5. Summary and conclusions
The goal of this work was to develop a simulation tool to investigate how different microbial functional groups and interactions among these groups impact the chemical composition of fluids and minerals in anoxic subsurface environments. Microbial metabolisms mark the environment with chemical signatures — the reactants and products unique to the pathways. In natural environments where diverse metabolisms proceed simultaneously, interactions among the pathways mask the metabolic signatures, which become less informative. Under such circumstances, models based on microbial physiology can be used to infer the occurrence and to quantify the significance of different metabolisms (Maurer and Rittmann, 2004).
The physiology-based model considers potential microbial functional groups capable of degrading ethanol to bicarbonate and methane. This model accounts for the common electron acceptors in anoxic subsurface environments, including nitrate, Fe(III) oxide, sulfate, and bicarbonate. Where significant O2, manganese IV, elemental sulfur, and other electron acceptors are present, the corresponding functional groups can be included accordingly (see Supporting Information Section S3).
Application of the model to the aquifer sediments of ORFRC Area 2 illustrates how physiology-based modeling, combined with laboratory experimentation, can reveal metabolic pathways that are significant in subsurface sediments. According to the best-fit modeling results (Table 2 and Fig. 2 and Fig. 3), only five catabolic reactions were significant in metabolizing ethanol in the sediment slurry experiment. Hence, to simulate field-scale ethanol metabolism in the aquifer at ORFRC Area 2, it may not be necessary to consider all catabolic reactions that potentially participate in ethanol metabolism. Instead, simulation time may be minimized, and data fitting simplified, by focusing (at least initially) on the active catabolic reactions identified in this study. This path toward field-scale reactive transport simulation offers the potential to integrate microbial physiology into the prediction of the long-term impact of in situ biostimulation strategies and can be applied to simulate in situ bioremediation with ethanol, acetate, or hydrogen-release compound (e.g., glycerol polylactate) as the electron donors.
5. Summary and conclusions
The goal of this work was to develop a simulation tool to investigate how different microbial functional groups and interactions among these groups impact the chemical composition of fluids and minerals in anoxic subsurface environments. Microbial metabolisms mark the environment with chemical signatures — the reactants and products unique to the pathways. In natural environments where diverse metabolisms proceed simultaneously, interactions among the pathways mask the metabolic signatures, which become less informative. Under such circumstances, models based on microbial physiology can be used to infer the occurrence and to quantify the significance of different metabolisms (Maurer and Rittmann, 2004).
The physiology-based model considers potential microbial functional groups capable of degrading ethanol to bicarbonate and methane. This model accounts for the common electron acceptors in anoxic subsurface environments, including nitrate, Fe(III) oxide, sulfate, and bicarbonate. Where significant O2, manganese IV, elemental sulfur, and other electron acceptors are present, the corresponding functional groups can be included accordingly (see Supporting Information Section S3).
Application of the model to the aquifer sediments of ORFRC Area 2 illustrates how physiology-based modeling, combined with laboratory experimentation, can reveal metabolic pathways that are significant in subsurface sediments. According to the best-fit modeling results (Table 2 and Fig. 2 and Fig. 3), only five catabolic reactions were significant in metabolizing ethanol in the sediment slurry experiment. Hence, to simulate field-scale ethanol metabolism in the aquifer at ORFRC Area 2, it may not be necessary to consider all catabolic reactions that potentially participate in ethanol metabolism. Instead, simulation time may be minimized, and data fitting simplified, by focusing (at least initially) on the active catabolic reactions identified in this study. This path toward field-scale reactive transport simulation offers the potential to integrate microbial physiology into the prediction of the long-term impact of in situ biostimulation strategies and can be applied to simulate in situ bioremediation with ethanol, acetate, or hydrogen-release compound (e.g., glycerol polylactate) as the electron donors.
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