Solid-phase arsenic speciation in aquifer sediments: A micro-X-ray absorption spectroscopy approach for quantifying trace-level speciation

Sarah L. Nicholas, Melinda L. Erickson, Laurel G. Woodruff, Alan R. Knaeble, Matthew A. Marcus, Joshua K. Lynch, Brandy M. Toner

Research output: Contribution to journalArticlepeer-review

27 Scopus citations


Arsenic (As) is a geogenic contaminant affecting groundwater in geologically diverse systems globally. Arsenic release from aquifer sediments to groundwater is favored when biogeochemical conditions, especially oxidation-reduction (redox) potential, in aquifers fluctuate. The specific objective of this research is to identify the solid-phase sources and geochemical mechanisms of release of As in aquifers of the Des Moines Lobe glacial advance. The overarching concept is that conditions present at the aquifer-aquitard interfaces promote a suite of geochemical reactions leading to mineral alteration and release of As to groundwater. A microprobe X-ray absorption spectroscopy (μXAS) approach is developed and applied to rotosonic drill core samples to identify the solid-phase speciation of As in aquifer, aquitard, and aquifer-aquitard interface sediments. This approach addresses the low solid-phase As concentrations, as well as the fine-scale physical and chemical heterogeneity of the sediments. The spectroscopy data are analyzed using novel cosine-distance and correlation-distance hierarchical clustering for Fe 1s and As 1s μXAS datasets. The solid-phase Fe and As speciation is then interpreted using sediment and well-water chemical data to propose solid-phase As reservoirs and release mechanisms. The results confirm that in two of the three locations studied, the glacial sediment forming the aquitard is the source of As to the aquifer sediments. The results are consistent with three different As release mechanisms: (1) desorption from Fe (oxyhydr)oxides, (2) reductive dissolution of Fe (oxyhydr)oxides, and (3) oxidative dissolution of Fe sulfides. The findings confirm that glacial sediments at the interface between aquifer and aquitard are geochemically active zones for As. The diversity of As release mechanisms is consistent with the geographic heterogeneity observed in the distribution of elevated-As wells.

Original languageEnglish (US)
Pages (from-to)228-255
Number of pages28
JournalGeochimica et Cosmochimica Acta
StatePublished - Aug 15 2017

Bibliographical note

Funding Information:
For funding, we thank the University of Minnesota (UMN) Center for Urban and Regional Affairs (BMT); the National Institutes for Water Resources and UMN Water Resources Center (BMT, Edward Nater); the UMN Office of the Vice President for Research (BMT); the UMN College of Food, Agricultural, and Natural Resource Sciences (BMT); and the UMN Undergraduate Research Opportunities Program (BMT, Sarah Baldvins). Sediment geochemical analysis costs were provided by USGS and were completed by the USGS contract chemistry laboratory. We thank Edward Nater (UMN), Paul Bloom (UMN), and Carrie Jennings (Freshwater Society) for providing mentorship to SLN. We thank Harvey Thorleifson for active collaboration with the Minnesota Geological Survey. We thank synchrotron scientists Sirine Fakra and Josep Roque-Rosell (Advanced Light Source, ALS, BL10.3.2), Mahaling Balasubramanian and Steve Heald (Advanced Photon Source, APS, PNC/XSD, 20-BM), and Matthew Newville (APS, 13-BM) for their support of this project. For assistance with preparation of the arsenic XAS reference standards, we thank Lindsey Briscoe and Shahida Quazi (UMN) and Barbara Lusardi of the Minnesota Geological Survey. We thank Lindsey Briscoe and Ryan Lesniewski (UMN), Carrie Jennings (Freshwater Society), James Berg (Department of Natural Resources), and Rick Ruhanen and Jordan Goodman (DNR Lands and Minerals Drill Core Library) for support in selecting and sampling of archived cores. We thank Sara Baldvins (UMN) and Sofia Oufqir (University of Mohamed V-Agdal, Morocco) for assistance with sample preparation. We thank Karen Johannesson (Tulane) for her help with arsenic enthalpies. Thank you to Richard Soule (Minnesota Department of Heath) and Randal Barnes (UMN) for contributing hydraulic conductivity data to EA Appendix 2. We thank Peter Croot, the Irish Centre for Research in Applied Geosciences, and the Department of Earth and Ocean Sciences at the National University of Ireland, Galway for allowing SLN to continue As data analysis into the first months of her postdoctoral fellowship. PNC/XSD facilities at the APS, and research at these facilities, are supported by the U. S. Department of Energy (DOE) - Basic Energy Sciences, a Major Resources Support grant from NSERC, the University of Washington, Simon Fraser University, and the APS. Use of the APS is also supported by the U. S. DOE, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357. The ALS is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. DOE under Contract No. DE-AC02-05CH11231. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Publisher Copyright:
© 2017 Elsevier Ltd


  • Arsenic
  • Chemical mapping
  • Glacial aquifer
  • Groundwater
  • X-ray microprobe


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