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Use of Geophysical Methods to Refine Geological Site Characterization at an Eastern Nebraska Landfill
Environmental and Engineering Geophysical Society (EEGS), 1994 Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP) proceedings.
Edward P. Pedersen, Jacobson Helgoth Consultants, Lakewood, Colorado, U.S.A.;
David Butler, MicroGeophysics Corporation, Wheat Ridge, Colorado, U.S.A.
Paper – [pdf] ENV_Butler_1994_Use_of_Geophysical_Methods_to_Refine_Geological_Site_Characterization
The goal of this project was geological site characterization of a closed landfill in eastern Nebraska. Geologic characterization is essential to the interpretation of the spatial distribution of aquifer characteristics. The desired final result of the geological characterization was collection of sufficient data to locate monitoring wells for long-term study of ground water quality. A phased approach of geological characterization was conducted at the site. The first phase consisted of a detailed review of the existing geologic data. Included in this review were regional geologic references, data from geotechnical borings in and adjacent to the landfill area, driller’s logs and water level data from nearby irrigation and water wells, and limited surface exposures. This review resulted in the development of a preliminary five-layer geologic model and identification of obvious data gaps.
The second phase of site characterization consisted of installing 12 new borings around the perimeter of the landfill. These boreholes sample the Paleozoic bedrock that underlies the site. Continuous cores were recovered from these borings to provide detailed lithologic information. In addition, selected intervals were sampled with Shelby tubes. These samples provided material for laboratory analysis to determine in-situ parameters, such as hydraulic conductivity and moisture content.
The boring data allowed the construction of a detailed site-specific nine-layer geologic model and a series of ten geologic cross-sections. As frequently happens in studies of this type, the increased knowledge of the site geology resulted in the formulation of additional questions. Answering many of these questions required collection of data beyond the landfill property boundaries. Based on cost and limited surface access, the installation of additional borings was not desirable at the time. It was decided to investigate the use of surface geophysical techniques to address these questions, to provide correlation of the geologic units between existing borehole locations and to extend the characterization into the surrounding area. Delineation of the landfill materials was not an objective.
The information provided by the geophysical data, especially in areas away from the boreholes, greatly increased the confidence level in locating the potential monitoring wells. Significant changes in the hydrogeologic interpretation were accomplished at a cost estimated to be at least one order of magnitude less than would have been required using a drill rig. Some of these changes were critical to meeting the project requirements for future ground water monitoring at the site.