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Hydrogeology and simulation of ground-water flow in the thick regolith-fractured crystalline rock aquifer system of Indian Creek Basin, North Carolina

Water-Supply Paper 2341-C
By C.C. Daniel, III, D.G. Smith, and J.L. Eimers

Full Report (PDF, 148 pages, 3 Mb)


Abstract

The Indian Creek Basin in the southwestern Piedmont of North Carolina is one of five type areas studied as part of the Appalachian Valleys-Piedmont Regional Aquifer-System analysis. Detailed studies of selected type areas were used to quantify ground-water flow characteristics in various conceptual hydrogeologic terranes. The conceptual hydrogeologic terranes are considered representative of ground-water conditions beneath large areas of the three physiographic provinces--Valley and Ridge, Blue Ridge, and Piedmont--that compose the Appalachian Valleys-Piedmont Regional Aquifer-System Analysis area. The Appalachian Valleys-Piedmont Regional Aquifer-System Analysis study area extends over approximately 142,000 square miles in 11 states and the District of Columbia in the Appalachian highlands of the Eastern United States. The Indian Creek type area is typical of ground-water conditions in a single hydrogeologic terrane that underlies perhaps as much as 40 percent of the Piedmont physiographic province.

The hydrogeologic terrane of the Indian Creek model area is one of massive and foliated crystalline rocks mantled by thick regolith. The area lies almost entirely within the Inner Piedmont geologic belt. Five hydrogeologic units occupy major portions of the model area, but statistical tests on well yields, specific capacities, and other hydrologic characteristics show that the five hydrogeologic units can be treated as one unit for purposes of modeling ground-water flow.

The 146-square-mile Indian Creek model area includes the Indian Creek Basin, which has a surface drainage area of about 69 square miles. The Indian Creek Basin lies in parts of Catawba, Lincoln, and Gaston Counties, North Carolina. The larger model area is based on boundary conditions established for digital simulation of ground-water flow within the smaller Indian Creek Basin.

The ground-water flow model of the Indian Creek Basin is based on the U.S. Geological Survey's modular finite-difference ground-water flow model. The model area is divided into a uniformly spaced grid having 196 rows and 140 columns. The grid spacing is 500 feet. The model grid is oriented to coincide with fabric elements such that rows are oriented parallel to fractures (N. 72° E.) and columns are oriented parallel to foliation (N. 18° W.). The model is discretized vertically into 11 layers; the top layer represents the soil and saprolite of the regolith, and the lower 10 layers represent bedrock. The base of the model is 850 feet below land surface. The top bedrock layer, which is only 25 feet thick, represents the transition zone between saprolite and unweathered bedrock.

The assignment of different values of transmissivity to the bedrock according to the topographic setting of model cells and depth results in inherent lateral and vertical anisotropy in the model with zones of high transmissivity in bedrock coinciding with valleys and draws, and zones of low transmissivity in bedrock coinciding with hills and ridges. Lateral anisotropy tends to be most pronounced in the north-northwest to south-southeast direction. Transmissivities decrease nonlineraly with depth. At 850 feet, depending on topographic setting, transmissivities have decreased to about 1 to 4 percent of the value of transmissivity immediately below the regolith-bedrock interface.

The model boundaries are, for the most part, specified-flux boundaries that coincide with streams that surround the Indian Creek Basin. The area of active model nodes within the boundaries is about 146 square miles and has about 17,400 active cells. The numerical model is designed not as a predictive tool, but as an interpretive one. The model is designed to help gain insight into flow-system dynamics. Predictive capabilities of the numerical model are limited by the constraints placed on the flow system by specified fluxes and recharge distribution.

Results of steady-state analyses that simulate long-term, average annual conditions indicate that the quantity of ground water flowing through model layers decreases with depth. In the top model layer, representing the soil and saprolite of the regolith, about 55 percent of recharge flows directly to streams, and 45 percent flows into layer 2. Lesser amounts flow into deeper layers. In the bottom model layer, the quantity of water moving in or out of the layer is about 2 percent of the maximum quantity that flows through the top layer. The quantity decreases with depth by about two orders of magnitude between layers 1 and 11, even though the bottom layer is 175 feet thick and the saturated thickness of the top layer is about 20 to 30 feet thick.

Flow-path and time-of-travel analyses show that most ground water flows through the shallower parts of the system close to streams, and that travel times in the regolith vary from less than 10 years to as much as 20 years from time of recharge to time of discharge in streams. Travel times along flow paths through the lower layers can take decades or even centuries; travel times approaching five centuries were computed in some areas for flow that passed through the bottom layer (675 to 850 feet below land surface).


Citation:

Daniel, C.C., III, Smith, D.G., and Eimers, J.L., 1997, Hydrogeology and simulation of ground-water flow in the thick regolith-fractured crystalline rock aquifer system of Indian Creek Basin, North Carolina, in Ground-water resources of the Piedmont-Blue Ridge Provinces of North Carolina: U.S. Geological Survey Water-Supply Paper 2341-C, 137 p.


For more information, contact
North Carolina Water Science Center
U.S. Geological Survey
3916 Sunset Ridge Road
Raleigh, North Carolina 27607
(919) 571-4000
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