Inverse microbial and geochemical reactive transport models in porous media

Microbial processes play a major role in controlling geochemical conditions in subsurface systems. Various laboratory and in situ experiments have been performed to evaluate the relevance of microbial processes and derive key microbial parameters. Such experiments are often interpreted in a subjective manner by trial-and-error curve fitting. A numerical model for the inverse problem of coupled flow, reactive solute transport, geochemical and microbial processes is presented here which overcomes the limitations of trial-and-error methods. It extends the capabilities of existing inverse models which deal mostly with flow and chemically-reactive solute transport. Our inverse model relies on the microbial reactive transport model of Zhang (2001) and Samper et al. (2006a) and improves the inverse reactive transport model of Dai and Samper (2004) by allowing the simultaneous estimation of geochemical and microbial parameters. The inverse model has been implemented in a finite element code, INVERSE-BIOCORE2D and its capabilities have been verified and tested with a synthetic experiment involving equilibrium speciation, kinetic sorption/desorption and kinetic biodegradation reactions. Model results indicate that both chemical and microbial parameters can be estimated accurately for error-free data. Estimation errors of microbial parameters are larger than those of kinetic sorption parameters and generally increase with increasing standard deviation of data noise. Estimation error of yield coefficient is the smallest of all microbial parameters and does not depend on data noise. The inverse model has been used also to estimate microbial parameters of DOC aerobic respiration responsible for oxygen consumption at the REX in situ experiment. Estimates of microbial parameters are found to be within the range of reported values and have small estimation errors.