Ground-water study of the central Coastal Plain, North Carolina
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Geology topics
W.L. Lyke
Publications and source records attributed to W.L. Lyke.
Hydrogeology of aquifers in Cretaceous and younger rocks in the vicinity of Onslow and southern Jones counties, North Carolina
Unconsolidated sediments in Onslow and Jones Counties, North Carolina overlie crystalline basement rocks and range in thickness from about 700 ft to more than 1,800 ft, thickening toward the east. This material is composed of permeable sand and limestone interlayered with relatively impermeable clay and silt beds. Sediments are divided into two groups: aquifers in Quaternary-, and Tertiary-aged rocks and aquifers in Cretaceous-aged rocks. Aquifers in the Cretaceous rocks provide most of the groundwater for public supplies and are the focus of this report. The aquifers in Cretaceous rocks are the Peedee, Black Creek, upper Cape Fear, and lower Cape Fear aquifers, which are composed of beds or groups of beds of sand and gravel. Each aquifer is overlain by a clay and silt bed, that impedes the flow of water between aquifers. The thickness of Cretaceous hydrogeologic units ranges from about 700 ft to more than 1, 300 ft. Hydrogeologic units are correlated using 60 geophysical logs and accompanying drillers ' logs along with water level and water quality data. Three hydrogeologic sections demonstrate the continuity of the aquifers and confining units, show water levels and chloride concentration in water from test intervals, and delineate where chloride concentration in water exceeds 250 mg/L within each aquifer. Maps of each aquifer in Cretaceous rocks show altitude of its top, thickness, sand percentage, and the transition from freshwater to saltwater. Maps of the confining units show thickness and sand percentage of each.
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Simulation of ground-water flow in aquifers in Cretaceous rocks in the central Coastal Plain, North Carolina
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.
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Ground-water pumpage and water-level declines in the Peedee and Black Creek aquifers in Onslow and Jones counties, North Carolina, 1900-86
Two aquifers in sediments of Cretaceous age, The Peedee and Black Creek aquifers, have become a major source of freshwater in Onslow and Jones Counties in North Carolina since about 1960. Prior to 1960, most water systems in this area withdrew water from younger sand or limestone beds that overlie the Peedee and Black Creek aquifers. Water-quality and economic considerations related to the treatment of water from these shallower aquifers led to increased use of the Peedee and Black Creek aquifers. Water withdrawals from the Black Creek and Peedee aquifers were about 10,000 gallons per day in 1933. By 1986, total withdrawals were about 7.8 million gallons per day, about 90 percent of which was supplied from the Black Creek aquifer. As a result of these withdrawals, ground-water levels have declined throughout Onslow and Jones Counties. The average rate of decline in static water levels in the Peedee aquifer is about 0.6 feet per year in central Jones County and about 1.4 feet per year in northern Onslow County. Rates of water level decline in the Black Creek aquifer average about 8.3 feet per year in Jones County to about 12 feet per year in northern Onslow County. Water levels in the Peedee aquifer have declined as much as 40 feet in Jones County and 80 feet in northern Onslow County since about 1900. During the same period, water levels in the Black Creek aquifer have declined as much as 120 and 160 feet in Jones and northern Onslow Counties, respectively.
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Aquifers in Cretaceous rocks of the central Coastal Plain of North Carolina
Aquifers in rocks of Cretaceous age are the major source of groundwater for public supplies in the central Coastal Plain. These aquifers consist of sand, gravel, and limestone beds of the Peedee, Black Creek, and the upper and lower Cape Fear aquifers, each separated by a confining unit composed of clay and silt beds. The aquifers and confining units (1) rest upon crystalline basement rocks; (2) dip and thicken to the east-southeast; and (3) are overlain by younger aquifers and confining units in deposits of Quaternary and Tertiary age. The top of the uppermost aquifer, the Peedee, ranges from 122 ft above to 595 ft below sea level. The maximum thickness of all aquifers and confining units in Cretaceous rocks is more than 1,600 ft. Aquifers and confining units were defined and correlated for this report using 125 geophysical logs and accompanying drillers' logs, water level data, and water quality information regarding chloride concentrations in water. This analysis allowed the construction of seven hydrogeologic cross sections that depict the continuity of all the aquifers and confining units. These cross sections also show water levels and chloride concentrations in water from various test intervals and describe where chloride concentration in water exceeds 250 mg/L within each aquifer. Detailed maps of each Cretaceous aquifer show altitude of its top, thickness, areas of selected sand percentages, transition zones from fresh to saltwater, and the thickness of the confining unit. H ydrogeologic data for all aquifers and confining units are presented in tabular form.
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Regional study of the Castle Hayne Aquifer of eastern North Carolina
The Castle Hayne aquifer is an eastward sloping and thickening wedge of limestone and sandstone, located in a 12,500 sq mi area in the eastern part of North Carolina. The Castle Hayne aquifer is the major source of freshwater for much of coastal North Carolina where the aquifers underlying the Castle Hayne contain saltwater. A regional study of the Castle Hayne aquifer was conducted to provide definitive information on the declines in water levels and of saltwater intrusion in the area. A series of geohydrologic maps produced from the study, show the altitude of the top of the aquifer, location of saltwater in the aquifer, aquifer thickness, aquifer transmissivity, and confining-unit thickness. The history of groundwater pumpage can be compared with historical water levels in the aquifer. These geohydrologic, pumpage, and water level data can be used to develop estimates of aquifer and confining-unit hydraulic coefficients and to develop a groundwater flow model for the Castle Hayne aquifer. The model will increase the understanding of the groundwater flow system and also can be used to assess various groundwater development scenarios for the entire Castle Hayne aquifer.
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