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Conceptualization and characterization of fractured-bedrock ground-water systems--Current research
By Melinda J. Chapman
Ground-water systems in crystalline bedrock aquifers often are difficult to characterize because of the inherent complexity of the geologic setting and fracture network of the flow system. In the Southeastern United States, the overlying regolith is considered the primary ground-water storage reservoir that supplies water to the fracture system in the underlying bedrock. Fractures have little storage capacity individually, but may be connected to a fracture network having higher storage capacity. Pumping wells initially withdraw water from storage in fracture networks and with continued pumping eventually encounter water stored in the overlying regolith. In some areas, a transition zone, consisting of partially weathered rock, may be present between the regolith and bedrock.
Conceptualization of ground-water flow in complex fractured-bedrock settings continues to evolve. In the Piedmont area of Georgia, studies have focused on analyses of geologic and topographic factors affecting high-yield wells, including geophysical analyses of fracture zones. In New Hampshire, detailed studies of fracture zones and networks at the U.S. Geological Survey Mirror Lake fractured-rock research site have resulted in the development of new subsurface characterization methods. In North Carolina, conceptual hydrogeologic frameworks were formulated and tested, and detailed statistical analyses of well data and numerical ground-water flow modeling was conducted in the Piedmont and Blue Ridge Provinces. A recently initiated ground-water resource study in North Carolina is focused on determining the factors and processes that affect ground-water flow in fractured-bedrock settings of the Piedmont and Blue Ridge Provinces. Type-area studies will be conducted in representative geologic settings using newly developed subsurface characterization methodology to evaluate conceptual models of the ground-water-flow system.
Factors controlling ground-water flow should be characterized in detail in studies of contaminant transport and protection of ground-water supplies. Fractured crystalline-bedrock aquifers are highly heterogeneous and often anisotropic. The regolith, being the shallow ground-water reservoir, is most likely to become contaminated from surface sources. However, in areas having high clay content, movement of contaminants may be slow as a result of low permeability. The transition zone may have the greatest permeability and, therefore, the most potential for lateral ground-water flow. Characterization of contaminant transport problems should address complex three-dimensional pathways within the bedrock fracture network. For example, in some fracture systems, contaminants may remain relatively undiluted and transported quickly in the fracture network. Connection of bedrock fractures with the overlying regolith is assumed in most hydrogeologic settings. Plans for the protection of water-supply wells should consider that ground-water flow gradients in the bedrock will be modified by pumping from a bedrock supply well. High pumping rates can induce large vertical gradients between the bedrock and overlying regolith and thus accelerate rates of contaminant movement. Drawdown associated with bedrock supply wells has been observed across surface-water drainage divides, as a result of fracture structure not reflected in surficial features.
Characterization of a fractured crystalline-bedrock aquifer at the local scale may include the evaluation of hydraulic gradients between the overlying regolith and bedrock; determination of bedrock fracture and fracture zone properties (depth, orientation, hydraulic head); geochemical analyses (comparison of regolith, transition zone, and bedrock ground-water quality); and physical flow-path studies. In recent years, technology has greatly improved our ability to measure physical properties of fractures and fracture zones in crystalline-bedrock wells. These methods include the application of new borehole geophysical technology, downhole packer equipment, improved surface geophysical data acquisition and processing, and ground-water age-dating and isotopic techniques. Determination of ground-water inflow and outflow, pressure heads, and fracture orientations can now be determined for each fracture zone within a well, improving our ability to physically describe ground-water flow in the complex crystalline-bedrock aquifer system. Geologic (lithologic) settings of fracture zones can be determined using traditional natural gamma logs, cuttings or core, and new digital video imaging (virtual core). Fracture orientation can be interpreted from three borehole geophysical methods: acoustical televiewer, oriented video, and directional radar. Analyses of water samples for chlorofluorocarbons and tritium/helium isotopic ratios, and stable isotopes (oxygen and nitrogen) can provide quantitative characterization of pathways of ground-water flow. Data synthesized from local studies in similar geologic and topographic settings may be transferable to interpretations of regional scale.
Chapman, M.J., 2001, Conceptualization and characterization of fractured-bedrock ground-water systems--Current research [abs.],
in Enigmas of ground water in crystalline rocks--Problems and solutions, symposium proceedings, February 16, 2001, Raleigh, N.C.: Carolina Sections of the Association of Engineering Geologists and the American Institute of Professional Geologists, 2 p.
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|North Carolina District
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|Available from the Association of Engineering Geologists or the American Institute
of Professional Geologists, proceedings of the symposium, Enigmas of Ground Water
in Crystalline Rocks--Problems and Solutions, February 16, 2001, Raleigh, N.C.