INDICATORS OF BIOGEOCHEMICAL CYCLING AT CENTIMETER-SCALE WATER-SEDIMENT, SEDIMENT-AQUIFER INTERFACES IN A WETLAND SYSTEM

A fundamental issue in chemical fate and transport is correlating linked hydrogeologic, microbiologic, and geochemical processes and determining their role on the redox state of a system. In subsurface environments, redox reactions are mediated by the metabolic activities of microorganisms through terminal electron accepting processes (TEAPs), which are in turn controlled by the delivery of electron acceptors and donors. Understanding the variable redox conditions of these environments is crucial to evaluating the fate and transport of nutrients and contaminants and thus in protecting human and ecosystem health.

The research site is a riparian wetland that runs parallel to a closed landfill in Norman, OK. A leachate plume extends from the landfill flowing underneath the wetland system. Our study focuses on the in-situ measurement of redox conditions and TEAPs in the groundwater/surface water mixing interfaces at cm-scale spatial resolution. In-situ characterization of redox chemistry has been limited by the volume of porewater necessary to perform chemical analyses. This study overcomes this limitation by using an emerging technology, capillary electrophoresis (CE), for the analysis of anions, cations, and organic acids. CE requires sample volumes as low as 5 microliters per chemical suite. A vertical profile of porewater, surface water and groundwater samples was collected using a passive-diffusion membrane (peeper) with half-centimeter resolution. These samples were analyzed in the field for alkalinity, Fe(II) and S2-, and in the laboratory for NH4, Ca, Na, K, Cl, SO4, and organic acids such as, acetate, butarate and propionate. Results show significant TEAP variability at cm-scale suggesting new boundaries of variable microbial processes occurring at the sediment/water, sediment/aquifer interface.