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Stable Isotope Fractionations in Biogeochemical Reactive Transport


Characterizing the reactivity and fate of solutes in groundwater presents a formidable challenge in that these environments are inherently isolated from the observer. Samples are commonly limited to sediment cores and fluid collected from discrete well bores, while the measurements from these locations must be used to infer geochemical processes occurring across continuous, dynamic and heterogeneous systems. Stable isotope fractionations are an important tool in the quantitative analysis of reactive transport processes in porous media because they provide information coupled to, but distinct from solute concentrations. Examples of the combined analysis of solute concentrations and stable isotope fractionations abound in the literature, but almost entirely involve analytical models that decouple isotopic fractionation from the full extent of reactive pathways contributing to water-rock interaction in porous media flow.

It is the purpose of this dissertation to apply analysis of stable isotopes to elucidate the processes of biogeochemical reactivity contributing to in situ bioreduction of uranium during electron donor augmentation in the Old Rifle aquifer in western Colorado. Bio-induced stabilization of heavy metals in subsurface systems involves a complex and concurrent series of dissimilatory reduction reactions, each with a unique metabolism and contaminant affinity. In such a complex system, application of simplified fractionation-reactivity relationships is shown to be inadequate to describe the behavior of multiple stable isotope systems. As a result, the methodology is developed to explicitly incorporate the isotopologues of these species into a multi-component reactive transport model of the system. The result is a means of mechanistically analyzing isotopic fractionation within an integrated framework encompassing the complete reactive network associated with electron-donor augmented bioreduction.

The chapters herein are roughly divided into two sections. In the first half, sulfur and calcium isotopes are measured during field scale acetate amendments at Old Rifle. The results of these analyses demonstrate the utility of isotopic fractionation to track the development of terminal electron accepting processes and the formation of secondary mineral precipitates. However, these studies also demonstrate the limitations incurred by restriction to discrete well bores. In the second half, a flow-though column study is designed to overcome these limitations and test the extent to which individual isotopic fractionations are conserved during reactive transport under Old Rifle conditions. The results provide the first carbon budget during electron donor augmentation, and demonstrate that while the &delta34S fractionation induced by biogenic sulfate reduction is conservative, ion exchange effects dampen the &delta44Ca fractionation associated with carbonate precipitation.

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