Using Hydrochemical Fades to Delineate Ground Water Flowpaths in Fractured Shale |
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| Authors: | M E Schreiber G R Moline J M Bahr |
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| Institution: | Madeline E. Schreiber is a Ph.D. candidate in the Department of Geology and Geophysics at the University of Wisconsin-Madison (1215 W. Dayton St., Madison WI 53706, phone: ( 262-9467;fax: 262-0693). Her research interests include bioremediation, low-temperature geochemistry, and geomicrobiology. She received her B.S. in geology from Yale University and her M.S. in geology from the University of Wisconsin-Madison.;Gerilynn R. Moline received her Ph.D. in geology with an emphasis in hydrogeology from the University of Wisconsin- Madison in 1992. Since that time she has been a research staff scientist in the Environmental Sciences Division at the Oak Ridge National Laboratory (P.O. Box 2008, Oak Ridge, TN 37831-6400) and also holds adjunct faculty positions at the University of Tennessee and the University of Utah. Her current research focuses primarily on subsurface flow and transport processes in fractured porous media, with an emphasis on the use of applied and environmental tracers to quantify mass transfer processes in complex heterogeneous systems.;Jean M. Bahr received her Ph.D. in applied earth sciences (hydrogeology) from Stanford University in 1987. She then joined the faculty at the University of Wisconsin-Madison (1215 W. Dayton St., Madison WI 53706), where she is currently a professor. She teaches courses in hydrogeology, contaminant hydrogeology, and hydrogeologic field methods. Her research interests include the field and modeling studies of contaminant transport, ground water-surface interactions, and chemical evolution of ground water. |
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| Abstract: | Large differences in chemistry between sampling points separated In short vertical intervals are often observed in contaminant plumes in both granular and fractured aquifers. However, most regional models assume that such differences will be reduced by dispersive mixing during transport. At a field site located in a discharge area on the Oak Ridge Reservation, Tennessee, ground water flows along discrete flowpaths, as evidenced by the presence of four distinct water types—Ca-HCO3, Ca-Na-HCO3, and Na-Ca-HCO3, and Na-Ca-HCO3-S04—in samples collected from shallow (< 3D in) multilevel wells. The preservation of distinct chemical signatures suggests that ground water must he contained in discrete flow zones during much of its transport time. The chemical composition of the water types can be explained primarily by strata-bound flow over varying flowpath lengths and secondarily by mixing of waters during cross-formational flow in a discharge zone. The hydrochemical facies identified by correlation of water types between the boreholes indicate the general orientation of ground water How paths. These inferred flowpaths are oblique to the orientation of the measured hydraulic gradient and are more closely aligned with bedding and the calculated flow direction. Results of this study indicate that discrete multilevel sampling for analysis of major ions, in addition to information gathered from tracer tests, borehole flow tests. and visual core observations, can provide valuable information on flow directions and preferential flowpaths for contaminant transport. |
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