AN INTEGRATED CONCEPTUAL MODEL TARGETING COMPLEX HYDROBIOGEOCHEMICAL INTERFACES WITHIN A CONTAMINATED AQUIFER-WETLAND SYSTEM

To understand and predict the fate and transport of numerous chemical species, including nutrients and contaminants, it is important to identify the most active zones of biogeochemical cycling within a system. In subsurface aqueous systems, the mixing interface between distinct water masses of differing redox state can be highly active zones of biogeochemical activity due to the availability of limiting electron acceptors such as oxygen, nitrate, or sulfate or electron donor such as low molecular weight organic acids. However, despite the well recognized importance of mixing interfaces on system-scale biogeochemical cycling, quantification of the complex suite of reactions initiated at the contact zone has been poorly documented. Mixing interfaces in a shallow aquifer-wetland system near the Norman Landfill in, Norman, OK provide an ideal location to investigate the coupled hydrogeological, microbiological, and geochemical processes that control biogeochemical cycling at mixing interfaces. Using cm-scale passive diffusion samplers (peepers) and shallow push-cores, water and sediment samples were collected in a depth profile to span interfaces between surface water and various sedimentary layers. Hydro-bio-geochemical data showed steep gradients of biogeochemical indicators corresponding to various types of mixing interfaces. Data suggest that the mixing interface between the aquifer and wetland is complex and comprised of several types of interfaces 1) hydrologic interfaces, such as the sediment-water interface or interfaces created at lithologic boundaries between sediment layers, 2) chemical interfaces, both mobile (e.g., contact between two distinct water masses such as recharge water floating on a contaminant plume) and immobile (e.g., distribution of particulate organic matter or solid phase electron acceptors such as iron oxides); and 3) microbiological interfaces. Due to the disparate nature of these interfaces, the controls on the spatial and temporal dynamics of each need to be considered independently to accurately describe and predict biogeochemical cycling.