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Simulation of ground-water flow in aquifers in Cretaceous rocks in the central Coastal Plain, North Carolina

Water-Resources Investigations Report 89-4153
By J.L. Eimers, W.L. Lyke, and A.R. Brockman

Abstract

The principal sources of water-supply in Cretaceous rocks in the central Coastal Plain of North Carolina are the Peedee, Black Creek, and upper Cape Fear aquifers. Ground-water withdrawals from these aquifers have increased from about 0.25 million gallons per day in 1910 to over 29 million gallons per day in 1986, causing water-level declines as much as 160 feet. The maximum rate of water-level decline in 1986 is about 11 feet per year in the Black Creek aquifer.

A quasi-three dimensional ground-water flow model was constructed and calibrated for the period 1900 to 1986 to simulate past water-level declines and to estimate the effects of future pumpage. Comparisons of 1,867 observed and model-computed heads were made at 323 well sites. The average difference between computed and observed water levels is -1 foot. About 68 percent of all the differences between computed and observed water levels falls in the range from -21.0 to 21.0 feet.

Simulation indicates that the 29 million gallons per day of pumpage in 1986 was supplied by (1) increased recharge (net discharge of 2 million gallons per day in 1900 changed to net recharge of 18 million gallons per day in 1986), (2) increased lateral inflow to the aquifers of about 8 million gallons per day, and (3) depletion of ground-water storage of about 1 million gallons per day. Two pumping scenarios simulated head changes through 1991 and were based on (1) constant pumpage at the 1986 rates in each aquifer, and (2) continuing increases in pumping rates from 1986 through 1991 and rates varying from 10 to 19 percent per year for the three pumped aquifers. For scenario 1, water-level declines exceeded 5 feet locally; however, water-level rises of about 1 foot occurred in two areas. For scenario 2, water-level declines ranged from 1 foot to 30 feet in some pumping centers.

Eimers, J.L., Lyke, W.L., and Brockman, A.R., 1990, Simulation of ground-water flow in aquifers in Cretaceous rocks in the central Coastal Plain, North Carolina: U.S. Geological Survey Water-Resources Investigations Report 89-4153, 101 p.