EVOLUTION OF BIOGEOCHEMICAL TRANSPORT MODELING OF AN EVOLVING HYDROCARBON PLUME

In the early 1980’s, Mary Jo Baedecker undertook the challenging task of characterizing and understanding the plume emanating from a crude-oil spill near Bemidji, Minnesota. She, with her colleagues, identified an anoxic plume downgradient from the oil body that was relatively stable in extent but becoming more reducing with time. Her unique work led to a comprehensive understanding of the natural biogeochemical evolution of the hydrocarbon plume. Dissolved organic carbon was undergoing aerobic biodegradation followed by anaerobic biodegradation processes including manganese reduction, iron reduction, and methanogenesis. These processes were coupled with complex geochemical processes including dissolution and precipitation of iron compounds, and outgassing of methane and carbon dioxide. Mary Jo’s first model of biogeochemical reactions and mass transfer along a flow path established a continuing effort to use models and the wealth of data collected at Bemidji to characterize the biogeochemical processes, reaction rates, and component mass balances for the plume. Building on Mary Jo’s conceptual model of the Bemidji plume, we were able to develop a reactive transport model to analyze biodegradation of volatile and nonvolatile fractions of dissolved organic compounds (DOC) by multiple terminal electron accepting processes (TEAPs) using Monod kinetics. This model reproduced the general features of the observed groundwater plume and bacterial concentrations. The model predicted that anaerobic biodegradation processes accounted for more than half of the removal of DOC at this site. A subsequent effort used inverse modeling to identify the hydrocarbon dissolution rate and estimate first-order approximations of biodegradation rates for individual components. Recent modeling has illustrated that the irreversible Monod rate law may predict forward reactions even when the free energy of reaction is positive, violating thermodynamic constraints. Thermodynamically correct simulations using either a partial equilibrium model or a completely kinetically controlled model reproduced the observed geochemical data. Mary Jo’s work at the Bemidji site measurably advanced the science of biogeochemistry, and through her spirit of collaboration developed the careers of many scientists.