| The evolution of redox conditions in an aquifer contaminated with waste fuels, chlorinated solvents, and fire fighting foams was evaluated at the former Wurtsmith Air Force Base in Oscoda, Michigan. Geochemical indicators of terminal electron accepting processes (TEAPs) were evaluated over 5 years in conjunction with hydrogeologic and microbiologic parameters to describe the spatial and temporal variability in redox processes. Observed redox variability was linked to transport processes, particularly recharge events, which deliver terminal electron acceptors (TEAS) to the reduced groundwater plume. To understand how recharge affects groundwater biogeochemical processes, multivariate statistics and graphical tools were used to analyze geochemical data before and after a major recharge event (spring snow melt, 2000) at both small (single well) and large (whole plume) scales. Results show that average TEA concentrations increased within the aquifer and that changes in TEAPs were initiated not in large redox zones but in smaller zones at the interfaces of plume boundaries. Important plume boundaries include the interfaces between the reduced plume and (1) surrounding groundwater, (2) the capillary fringe zone, and (3) recharge waters. Due to the small and transient nature of these important zones, it is difficult to directly measure changes. Therefore, a series of field experiments were designed to replicate as closely as possible a recharge event in the aquifer. These experiments revealed that, when TEAS are introduced simultaneously into an anaerobic aquifer where fermentative products such as acetate and HZ gas have accumulated, TEAS could be consumed simultaneously and at similar rates. However, consumption rates were more rapid than observed in natural-gradient, large-scale experiments, likely due to a lack of readily available electron donor (i.e., accumulation of fermentative products) in those systems.; Results from the aforementioned analyses and experiments led to the development of a model for the evolution of redox zonation in contaminated aquifer systems. The conceptual model proposes that during initial contaminant plume development, the rates of TEAPs and therefore redox zonation are controlled by the production of readily available electron donors (e.g., acetate, H2) and therefore by fermentation rates. As the plume continues to evolve, TEAS are depleted sequentially based on thermodynamically predicted free energies. Once a system has reached a state of maturity where organic carbon inputs balance biodegradation reactions, a stable plume length is achieved. Under these conditions, TEAs will continue to be depleted and fermentative products will accumulate such that distinct redox zones down flow path cannot be sustained. Spatial variability in TEAPs occurs in small interface zones near plume boundaries where TEAS are introduced either by diffusion and dispersion from the sides of the plume or by recharge from precipitation and upgradient flow. TEAP rates are no longer limited by fermentation rates but rather by TEA reduction rates. In addition, TEAS would not necessarily be degraded sequentially given the excess of available donor. The conceptual model was verified by incorporating its key elements and linkages in a 3-D flow and transport model, which successfully represented the early stages of plume evolution. |
