QUANTIFYING THE BIOGEOCHEMICAL EVOLUTION OF REDOX ZONES AND CONTAMINANT TRANSPORT IN A SHALLOW SANDY AQUIFER USING A NOVEL REACTIVE TRANSPORT CODE BKTK3D 2.0

Hydrogeological, microbiological, and geochemical processes operating in a shallow sandy aquifer contaminated by waste fuels and chlorinated solvents were integrated using high-resolution mechanistic models. A 3-D, transient, reactive transport model was developed to quantitatively describe coupled processes via thermodynamic and kinetic arguments. The model was created by linking the hydrodynamic model MODFLOW (McDonald and Harbaugh, 1988), with advection, dispersion and user defined kinetic reactions based on RT3D 2.0, (Clement and Jones, 1998) and geochemical model PHREEQC (Parkhurst and Appelo, 1999). This model, BGTK3D 2.0, describes 1) the biodegradation of organic matter based on solute transport processes and microbial growth, 2) a complex suite of geochemical reactions, and 3) sharp chemical gradients. Some key features are an ability to incorporate natural solid phases to describe mineral-water interactions, and an ability to accurately describe small-scale biogeochemical cycling observed in the field without oscillations or excessive numerical damping.

BGTK3D was used to test hypotheses regarding the evolution of redox chemistry in a contaminated aquifer. The conceptual model that terminal electron accepting processes (TEAPs) distribute themselves sequentially into zones down flow is often used to interpret how and at what rates contaminants will be degraded. Geochemical and microbiological data collected from a mixed contaminant plume at the former Wurtsmith AFB in Oscoda, MI suggests that under “mature” plume conditions, sequential redox zonation may not be a realistic model of TEAP distribution and thus may not be the best model to evaluate biodegradation. Based on these data, a model of TEAP evolution was established that proposes that during initial plume development terminal electron acceptors O2, Fe3+, NO3, and SO4, are consumed sequentially based on thermodynamic arguments until a balance between organic degradation rates and source inputs and thus a stable plume length can be achieved. Once this “mature” state has been achieved, distinct redox zones can no longer be sustained and methanogenesis will dominate except in portions of the aquifer impacted by recharge water and diffusion of TEAs at mixing interfaces. Under these conditions, TEAPs will not proceed sequentially.