Simulation of
ground-water flow in the Coastal Plain aquifer system of North
Carolina
Open-File Report 90-372
By G.L. Giese, J.L. Eimers, and R.W. Coble
Abstract
A 3-D finite difference digital model was used to simulate
groundwater flow in the 25,000 sq mi aquifer system of the North
Carolina Coastal Plain. The model was developed from a
geohydrologic framework that is based on an alternating sequence
of 10 aquifers and 9 confining units, which comprise a
seaward-thickening wedge of sediments that form the Coastal
Plain aquifer system in North Carolina. The model was calibrated
by comparing observed and simulated water levels. The maximum
transmissivity of an individual aquifer in the calibrated model
is 200,000 sq ft/d in a part of the Castle Hayne aquifer, which
is composed predominately of limestone. The maximum simulated
vertical hydraulic conductivity in a confining unit was 2.5 ft/d
in a part of the confining unit overlying the upper Cape Fear
aquifer. Analysis indicated the model is highly sensitive to
changes in transmissivity and leakage near pumping centers; away
from pumping centers, the model is only slightly sensitive to
changes in transmissivity but is moderately sensitive to changes
in leakance. Recharge from precipitation to the surficial
aquifer ranges from about 12 in/yr in areas having clay at the
surface, to about 20 in/yr in areas having sand at the surface.
Most of this recharge moves laterally to streams, with only
about 1 in/yr moving down to the confined parts of the aquifer
system. Groundwater level declines, which are the result of
water taken from storage, are extensive in some area and minimal
in others. Water level declines exceeding 100 ft have occurred
in the Beaufort County area because of withdrawals for a mining
operation and water supplies for a chemical plant. Head declines
have been less than 10 ft in the shallow surficial and Yorktown
aquifers and in the updip parts of the major confined aquifers
distant from areas of major withdrawals. A water-budget analysis
using the model simulations indicates that much of the water
removed from the groundwater system by pumping ultimately is
made up by a reduction in water leaving the aquifer system,
which discharges to streams as base flow.
Citation:
Giese, G.L., Eimers, J.L., and Coble, R.W., 1991, Simulation of ground-water flow in the Coastal Plain aquifer system of North Carolina: U.S. Geological Survey Open-File Report 90-372, 178 p.
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