South Carolina Water Science Center
May River, Beaufort County
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Groundwater Availability of the Atlantic Coastal Plain Aquifers of North and South Carolina
Project Number: 2519-BC701
Groundwater withdrawals from Atlantic Coastal Plain aquifers in North Carolina (NC) and South Carolina (SC) (fig 1) have increased over the past 15 years in response to demands for water for a rapidly growing population. The 2000 Census reported that the combined populations of Coastal Plain counties in NC and SC totaled nearly 6 million people with 3.2 million located in NC and 2.5 million in SC. These respective populations comprised about 40 percent of NCís total State population and about 63 percent of SCís total population. Overall, the populations of both states have grown rapidly. In NC, the population grew by 21.4 percent from 1990-2000 (Perry and Mackun, 2001). The population is projected to grow another 13.7 percent by the year 2 (Campbell, 1997). The numbers are similar in SC with a population increase of 15.1 percent from 1990-2000 and a projected 13.2 percent increase by 2015. While NC and SC endeavor to increase their development of surface-water supplies in response to the rapid growth in these coastal populations, both States recognize that they are facing a number of unanswered questions regarding their groundwater supplies. For instance, the effects of groundwater withdrawals on the quantity of freshwater discharge to streams, estuaries, and wetlands are unknown. Further complicating these issues are regional concerns about saltwater intrusion, which is already occurring in some areas along the SC coast.
The problem of adequate groundwater supplies and declining water levels in the Coastal Plain of NC and SC date back to the early part of the 20th century. For example, groundwater from the Middendorf aquifer had been used since 1879 to supply water to the Charleston, SC, area. When water levels and production began to decline in the 1920ís, however, Charleston was forced to abandon use of the aquifer and switch to a surface-water source to ensure a sufficient supply of water for Charlestonís population. In another case, groundwater withdrawals from the Lower Cape Fear aquifer in southeastern Virginia (VA) since the 1940ís caused groundwater levels to decline sharply in northeastern North Carolina. More recently in 1967, a phosphate mining operation in NC caused the dewatering of a part of the Castle Hayne aquifer. As a result, a capacity-use area (CUA) was established to regulate groundwater withdrawals from the Castle Hayne aquifer in the area of the mining operation. In 2002, a second CUA was established in 15 counties in the central Coastal Plain region (fig. 2) to regulate withdrawals from the Black Creek and Upper Cape Fear aquifers. Under the 2002 CUA, a number of counties and municipalities within the central Coastal Plain region must reduce withdrawals by 25 percent within the next 6 years and by 75 percent within the next 16 years. Land subsidence measuring as much as 7 inches has been documented (during the 33-year period from 1935-68) in the central Coastal Plain of NC, and overall water-level declines are estimated to be as much as 200 feet (ft) near pumping centers. A third CUA has bproposed for the Upper Cape Fear aquifer in Bladen County in the southern NC Coastal Plain.
The following web site contains detailed information on the Central Coastal Plain Capacity Use Area of NC: http://www.ncwater.org/Permits_and_Registration/Capacity_Use/Central_Coastal_Plain/
Facing similar problems with declining water levels, SC also has instituted use of Capacity Use Areas. In 1979, a CUA was established in the Myrtle Beach, SC, area, because of 200-ft drawdowns from predevelopment levels in the Black Creek aquifer. In 1981, the Hilton Head, SC, area was designated as a CUA because of a 130-ft-deep cone of depression centered at Savannah, Georgia (GA), which is contributing to saltwater encroachment in the Upper Floridan aquifer. More recently in 2002, the Charleston, SC, area was designated as a CUA because of 180-ft drawdowns in the Middendorf aquifer.
The recent drought (1998-2002) experienced in the East has further exacerbated the problem of declining water levels. During the drought, groundwater levels in the Coastal Plain of the Carolinas declined to some of the lowest levels on record. The Pee Dee River in SC essentially stopped flowing due to the effects of the drought (Henderson, 2002), causing NC and SC State officials to enter into negotiations over how much water from the Pee Dee River should be allowed to cross the NC border into SC. Freshwater intakes on the lower Pee Dee near Georgetown, SC, were shut down due to saltwater encroachment in the river as a result of the low-flow conditions.
A combined NC-SC model would be useful to address interstate groundwater issues, such as the development of the Black Creek aquifer in Horry County, SC, the Castle Hayne aquifer in Brunswick County, NC, and the Upper Cape Fear aquifer in the Bladen County, NC area. Coupling an updated model with the flow model that currently is under development for the Savannah, GA, and Hilton Head Island, SC, areas and with the updated VA Coastal Plain model, which includes northeastern NC, would result in a substantial regional management tool that the States could use to address interstate water issues involving the Atlantic Coastal Plain between SC, GA, NC, and VA.
Increased groundwater withdrawals related to population growth and drought-related problems of the last few years have emphasized the need for more accurate, detailed information describing the groundwater resources in the Coastal Plain region. Both NC and SC recognize the need for cooperation to address these critical issues. The States further recognize the need for current water-management tools, such as an updated groundwater flow model for the Coastal Plain region. Groundwater availability and use in the Coastal Plain are formidable issues, having caused at least the State of SC to formally address. The draft 2003 State Water Plan in SC calls for the construction of an up-to-date groundwater flow model to be used to assist in answering resource management questions (Bud Badr, SCDNR, personal commun., 2003). Currently, neither NC nor SC have up-to-date groundwater flow models of the Coastal Plain, although since completion of the Regional Aquifer System Analysis (RASA) models in both States, an abundance of groundwater pumpage, water-level, and hydrogeologic framework data have been collected.
OBJECTIVES AND SCOPE
Land surface elevations of the study area range from 0 to 700 feet
The objectives of revitalizing and updating the NC-SC RASA models are to improve our understanding of the Atlantic Coastal Plain aquifer system flow paths and recharge; and to evaluate groundwater and surface-water interactions and the potential for reduction in both stream base flow and discharge to wetlands and estuaries as a result of increased groundwater withdrawals. A second important objective is to provide a scientifically based management tool for optimizing conjuctive water-use strategies and for optimizing groundwater withdrawals in order to mitigate saltwater intrusion. Specifically, the proposed updated model would enable the States to: determine optimal locations of pumping centers and magnitudes of withdrawals that would reduce interference of water-resource demands; test possible scenarios for pumpage of alternative aquifers and withdrawals from surface-water resources; evaluate potential reduction of base flow in rivers and wetlands from overpumpage of the aquifers; and determine potential susceptibility of the shallow aquifers to contamination, saltwater intrusion, and induced leakage from overpumpage of deeper, confined aquifer.
The scope of the modeling effort is the Atlantic Coastal Plain area extending from eastern GA through SC and NC, and possibly into southern VA, and bridging between several on-going modeling efforts of the Atlantic Coastal Plain aquifer system. The modeling effort would include the surficial, Tertiary, and Cretaceous-age Coastal Plain aquifer systems.
USGS Hydrologist collecting data from a groundwater production well in the Chesterfield County study area (USGS Photo)
The approach of the proposed study focuses on a regional groundwater-flow modeling effort to address critical groundwater supply issues in the Atlantic Coastal Plain areas of North and South Carolina and, in particular, provide a regional hydrogeological synthesis at the shared state boundary. An essential part of this effort will be interaction with cooperators and stakeholders in both states and formation of several project liaison committees early in the project. In addition, USGS, state, and local agency databases will be reviewed for inclusion in the updated hydrogeologic framework and groundwater use database for the model.
The groundwater flow model will be developed using MODFLOW-2000 (Harbaugh and others, 2000), and will use a commercial Graphical User Interface (GUI) to enhance pre- and post-processing tasks, as well as to allow for ease of use, model refinement, and updating. U.S. Geological Survey scientists from North and South Carolina will develop the model jointly, and will provide technical and scientific expertise for geohydrologic modeling. Members of the TAC will provide specific hydrogeologic data and modeling concepts for use in constructing the model, and will participate in all aspects of the project including acquisition and interpretation of geologic information, development and calibration of the MODFLOW model, and preparation of management scenarios. The project will be organized to emphasize technology transfer to state and local scientists, engineers, and water managers.
The study includes syntheses of streamflow data, aquifer boundary conditions, and hydraulic properties of the aquifer, such as aquifer transmissivity. During construction of the groundwater flow model, the study will test initial model parameters and model sensitivity, and perform a steady-state calibration of parameters. In subsequent iterative runs, the steady-state model will be used to refine a transient-model calibration, which will compare model results with the hydrologic data available.
Open File Report 2006-1298
Proceedings of the 2008 South Carolina Water Resources Conference
Professional Paper 1773
MODEL INPUT/OUPUT FILES
Input files used with the model and resulting output files
Aucott, W.R., 1996, Hydrology of the Southeastern Coastal Plain aquifer system in South Carolina and parts of Georgia and North Carolina: U.S. Geological Survey Professional Paper 1410-E, 83p.
Campbell, B.G., and van Heeswijk, M., 1996, Ground-water hydrology, historical water use, and simulated ground-water flow in Cretaceous-age Coastal Plain aquifers near Charleston and Florence, South Carolina: U.S. Geological Survey Water-Resources Investigations Report 96-4050, 100 p.
Campbell, P., 1997, Population projections: States, 1995-2025: Current Population Reports, U.S. Census Bureau P25-1131, 6 p.
Clark, W.B., Miller, B.L., Stephenson, L.W., Johnson, B.L., and Parker, H.N., 1912, The Coastal Plain of North Carolina: North Carolina Geological and Economic Survey Bulletin 3, 372p.
Giese, G.L., Eimers, J.L., and Coble, R.W., 1997, Simulation of ground-water flow in the Coastal Plain aquifer system of North Carolina, in Regional Aquifer-System AnalysisóNorthern Atlantic Coastal Plain: U.S. Geological Survey Professional Paper 1404ĖM, 142 p.
Lautier, J.C., 2003, Hydrogeologic framework and ground-water conditions in the North Carolina central Coastal Plain: North Carolina Department of Environment and Natural Resources, Division of Water Resources Report, 46 p.
Lyke, W.L., Winner, M.D., Jr., and Brockman, A.R., 1989, Potentiometric surface of the Black Creek aquifer in the central coastal plain of North Carolina, December, 1986: U.S. Geological Survey Water Resources Investitations Report 87-4233, one sheet.
Newcome, Roy, Jr, 1993, Pumping tests of the Coastal Plain aquifers in South Carolina: South Carolina Water Resources Commission Report 174, 52p.
Newcome, Roy, Jr., 2000, Results of pumping tests in the Coastal Plain of South Carolina (Supplement to Water Resources Commission Report 174): South Carolina Department of Natural Resources Open-File Report 5.
Perry, M.J., and Mackun, P.J., 2001, Population change and distribution - 1990 to 2000: U.S. Census Bureau, Census 2000 brief, 7 p.
Stricker, V.A., 1983, Base flow of streams in the outcrop area of the Southeastern sand aquifer—South Carolina, Georgia, Alabama, and Mississippi: U.S. Geological Survey Water-Resources Investigations Report 83-4106, 17 p.
Strickland, A.G., 2000, Water-level conditions in the Black Creek aquifer, 1992-98, in parts of Bladen, Hoke, Robeson, and Scotland Counties, North Carolina: U.S. Geological Survey Water-Resources Investigations Report 00-4138, 23 p. + 1 pl.
Stringfield, W.J., and Campbell, B.G., 1993, Potentiometric surfaces of the November 1989 and declines in the potentiometric surface between November 1982 and November 1989 for the Black Creek and Middendorf aquifers in South Carolina: U.S. Geological Survey Water Resources Investigation Report 92-4000, 4 sheets.
Winner, M.D., Jr., Lyke, W.L., and Brockman, A.R., 1989b, Potentiometric surface of the Lower Cape Fear aquifer in the central coastal plain of North Carolina, December 1986: U.S. Geological Survey Water Resources Investigations Report 87-4234, 1 sheet.