MD-DE-DC Toxics Programs

Objectives

The purpose of this proposed research is twofold: (1) define the interaction of the slough (wetland) with the landfill leachate plume in the aquifer and its effect on the spatial and temporal changes in the wetland porewater biogeochemistry (2) provide further development and testing of the microelectrode technology for applications in wetland and streambottom environments. The objectives of this study for FY04 are to (1) optimize the electrochemical methods to collect in situ data at the site (2) interpret the data that has been collected at this site for the last two years.

Statement of Problem

Characterizing the distribution of microbially mediated redox reactions in ground water is critical in evaluating the fate of organic contaminants because the rate and extent of biodegradation depends largely on redox conditions. The increasing use of monitored natural attenuation as a remediation method for contaminated ground water has increased the need for methods to accurately characterize redox conditions in ground water. Redox conditions often are determined by individual analyses of key biogeochemical constituents that are electron acceptors or reaction products, including dissolved oxygen, nitrate, ammonia, manganese, ferrous and ferric iron, sulfate, sulfide, and methane. The occurrence of steep vertical concentration gradients and the unstable nature of many of the redox-sensitive species complicates redox characterization in contaminated ground water. Large temporal variability also is typical in shallow ground-water/surfacewater environments from seasonal, diurnal, and/or tidal effects. Redox and contaminant fate characterization is further complicated in these areas of ground-water/surface-water interaction, which typically contain silty or clayey sediments, by the difficulty in obtaining sufficient porewater volumes for all the necessary chemical analyses without changing hydraulic gradients and mixing porewater from different zones. A gold amalgam (Au/Hg) voltammetric microelectrode system (Model DLK-100 Electrochemical Analyzer) is a promising new technique to measure in situ concentrations of several major redox species and metals simultaneously at millimeter- and centimeter-depth intervals using a single working electrode. Environmental testing and application of this type of microelectrode technology, however, is very limited, although the electrochemical theory is well developed and has been used in other applications.

We will use the voltamemetric microelectrode system to determine major redox species and potential tracers of contamination in porewater of the slough, a shallow intermittent stream and wetland, that overlies the leachate plume at a closed municipal landfill near Norman, Oklahoma, which was established in 1995 as a research site under the U.S. Geological Survey Toxic Substances Hydrology Program. This study will be performed in collaboration with Isabelle Cozzarelli, USGS, Reston, VA. Little information is available on the quality or quantity of ground water discharging to the slough or on the biogeochemical processes occurring in the slough. Determination of redox species is critical for evaluating the effect of the slough on natural attenuation of organic contaminants in the leachate plume.

Strategy and Approach

Redox and leachate discharge characterization of the slough porewater will be completed where the established transect of multilevel samplers borders the slough. The microelectrode system, porous diffusion samplers (peepers), and existing multi-level samplers piezometers will be used to obtain redox measurements across the width of the slough and to a depth of about 2 m below land surface at each site (to the top of the alluvial aquifer). The microelectrode will be used to obtain detailed depth profiles at each site as deep as possible by direct push into the sediment (at least 50 cm based on our previous work). Deeper measurements will be obtained with the microelectrode by downhole measurements in existing multilevel samplers or newly installed temporary piezometers, and by direct measurements in the peeper water once they are removed from the sediment.