Modeling the Organic Carbon Oxidation and Redox Sequence Under the Partial‐Equilibrium Approach: A Discussion by Means of a Semi‐Analytical Solution

Abstract

AbstractIn this work, we have developed a semi‐analytical solution for organic carbon oxidation coupled to the reduction‐oxidation sequence assuming the Partial Equilibrium Approach (PEA) and using the decoupling procedure of De Simoni et al. (2005), https://doi.org/10.1029/2005WR004056. Our solution was applied to two very simple cases. The first assumes only advective transport and the second only diffusive transport. Comparison with a numerical solution showed the adequacy of our analytical solution to be implemented in several scenarios, for example, in organic carbon oxidation in the unsaturated zone or in highly heterogeneous advective domains. We found that for the diffusion case the PEA produced spurious reactions, such as oxidation of N2 by O2 when compared with an approach using full kinetics. These reactions do not occur in the advection case. An analysis with the semi‐analytical solution revealed that they are the result of a combination of diffusive fluxes and the fact that the PEA assumes the electron acceptors to react with each other in equilibrium. Our analytical solutions are capable to quantify this shortcoming, becoming a tool to validate numerical models using PEA to describe organic carbon oxidation.