Arctic GIS Workshop Poster Abstract22-24
January 2001 |
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Combinging Geophysical Techniques for Hydrogeologic Modeling at Fort Fichardson, Alaska
Using a combination of geophysical techniques provides an opportunity to compare several different methods and collect detailed data on subsurface stratigraphy and buried objects. These geospatial data can then be modeled in two and three dimensions to provide detailed hydrogeology. Here we present a study from Fort Richardson, Alaska, where a contaminant plume is migrating from an unknown source through an alluvial aquifer. Our objective is to locate possible sources of contamination and to model the site hydrogeology. We used ground-penetrating radar (GPR) and electromagnetics with an EM61 to locate buried metal and former trench locations that are potential sources for contaminants, and DC resistivity to determine site geology and locate a discontinuous confining unit. We also performed a seismic survey in one area of the site to map the confining unit extent and to test the method for use in areas with similar geology. Former trench locations and buried metal were mapped in two- and three-dimensions to aid in contaminant source determination and remediation. Borehole data, DC resistivity, and seismic data were used to characterize the subsurface geology of the site and to model the confining layer in three dimensions with Dynamic Graphics software EarthVision. Ground water elevations in monitoring wells were measured to model the piezometric surface and ground water flow directions. A hydrogeologic model of the site was produced using geophysical, borehole and ground water elevation data. The results of this analysis will be entered as coverages into the Army's environmental GIS and used to plan remediation activities.
Permafrost and Ground Water Pathway Delineation Using Ground Resistivity and Ground-Penetrating Radar, Fort Wainright, Alaska
Geohydrologic investigations can be complicated by the presence of discontinuous permafrost and bedrock fractures because both can alter the ground water flow direction. We conducted ground resistivity (GR) and ground-penetrating radar (GPR) investigations in an area of discontinuous permafrost and shallow, fractured bedrock on Fort Wainwright, Alaska, where a contamination plume has been detected by soil and ground water sampling. The stratigraphy consists of the Chena Alluvium and weathered Birch Hill Schist draped by loess above the Chena Alluvium on Birch Hill. Our purpose was to detect features that might influence ground water flow directions on the site and to model potential contaminant migration pathways to aid in remediation planning. All data were entered digitally into a geospatial database for subsequent analysis in ReMoTe, an environmental GIS. GPR profiles were recorded using 50-, 100-, and 400-MHz antennas. Analysis revealed that the seasonally frozen layer was detected using the 400-MHz antennas while the 50- and 100-MHz antennas recorded intermittent reflections interpreted to be permafrost and bedrock. On Birch Hill, the GPR signals were attenuated quickly due to high dielectric loss in the loess and weathered bedrock. The GR data mapped the permafrost zones in the Chena Alluvium with resistivities within the permafrost on the order of 1,000 to 15,000 ohm-m. In contrast, the unfrozen Birch Hill Schist ranges from 80 to 1800 ohm-m, the loess ranges from 170 to 340 ohm-m and the unfrozen silty sands and gravels of the Chena Alluvium range from 100 to 300 ohm-m. The unfrozen zones are interpreted to be potential ground water pathways. Low resistivity zones in the bedrock line up with known fractures in the schist. As the project continues, additional data will be compared via the GIS for 3-dimensional analysis of the aquifer system, as well as to provide project managers easy access to these data and the models generated from it.
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