KARST PATHWAY DELINEATION USING COMBINED SPATIAL AND GEOPHYSICAL ANALYSIS AT CAMP CROWDER, MISSOURI

Camp Crowder, a Missouri Army National Guard training facility located south of Neosho, Missouri, is part of the Pool's Prairie NPL site. DNAPL contamination found within the soil and groundwater is suspected of migrating off-post through conduit and fracture systems that have developed within the underlying karst bedrock. Dye tracer studies have confirmed groundwater movement towards the contaminated wells, though the actual route of migration is not known. The overburden is dominantly a cherty residuum, through which the surface water and contaminants percolate into the underlying karst aquifer. An integrated approach using photo-geologic analysis, airborne electromagnetic surveys, seismic profiling, and two-dimensional electrical resistivity imaging (2D-ERI) was conducted to map subsurface changes associated with this complex karst system.

Surface based geophysical surveys were able to map the character of the overburden and underlying bedrock. Seismic refraction data proved best in resolving the top of the competent bedrock (below the weathering layer), whereas the resistivity data imaged the top of the weathered horizon (transition from low-to-high resistivity) and coarser grained intervals within the overburden. In many instances, lineaments interpreted from aerial photographs coincide with disruptions observed in the ground-based geophysics. Interpretation of the surface geophysical data suggests that a mantle of friable, heavily fractured, and/or weathered rock overlies the competent bedrock and may act as a zone for contaminant storage, and for migration over short distances.

Airborne multi-frequency electromagnetic mapping provided a regional view of the site. Conductivity inversion of the data correlated well with the surface geophysical data and indicated that the mantle of friable material extended farther than originally anticipated. This extension brought potential contaminant pathways into contact with several additional lineaments. Derived bedrock topography indicates that the contaminant source area is situated on a transition zone between deep and shallow residuum, with paths and pools leading in several directions.