Research highlights

Exported organic carbon promotes reducing conditions and redox cycling in oxic aquifers

N. Kumar, et al. “Redox heterogeneities promote thioarsenate formation and release into groundwater from low arsenic sediments.” Environ. Sci. Te

chnol. 54, 6, 3237–3244 (2020). [DOI:10.1021/acs.est.9b06502] thioarsenate-formation-release-groundwater-from-low-arsenic-sediments

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The Science: Aquifers contain abundant organic-enriched, fine-grained, and sulfidic lenses. While it is widely understood that these heterogeneities are important sources of organic carbon, Fe(II), and S, the reactive transport mechanisms.... Read more >

Soil organic matter controls Pb release during redox cycles in floodplain soils

Dewy, C. et al. “Porewater Lead Concentrations Limited by Particulate Organic Matter Coupled with Ephemeral Iron(III) and Sulfide Phases during Redox Cycles within Contaminated Floodplain Sediments.” Environ. Sci. Technol. 9, 5878-5886 (2021).

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The Science: Lead contamination in soils is a major threat to water quality. Although Pb tends to occur in sparingly soluble minerals, changes in dissolved oxygen concentrations can promote dissolution of these minerals, potentially causing... Read more >


Spatial and Compositional Heterogeneities Control Zn Retention Mechanisms in a Simulated Aquifer

Engel, M., Boye, K., Noël, V., Babey, T., Bargar, J.R., Fendorf, S. (2021) Simulated Aquifer Heterogeneity Leads to Enhanced Attenuation and Multiple Retention Processes of Zinc. Environmental Science & Technology

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The Science: Alluvial aquifers are an essential source of groundwater worldwide, particularly for water storage purposes.  Fine-grained lenses of clay and organic matter, enriched in iron and sulfur, are abundant within aquifers and support cycling of... Read more >

Calcium-Uranyl-Carbonato Species Kinetically Limit U(VI) Reduction by Fe(II) and Result in U(V)-Bearing Ferrihydrite

Dewey, C., Sokaras, D., Kroll, T., Bargar, J.R., Fendorf, S. Calcium-Uranyl-Carbonato Species Kinetically Limit U(VI) Reduction by Fe(II) and lead to U(V) - Bearing Ferrihydrite. Environmental Science and Technology (Accepted Manuscript).

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The Science  Uranium (U) solubility determines the dissolved concentrations and thus its threat to water quality. It is therefore essential to understand the controls on U solubility to predict and mitigate the impacts of U contamination. Read more >

Microbial Communities in Floodplain Soils Remain Unchanged Throughout Seasonal Redox and Water Table Flux

Tolar, B.B., Boye, K., Bobb, C., Maher, K., Bargar, J.R., Francis, C.A. Stability of floodplain subsurface microbial communities through seasonal hydrological and geochemical cycles. Frontiers in Earth Science – Biogeoscience (in revision).

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The Science  Microbial communities play a crucial role in environmental systems, mediating biogeochemical reactions through metabolic processes that can vary depending on environmental conditions. Read more >

Diverse Ammonia-Oxidizing Archaea Dominate Subsurface Nitrifying Communities in Semi-Arid Floodplains

Cardarelli, E. L., Bargar, J.R., Francis, C.A. Diverse Thaumarchaeota dominate subsurface ammonia-oxidizing communities in semi-arid floodplains in the Western United States. Microbial Ecology (in press)


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Ammonia oxidation may be driven by archaea rather than bacteria within the riparian subsurface. Read more >

FeS Nanoclusters Can Mobilize Fe and S from sediment to the Groundwater

Noël, V., Kumar, N., Boye, K., Barragan, L., Lezama-Pacheco, J., Chu, R., Tolic, N., Gordon E. Brown Jr., G.E, Bargar, J.R. (2020) FeS Colloids – Formation and Mobilization Pathways in Natural Waters. Environmental Science Nano.,  Accepted Manuscript.  

DOI: 10.1039/C9EN01427F

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Ferrihydrite Sulfidation Promotes FeS Nanocluster Formation.   Read more >

ComBone_ESTplexation by Organic Matter COntrols Uranium Mobility in Anoxic Sediments

Bone, S.E.; Cliff, J.; Weaver, K.; Takacs, C.J.; Roycroft, S.; Fendord, S.; Bargar, J.R. Uranium complexation by organic matter and clay minerals in anoxic contaminated sediments. Environmental Science and Technology 2020, 54, 1493-1502.

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Uranium(IV) adsorbs to organic matter in anoxic alluvial sediment, which may be mobilized through desorption and colloidal release. Read more >


Redox Interfaces Can Produce Toxic Arsenic Levels in Groundwater from Low Arsenic-Abundance Sediments

Kumar, N., Noël,V., Planer-Friedrich, B., Besold, J., Lezama-Pacheco, J., Bargar, J.R., Brown, G. E., Fendorf, S. Boye, K Redox Heterogeneities Promote Thioarsenate Formation and Release into Groundwater from Low Arsenic Sediments. Environmental Science & Technology, 54 (2020), 3237-3244;

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New process observed for uranium accumulation and release at contaminated DOE sites. Read more >



Hexavalent uranium storage mechanisms in wet-dry cycled sediments at contaminated DOE sites in the Western U.S

Noël, V.; Boye, K.; Kukkadapu, R. K.; Li, Q.; Bargar, J. R. Uranium storage mechanisms in wet-dry redox cycled sediments. Water Research. Water Research 152 (2019), 251-263;

Sulfate-rich groundwater promote formation of thioarsenates at fine-coarse sediment interfaces and increases arsenic solubility Read more >



A Simplified Way to Predict the Function of Microbial Communities

K. Boye, A.H. Hermann, M.V. Schaefer, M.M. Tfaily, and S. Fendorf, “Discerning microbially mediated processes during redox transitions in flooded soils using carbon and energy balances.” Frontiers in Environmental Science (2018) [DOI: 10.3389/fenvs.2018.00015]

This pioneering study offers an easier approach to study how microbes work and could help scientists advance models of the cycling of elements and nutrients in frequently flooded soils. Read more >


Sorption to Organic Matter Controls Uranium Mobility

Bone SE, Dynes JJ, Cliff J, & Bargar JR “Uranium(IV) adsorption by natural organic matter in sediments.” Proceedings of the National Academy of Sciences of the United States of America 114(4), 711-716. [10.1073/pnas.1611918114]

A new multi-technique study using X-ray absorption spectroscopy at SSRL, nanoscale secondary Ion mass spectroscopy at EMSL, and scanning transmission X-ray microscopy at the CLS has revealed crisp new details about the mechanisms of uranium binding in sediments.  Surfaces of natural organic matter bind uranium more strongly than minerals under field-relevant conditions. Read more >

Thermodynamic preservation of carbon in anoxic environments

Boye, K., Noël, V., Tfaily, M.M., Bone, S.E., Williams, K.H., Bargar, J.R., Fendorf, S. (2017) Thermodynamically controlled preservation of organic carbon in floodplains. Nature Geoscience [10.1038/ngeo2940]

A new study combining X-ray absorption spectroscopy (XAS) at SSRL with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) at EMSL provides new insights on why carbon persists in waterlogged soil and subsurface sediments . Energetic constraints prevent microbial respiration of certain organic carbon compounds, leaving a pool of water-soluble carbon that is susceptible to oxidation or export and subsequent decomposition in downstream, aerated environments. Read more >

U Release from NRZ sediments is Inhibited by Transport and Geochemistry

Bone, S.E.; Cahill, M.R.; Jones, M.E.; Fendorf, S.; Davis, J.A.; Williams, K.H.; Bargar, J.R. Oxidative Uranium Release from Anoxic Sediments under Diffusion-Limited Conditions. Environmental Science and Technology 2017, 51, 11039−11047.

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The Science

Floodplain aquifers undergo seasonal water table fluctuations that drive redox cycling of subsurface species, including contaminants such as uranium. We posited that seasonal intrusion of O2 into anoxic sediments will stimulate production of NO3- followed by denitrification. Read more >

Iron and sulfur cycling in NRZs controlled by sediment textural and hydrology

Noël V, Boye K, Kukkadapu RK, Bone SE, Lezama Pacheco JS, Cardarelli E, Janot N, Fendorf S, Williams KH, Bargar JR (2017) Understanding controls on redox processes in floodplain sediments of the Upper Colorado River Basin. Science of The Total Environment,

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Detailed characterization of the molecular structures of Fe and S, on depth-resolved field samples from floodplain sediments, were performed using x-ray absorption spectroscopy (XAS) and X-ray microspectroscopy at SSRL combined with mössbauer spectroscopy at EMSL. The results revealed that organic carbon content, moisture, and particle size control the distribution and reactivity of redox constraints in the floodplain subsurface. Read more >

A regional model for uranium redox and mobility

Noël, V.; Boye, K.; Dynes, J.; Lezama-Pacheco, J. S.; Bone S.; Janot, N.; Cardarelli, E.; Williams, K. H.; Bargar, J. R. Redox constraints over U(IV) mobility in the floodplains of Upper Colorado River basin. Environmental Science & Technology. 2017b, 51, 10954-10964.

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The Science                       

Two new studies shed light on an important and previously underappreciated biogeochemical redox ‘engine’ believed to mediate groundwater quality in floodplains within the upper Colorado River Basin (CRB). Sediments enriched in organic carbon were found to be common within saturated zones and capillary fringes, to be highly redox active, and to strongly accumulate sulfide and uranium. Read more >


Long-Term in Situ Oxidation of Biogenic Uraninite in an Alluvial Aquifer: Impact of Dissolved Oxygen and Calcium

J.S. Lezama-Pacheco, J. Cerrato, H. Veeramani, D.S. Alessi, E.I. Suvorova, R. Bernier-Latmani, D.E. Giammar, P.E. Long, K.H. Williams, and J.R. Bargar, Long-Term in Situ Oxidation of Biogenic Uraninite in an Alluvial Aquifer: Impact of Dissolved Oxygen and Calcium. Environ. Sci. Technol. 2015, 49, 12, 7340–7347

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The science.

Oxidative dissolution of U(IV) to U(VI) controls the release of uranium from reduced sediments to groundwater. In this study, uraninite (UO2) was used as a proxy for all forms of subsurface U(IV) in order to study oxidative dissolution of U(IV) under bona fide aquifer conditions. Read more >