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Isotope Modeling as Bioremediation Tool

Source: Jennifer Druhan and Dan Hawkes

Druhan_fig2Dissimilatory sulfate reduction (DSR) may occur in any anaerobic environment where both sulfate and labile organic carbon are available, such as in marine sediments and poorly drained soils. As a result, DSR exerts a primary role in the global sulfur cycle. During amended subsurface biostimulation, introduction of organic carbon sources leads to rapid microbial growth and reduction of available electron acceptors. Recent work at the Old Rifle site, a uranium-contaminated aquifer in Western Colorado, USA, has shown substantial enrichment in sulfate δ34S associated with the onset of sulfate reduction

A team of ESD scientists headed by Jennifer Druhan (also of Stanford University), and including Carl Steefel, Mark Conrad, Don DePaolo, recently incorporated the stable isotopes of sulfur within a reactive-transfer code to model the range of microbially mediated redox processes affecting kinetic isotope fractionation. Improving on previous models that could not accurately simulate sulfur isotope fractionation over a wide range of substrate concentrations, the team developed a model that successfully captures the behavior of 32S and 34S isotopes over a broad range, using a constant fractionation factor. The model was then used to simulate a large-scale column study designed to replicate the field-scale conditions of a biostimulation experiment at the Old Rifle site in western Colorado. Results demonstrate an initial period of iron reduction that transitions to sulfate reduction, in agreement with field-scale behavior observed at the Old Rifle site.

The team’s work is potentially applicable to a variety of stable isotope systems associated with both steady-state and transient biogenic redox environments. Furthermore, the ability of this model treatment to predict the isotopic composition of secondary minerals accumulated as a result of fractionating processes offers an important means of interpreting solid-phase isotopic compositions and tracking the long-term stability of precipitates.

To read further, go to:

Citation: Druhan, J.L., C.I. Steefel, M.E. Conrad, and D.J. DePaolo (2014). A large column analog experiment of stable isotope variations during reactive transport: I. A comprehensive model of sulfur cycling and d34S fractionation. Geochimica et Cosmochimica Acta, 124, 366-393; DOI: 10.1016/j.gca.2013.08.037.

Funding: BER, SS SFA