SIMULATION OF GROUND-WATER FLOW IN THE SHENANDOAH VALLEY OF VIRGINIA USING VARIABLE ANISOTROPY TO REPRESENT GEOLOGIC STRUCTURE

Folded and faulted sedimentary rocks in the Shenandoah Valley of Virginia underlie the Massanutten synclinorium and form a hydrogeologic basin 200-km long, 30 km-wide, and about 5-km deep. The folded beds allow ground water to penetrate deeply below land surface—fresh water is obtained from wells over 800 ft deep. In a novel approach developed by this study, hydraulic connections along bedding planes within the rocks are represented by specifying variable anisotropy based on bedding form-surfaces interpolated from generalized geologic cross-sections. The directions of maximum and medium permeability conform to the strike and dip of the bedding, respectively, while the minimum permeability direction is normal to the strike and dip of the bedding. These three directions define the permeability tensor in individual mesh elements that compose a three-dimensional model developed with a modified version of the U.S. Geological Survey ground-water flow and transport simulator SUTRA.

The ground-water flow model represents a 7500-km2 area that contains three hydrogeologic units that correspond to the major rock types in the Shenandoah Valley (siliciclastic rocks, carbonate rocks, and crystalline rocks). The model was calibrated to baseflow in major streams and water levels measured in wells. The sensitivity of predicted ground-water flow paths and ages is investigated by changing (1) the degree of anisotropy, (2) the distribution of permeability with depth, and (3) the representation of folds and faults. The method used to represent hydraulic connections in this study is applicable to simulation studies of other fractured rock terranes, such as in the Valley and Ridge province and the Mesozoic Basins in the Piedmont province.