Journal
GEOCHIMICA ET COSMOCHIMICA ACTA
Volume 211, Issue -, Pages 228-255Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.gca.2017.05.018
Keywords
X-ray microprobe; Chemical mapping; XANES; Glacial aquifer; Groundwater; Arsenic
Categories
Funding
- University of Minnesota (UMN) Center for Urban and Regional Affairs (BMT)
- National Institutes for Water Resources and UMN Water Resources Center (BMT, Edward Nater)
- UMN Office of the Vice President for Research (BMT)
- UMN College of Food, Agricultural, and Natural Resource Sciences (BMT)
- UMN Undergraduate Research Opportunities Program (BMT, Sarah Baldvins)
- U.S. Department of Energy (DOE) - Basic Energy Sciences
- NSERC
- University of Washington
- Simon Fraser University
- APS
- U.S. DOE, Office of Basic Energy Sciences [DE-AC02-06CH11357, DE-AC0205CH11231]
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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 (mu XAS) 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 mu XAS 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. (C) 2017 Elsevier Ltd. All rights reserved.
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