TEAPREV: A NUMERICAL TOOL FOR ASSESSING RATES OF TERMINAL ELECTRON-ACCEPTING PROCESSES IN A REPRESENTATIVE ELEMENTARY VOLUME OF SUBSURFACE SEDIMENT

A numerical model of Terminal Electron Accepting Processes (TEAP) and other biogeochemical reactions in a hypothetical Representative Elementary Volume (REV) of subsurface sediment was developed as a tool to aid in quantitative interpretation of laboratory- and (ultimately) field-scale studies of subsurface microbial processes.  The model envisions flow of dissolved organic carbon (DOC)-containing fluid through a single reactor cell (the fluid flow rate is set equal to zero to model batch experiments).  The incoming fluid contains soluble electron acceptors (O₂, NO₃^-, SO₄^2-) whose abundance, together with the abundance of solid-phase electron acceptors (MnO₂, FeOOH, S⁰) in the sediment, control the rates of terminal electron accepting processes (TEAP) and other biogeochemical reactions over time in the reactor.  The model accounts for complete (to HCO₃^-) or incomplete (to acetate) oxidation of DOC via 7 core TEAP pathways.  Additional TEAP reactions (e.g. As(V) or U(VI) reduction) can easily be added to expand the scope of the model.  Each of the TEAP reactions is dependent on the biomass of one or more distinct microbial populations chosen based on current knowledge of the kinds of organisms likely to be involved in subsurface organic carbon metabolism.  Growth of these populations is described using the bioenergetics-based approach in which the partitioning of electron flow between energy generation and cell biomass production is dependent on the free energy of the corresponding TEAP.  This approach alleviates the need for making a priori assumptions about the biomass yield for the different physiological functional populations.  Each of the TEAPs results in production of various inorganic compounds, which either accumulate in solution or undergo reactions (sorption and/or mineral precipitation) with the solid-phase.  The optimized model reproduced the basic patterns of organic substrate metabolism, consumption of electron acceptors (NO₃^-, Fe(III), SO₄^2-), and accumulation of reduced end-products (Fe(II) and CH₄) of anaerobic respiration in a slurry experiment with ethanol-amended subsurface sediment.  The general agreement between the simulation and the data suggests that the developed reaction network is appropriate for incorporation into field-scale simulations of natural or engineered subsurface microbial processes.