Wednesday, March 11, 2015 — 2:00 PM EDT

Tom Johnson

University of Illinois at Urbana-Champaign

Tracing groundwater contaminant transformation, immobilization, and transport (and doing some other fun redox geochemistry) using Cr, U, and Se isotope ratios"

EIT 3142

Followed by reception, first floor, EIT building

ABSTRACT: Chromium (Cr), selenium (Se), and uranium (U) are all mobile and toxic in their hexavalent, oxidized states but immobile and much less dangerous in more reduced valences.  Accordingly, detecting and quantifying reduction reactions can be critically important in determining impacts of these contaminants on surface water and groundwater resources and/or monitoring remedial actions that involve reduction.  Isotopic fractionation accompanies Cr, Se, and U reduction reactions, and 53Cr/52Cr, 82Se/76Se, and 238U/235U measurements appear to provide direct evidence for progress of reduction reactions in laboratory and real-world systems: Isotope ratios in the remaining contaminant change predictably with increasing extent of reduction.  The ratios also promise to help us determine redox conditions in the earth’s past, including critical time periods such as the “Great Oxidation Event” about 2.3 billion years ago.

In modern groundwater systems, the general relationship between isotope ratios and extent of reduction allows qualitative or semi-quantitative interpretation of field data, but several challenges must be overcome to enable more quantitative interpretations.   First, the isotopic fractionation is a kinetic phenomenon and isotopic fractionation factors show considerable variability, depending on reaction mechanism and conditions.  Second, natural systems are heterogeneous and reduction may be confined to certain subdomains of the system.  Simple models for heterogeneous systems suggest that the effective fractionation, observed at the field scale, may be a factor of two or more smaller than the intrinsic fractionation (δproduct - δreactant) occurring at the micro scale.  A further complication may arise if isotopic exchange between oxidized and reduced species occurs and alters isotopic compositions after the initial reduction. 

The talk will cover the general concepts behind these new tools, recent work addressing some of the challenges, and examples of field applications.

Biography

Prof. Johnson is Professor and Department Head of Geology at the University of Illinois main campus at Urbana-Champaign.  He received his Ph.D. in 1995 from UC Berkeley, working in the isotope geochemistry group of Don DePaolo, and has been at Illinois since 1997.  Most of his work has focused on development of methods for measurement and/or application of stable isotope ratios of elements heavier than sulphur, and specifically “redox sensitive” elements whose geochemistry changes strongly with changes of valence.

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