CORRELATING BIOGEOCHEMICAL PROCESSES AT SMALL-SCALE MIXING INTERFACES IN A CONTAMINATED WETLAND SYSTEM

A fundamental goal in determining chemical fate and transport is to correlate linked hydrogeologic, microbiologic, and geochemical processes and their role in 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 in these environments is crucial to evaluating the fate and transport of nutrients and contaminants and thus in protecting human and ecosystem health.

This study was conducted in a riparian wetland that parallels a closed landfill in Norman, OK. A leachate plume from the landfill underlies the wetland system. Our study focuses on the in-situ measurement of redox conditions and TEAPs in the groundwater/surface water mixing interfaces at small-scale spatial resolution. Observed small-scale changes in porewater redox conditions were correlated with the stable C, N and S isotopes in sedimentary organic matter to delineate microbial processes within the sediments such as sulfate reduction and methanogenesis.

In-situ characterization of redox chemistry has been limited by the volume of porewater necessary to perform chemical analyses. This study overcomes this constraint 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 sulfide. Sediment cores collected near the peepers were sectioned at high (mm scale) resolution and analyzed for bulk organic constituents. Stable sulfur isotopes will be determined in order to discern sulfate reduction boundaries. Isotopic ratios will be used to determine the source of the organic matter in the sediments (i.e. groundwater inputs or overlying wetland material). Results indicate significant TEAP variability at the cm-scale suggesting new boundaries of variable microbial processes occurring at the sediment/water interface.