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Mapping Structural Pathways for DNAPL Transport in Karst

Publisher –
Environmental and Engineering Geophysical Society (EEGS), 2003 Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP) proceedings.

Authors –
Larry J. Hughes, EnSafe Inc., Memphis, Tennessee, U.S.A.;
Norman R. Carlson, Zonge Engineering & Research Organization, Inc., Tucson, Arizona, U.S.A.

Paper – [pdf] ENV_DNAPLPaperSAGEEP2003

A capped landfill in central Tennessee lies atop regolith, thought to be some 20 meters thick, which overlies limestone bedrock with extensive karst weathering. Fractures and joints are pervasive in bedrock, following three well-defined trends. Trichloroethene (TCE) has been detected in an offsite spring, and the State wanted to drill and sample a deep structure beneath the landfill to maximize the chances of finding this dense contaminant. An induced polarization (IP) survey in an agreed-to test section of the landfill produced a 3D map of subtle linear anomalies, interpreted as structures and karst development. The structures are enhanced beneath thicker parts of the landfill. Successive depth slices show
that the enhancement effect progresses from broad areas beneath the landfill to narrower, deeper, linear zones downgradient from it. We hypothesize that an IP effect occurs because of clays within the structures, but also that migrating conductive leachate preferentially “lights up” these structures to IP, providing an indirect map of contaminant migration pathways. Seven boreholes were drilled on and off IP anomalies: nominal depths to bedrock (24 to 35 meters) were observed away from the IP-defined structures, but deep bedrock (>99 meters in one case) occurred within pockets of high IP response. These results were used to site two monitoring wells for which TCE data will be forthcoming.