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2003, Integration of geophysics, geologic mapping, and water-level monitoring to characterize the hydrology of a fractured bedrock site in

By James R. Degnan and U. S. Geological Survey


Elemental mercury is found in fractured depressions and potholes on the bedrock surface, in the overburden, and is dissolved in ground water at the site of a former chlor-alkali plant (cell house) along the bank of the Androscoggin River, in Berlin, N.H. The cell house has been demolished, ground-water flow impeded by installation of an up-gradient-slurry wall, and capped with an impermeable geomembrane. Mercury has been removed from fractures at the surface on the riverbank five times; however, more continues to be found. Potential pathways for movement of mercury and ground water were indicated with the results of a geohydrologic study. The study demonstrates the integration of geophysical surveys, geologic mapping, and water-level monitoring in three areas: (1) the riverbank, (2) the site perimeter, and (3) the capped area. Results from ground-penetrating radar and two-dimensional-resistivity surveys along the riverbank indicate electrically conductive zones that are interpreted as bedrock fractures, which are potential ground-water-flow paths. Geologic mapping along the riverbank shows that bedrock fracturing is more prevalent in gneiss than pegmatite and is associated with schist in shear zones that correspond to geophysical anomalies. Nearly horizontal anomalies in ground-penetrating-radar survey results beneath areas mapped as pegmatite may represent locations where vertical fractures in gneiss terminate on a horizontal fractured contact between gneiss and pegmatite. Significant hydraulic connections were identified by analysis of stage changes in the Androscoggin River and bedrock water levels. Assessment of hydraulic heads in bedrock and overburden show vertical ground-water gradients in both directions. Resistivitysurvey results along the site perimeter indicate fractures and overburden-filled bedrock troughs where ground water may bypass a slurry wall. Overburden water levels and discharge from a drainage pipe suggest that ground water is passing through or beneath the slurry wall. Response of overburden water levels do not show connections with the river

Year: 2010
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