural and synthetic solids were analyzed with X-ray absorption spectroscopy and a suite of other techniques. In synthetic amorphous silica, U was coordinated by silicate in a double corner-sharing coordination geometry (Si at ∼ 3.8-3.9 Å) and a small amount of uranyl and silicate in a bidentate, mononuclear (edge-sharing) coordination (Si at ∼ 3.1-3.2 Å, U at ∼ 3.8-3.9 Å). In iron-bearing synthetic solids, U was adsorbed to iron (hydr)oxide, but the coordination environment also contained silicate in both edge-sharing and corner-sharing coordination. Uranium local coordination in synthetic solids is similar to that of natural U-bearing opals that retain U for millions of years. The stability and extent of U incorporation into opaline and amorphous silica represents a long-term repository for U that may provide an alternative strategy for remediation of U contamination.

    View details for DOI 10.1021/es501064m

    View details for Web of Science ID 000340080600039

    View details for PubMedID 24984107

  • Olivine dissolution and carbonation under conditions relevant for in situ carbon storage CHEMICAL GEOLOGY Johnson, N. C., Thomas, B., Maher, K., Rosenbauer, R. J., Bird, D., Brown, G. E. 2014; 373: 93-105

    View details for DOI 10.2475/04.2014.01

    View details for Web of Science ID 000345105100001

  • Hydrologic regulation of chemical weathering and the geologic carbon cycle. Science Maher, K., CHAMBERLAIN, C. P. 2014; 343 (6178): 1502-1504


    Earth's temperature is thought to be regulated by a negative feedback between atmospheric CO2 levels and chemical weathering of silicate rocks that operates over million-year time scales. To explain variations in the strength of the weathering feedback, we present a model for silicate weathering that regulates climatic and tectonic forcing through hydrologic processes and imposes a thermodynamic limit on weathering fluxes, based on the physical and chemical properties of river basins. Climate regulation by silicate weathering is thus strongest when global topography is elevated, similar to the situation today, and lowest when global topography is more subdued, allowing planetary temperatures to vary depending on the global distribution of topography and mountain belts, even in the absence of appreciable changes in CO2 degassing rates.

    View details for DOI 10.1126/science.1250770

    View details for PubMedID 24625927

  • Uranium isotopes in soils as a proxy for past infiltration and precipitation across the western United States AMERICAN JOURNAL OF SCIENCE Maher, K., Ibarra, D. E., Oster, J. L., Miller, D. M., Redwine, J. L., Reheis, M. C., Harden, J. C. 2014; 314: 821-857
  • Relationships between the transit time of water and the fluxes of weathered elements through the critical zone Geochemistry of the Earth's Surface (GES) Meeting Maher, K., Druhan, J. ELSEVIER SCIENCE BV. 2014: 16–22
  • A model linking stable isotope fractionation to water flux and transit times in heterogeneous porous media Geochemistry of the Earth's Surface (GES) Meeting Druhan, J. L., Maher, K. ELSEVIER SCIENCE BV. 2014: 179–188
  • Rise and fall of late Pleistocene pluvial lakes in response to reduced evaporation and precipitation: Evidence from Lake Surprise, California GEOLOGICAL SOCIETY OF AMERICA BULLETIN Ibarra, D. E., Egger, A. E., Weaver, K. L., Harris, C. R., Maher, K. 2014; 126 (11-12): 1387-1415

    View details for DOI 10.1130/B31014.1

  • Uranium incorporation into amorphous silica ENVIRONMENTAL SCIENCE AND TECHNOLOGY Massey, M., Lezama-Pacheco, J. S., Nelson, J. M., Fendorf, S., Maher, K. 2014; (in press)
  • A Teaching Exercise To Introduce Stable Isotope Fractionation of Metals into Geochemistry Courses JOURNAL OF CHEMICAL EDUCATION Weiss, D. J., Harris, C., Maher, K., Bullen, T. 2013; 90 (8): 1014-1017

    View details for DOI 10.1021/ed300370d

    View details for Web of Science ID 000323462900010

  • Environmental Speciation of Actinides INORGANIC CHEMISTRY Maher, K., Bargar, J. R., Brown, G. E. 2013; 52 (7): 3510-3532


    Although minor in abundance in Earth's crust (U, 2-4 ppm; Th, 10-15 ppm) and in seawater (U, 0.003 ppm; Th, 0.0007 ppm), light actinides (Th, Pa, U, Np, Pu, Am, and Cm) are important environmental contaminants associated with anthropogenic activities such as the mining and milling of uranium ores, generation of nuclear energy, and storage of legacy waste resulting from the manufacturing and testing of nuclear weapons. In this review, we discuss the abundance, production, and environmental sources of naturally occurring and some man-made light actinides. As is the case with other environmental contaminants, the solubility, transport properties, bioavailability, and toxicity of actinides are dependent on their speciation (composition, oxidation state, molecular-level structure, and nature of the phase in which the contaminant element or molecule occurs). We review the aqueous speciation of U, Np, and Pu as a function of pH and Eh, their interaction with common inorganic and organic ligands in natural waters, and some of the common U-containing minerals. We also discuss the interaction of U, Np, Pu, and Am solution complexes with common Earth materials, including minerals, colloids, gels, natural organic matter (NOM), and microbial organisms, based on simplified model system studies. These surface interactions can inhibit (e.g., sorption to mineral surfaces, formation of insoluble biominerals) or enhance (e.g., colloid-facilitated transport) the dispersal of light actinides in the biosphere and in some cases (e.g., interaction with dissimilatory metal-reducing bacteria, NOM, or Mn- and Fe-containing minerals) can modify the oxidation states and, consequently, the behavior of redox-sensitive light actinides (U, Np, and Pu). Finally, we review the speciation of U and Pu, their chemical transformations, and cleanup histories at several U.S. Department of Energy field sites that have been used to mill U ores, produce fissile materials for reactors and weapons, and store high-level nuclear waste from both civilian and defense operations, including Hanford, WA; Rifle, CO; Oak Ridge, TN; Fernald, OH; Fry Canyon, UT; and Rocky Flats, CO.

    View details for DOI 10.1021/ic301686d

    View details for Web of Science ID 000317094300009

    View details for PubMedID 23137032

  • Uranium comminution ages: Sediment transport and deposition time scales COMPTES RENDUS GEOSCIENCE DePaolo, D. J., Lee, V. E., Christensen, J. N., Maher, K. 2012; 344 (11-12): 678-687
  • Influence of eolian deposition and rainfall amounts on the U-isotopic composition of soil water and soil minerals GEOCHIMICA ET COSMOCHIMICA ACTA Oster, J. L., Ibarra, D. E., Harris, C. R., Maher, K. 2012; 88: 146-166
  • The role of fluid residence time and topographic scales in determining chemical fluxes from landscapes EARTH AND PLANETARY SCIENCE LETTERS Maher, K. 2011; 312 (1-2): 48-58
  • Evolution of hillslope soils: The geomorphic theater and the geochemical play APPLIED GEOCHEMISTRY Yoo, K., Weinman, B., Mudd, S. M., Hurst, M., Attal, M., Maher, K. 2011; 26: S149-S153
  • The dependence of chemical weathering rates on fluid residence time EARTH AND PLANETARY SCIENCE LETTERS Maher, K. 2010; 294 (1-2): 101-110
  • Climatic and vegetation control on sediment dynamics during the last glacial cycle GEOLOGY Dosseto, A., Hesse, P. P., Maher, K., Fryirs, K., Turner, S. 2010; 38 (5): 395-398

    View details for DOI 10.1130/G30708.1

    View details for Web of Science ID 000277220900003

  • Uranyl-chlorite sorption/desorption: Evaluation of different U(VI) sequestration processes GEOCHIMICA ET COSMOCHIMICA ACTA Singer, D. M., Maher, K., Brown, G. E. 2009; 73 (20): 5989-6007
  • Combined ecological and geologic perspectives in ecosystem studies Preface CHEMICAL GEOLOGY Holloway, J. M., Ewing, S. A., Maher, K. 2009; 267 (1-2): 1-2
  • The role of reaction affinity and secondary minerals in regulating chemical weathering rates at the Santa Cruz Soil Chronosequence, California GEOCHIMICA ET COSMOCHIMICA ACTA Maher, K., Steefel, C. I., White, A. F., Stonestrom, D. A. 2009; 73 (10): 2804-2831
  • Chemical weathering of a marine terrace chronosequence, Santa Cruz, California. Part II: Solute profiles, gradients and the comparisons of contemporary and long-term weathering rates GEOCHIMICA ET COSMOCHIMICA ACTA White, A. F., Schulz, M. S., Stonestrom, D. A., Vivit, D. V., Fitzpatrick, J., Bullen, T. D., Maher, K., Blum, A. E. 2009; 73 (10): 2769-2803
  • Fluid-Rock Interaction: A Reactive Transport Approach 19th Annual V M Goldschmidt Conference Steefel, C. I., Maher, K. MINERALOGICAL SOC AMER. 2009: 485–532
  • Field evidence for strong chemical separation of contaminants in the Hanford vadose zone VADOSE ZONE JOURNAL Conrad, M. E., DePaolo, D. J., Maher, K., Gee, G. W., Ward, A. L. 2007; 6 (4): 1031-1041
  • Th-230-U dating of surficial deposits using the ion microprobe (SHRIMP-RG): A micro stratigraphic perspective Conference on Dating Quaternary Sediments and Landforms in Drylands Maher, K., Wooden, J. L., Paces, J. B., Miller, D. M. PERGAMON-ELSEVIER SCIENCE LTD. 2007: 15–28
  • U-Sr isotopic speedometer: Fluid flow and chemical weathering rates in aquifers GEOCHIMICA ET COSMOCHIMICA ACTA Maher, K., DePaolo, D. J., Christensen, J. N. 2006; 70 (17): 4417-4435
  • Sediment transport time measured with U-series isotopes: Results from ODP North Atlantic drift site 984 EARTH AND PLANETARY SCIENCE LETTERS DePaolo, D. J., Maher, K., Christensen, J. N., McManus, J. 2006; 248 (1-2): 394-410
  • Dissolution rates and vadose zone drainage from strontium isotope measurements of groundwater in the Pasco Basin, WA unconfined aquifer JOURNAL OF HYDROLOGY Singleton, M. J., Maher, K., DePaolo, D. J., Conrad, M. E., Dresel, P. E. 2006; 321 (1-4): 39-58
  • The mineral dissolution rate conundrum: Insights from reactive transport modeling of U isotopes and pore fluid chemistry in marine sediments GEOCHIMICA ET COSMOCHIMICA ACTA Maher, K., Steefel, C. I., DePaolo, D. J., Viani, B. E. 2006; 70 (2): 337-363
  • Rates of silicate dissolution in deep-sea sediment: In situ measurement using U-234/U-238 of pore fluids GEOCHIMICA ET COSMOCHIMICA ACTA Maher, K., DePaolo, D. J., Lin, J. C. 2004; 68 (22): 4629-4648
  • Identifying the sources of subsurface contamination at the Hanford Site in Washington using high-precision uranium isotopic measurements ENVIRONMENTAL SCIENCE & TECHNOLOGY Christensen, J. N., Dresel, P. E., Conrad, M. E., Maher, K., DePaolo, D. J. 2004; 38 (12): 3330-3337


    In the mid-1990s, a groundwater plume of uranium (U) was detected in monitoring wells in the B-BX-BY Waste Management Area 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, high-precision uranium isotopic analyses were conducted of samples of vadose zone contamination and of groundwater. The isotope ratios 236U/238U, 234U/238U, and 238U/235U are used to distinguish contaminant sources. On the basis of the isotopic data, the source of the groundwater contamination appears to be related to a 1951 overflow event at tank BX-102 that spilled high-level U waste into the vadose zone. The U isotopic variation of the groundwater plume is a result of mixing between contaminant U from this spill and natural background U. Vadose zone U contamination at tank B-110 likely predates the recorded tank leak and can be ruled out as a significant source of groundwater contamination, based on the U isotopic composition. The locus of vadose zone contamination is displaced from the initial locus of groundwater contamination, indicating that lateral migration in the vadose zone was at least 8 times greater than vertical migration. The time evolution of the groundwater plume suggests an average U migration rate of approximately 0.7-0.8 m/day showing slight retardation relative to a groundwater flow of approximately 1 m/day.

    View details for DOI 10.1021/es034700q

    View details for Web of Science ID 000222051400028

    View details for PubMedID 15260332

  • Evaporation effects on oxygen and hydrogen isotopes in deep vadose zone pore fluids at Hanford, Washington VADOSE ZONE JOURNAL DePaolo, D. J., Conrad, M. E., Maher, K., Gee, G. W. 2004; 3 (1): 220-232
  • Vadose zone infiltration rate at Hanford, Washington, inferred from Sr isotope measurements WATER RESOURCES RESEARCH Maher, K., DePaolo, D. J., Conrad, M. E., Serne, R. J. 2003; 39 (8)