EVOLUTION OF REDOX ZONATION IN A CONTAMINATED AQUIFER: DEVELOPMENT OF A 3-D COUPLED PARTIAL EQUILIBRIUM MODEL

The concept that terminal electron accepting processes (TEAPs) distribute themselves sequentially into redox zones down flow path in aqueous systems is often used to interpret how and at what rates organic compounds will be degraded in the environment. Geochemical and microbiological data collected from a mixed contaminant plume at the former Wurtsmith Air Force Base in Oscoda, Michigan suggests that under steady-state, mature plume conditions, traditional redox zonation may not be a realistic model of the distribution of TEAPs and therefore may not be the best model to evaluate the potential degradation of organic compounds. Based on these data, a conceptual model of TEAP evolution in contaminated systems was established. This model proposes that during initial plume development, O2, Fe3+, NO3, and SO4 (TEAs), will be consumed sequentially based on thermodynamic arguments until the balance of organic degradation rates and source inputs can be achieved. Once this state has been achieved, distinct redox zones will no longer be sustained and the bulk of the plume will be depleted with respect to TEAs allowing methanogenesis to dominate. Under these conditions, solute inputs at contaminant plume boundaries become important controls on TEAP distribution and contaminant degradation, particularly during periods of recharge.

This conceptual model was evaluated quantitatively through the development of a 3-D partial equilibrium reactive transport model. The numerical model was created by linking the geochemical modeling code PHREEQC with the reactive transport code RT3D. This coupled model considers 1) biodegradation of organic matter based on the influence of transport on microbial growth and 2) a complex suite of interconnected biogeochemical reactions operating in the aquifer as constrained by thermodynamic and kinetic arguments.