Coupling Isotopic Fractionation to Multiple-Continuum Reactive Transport Models of Biogeochemical Systems

Stable isotopic systems often show an unexpected range in observed fractionation factors associated with biogeochemical systems. In particular, the ranges in such isotopic systems as Cr, Ca, Li, and C have often been attributed to kinetic effects as well as different biogeochemical mechanisms. Reactive transport models developed to capture the sub-micron-scale transport and reaction processes within the macroscale system (e.g., biofilm to cm-scale) have been successful in simulating the biogeochemical processes associated with bacterial growth and the resultant changes in pore-fluid chemistry and redox conditions. Once such multicontinuum reactive transport models are extended to include equilibrium and kinetic isotopic fractionation, diffusive transport, and fluid-gas equilibria, it becomes possible to quantitatively interpret the isotopic changes observed in experimental and natural or engineered biogeochemical systems. We combine a solid-solution approach for isotopic substitution in minerals with the multiple-continuum reactive-transport approach to interpret the effective fractionation factor observed in experimental systems. Although such systems often have poorly constrained inputs (such as the equilibrium fractionation factor and many of the parameters associated with bacterial growth), by combining several independent contraints on reaction rates (such as lactate consumption, 13C/12C and 87Sr/86Sr in calcite), the range of possible interpretations can often be greatly narrowed. Here we present examples of the modeling approaches and their application to experimental systems to examine why the observed fractionation factors are often different from the theoretical values.