Atlantic Coastal Plain Ground-Water Availability

Approximate model boundary of the North and South Carolina Coastal Plain Click on image for larger map

Background

In January 2004, the U.S. Geological Survey began a 3-year study to combine and update the Regional Aquifer System Analysis (RASA) models of North and South Carolina in order to improve the understanding of ground-water availability in the Atlantic Coastal Plain aquifer system of both States. Increased ground-water withdrawals related to population growth and drought of the last few years have emphasized the need for more accurate, detailed information describing the ground-water resources in the Coastal Plain. Currently North Carolina and South Carolina do not have up-to-date ground-water flow models of the entire Coastal Plain aquifer system. Since completion of the RASA models, however, additional ground-water pumpage, water-level, and hydrogeologic framework data have been collected.

The North Carolina RASA model was one of the first developed in the national RASA Program (Giese and others, 1997). The Trescott (1975) code modified by Leahy (1982) was used in the computer model that was calibrated to predevelopment and 1980 ground-water conditions. The South Carolina RASA model was developed by Aucott (1988; 1996), calibrated to predevelopment and 1982 conditions, and later recalibrated by Campbell and van Heeswijk (1996) using data collected in 1989.

Objectives

The principal objective of this study is to provide a tool for assessing ground-water availability in the Coastal Plain aquifer system of North and South Carolina. Specifically, the study will develop a model that will improve our understanding of the aquifer-system flow paths and recharge; evaluate ground- and surface-water interaction and the potential for base-flow reduction in streams as a result of increased ground-water withdrawals; and provide a scientifically based management tool for optimizing conjunctive water-use strategies. In addition to providing scientists with an improved database and better understanding of the ground-water-flow system, the model will provide a framework for facilitating natural-resource protection and water-management decisionmaking, and a tool for regulatory agencies to test the effects of alternative management scenarios before enactment. The model will be developed with carefully considered input simplifications and graphics-rich output to make it user-friendly for scientists and non-scientists alike.

Approach

The scope of the modeling effort is the Atlantic Coastal Plain area extending north from eastern Georgia through South and North Carolina and possibly into southern Virginia, including the surficial, Tertiary, and Cretaceous aquifer systems. The ground-water flow model will be developed using MODFLOW-2000 (Harbaugh and others, 2000), and will use the GMS graphical user interface. New USGS and state agency aquifer-property, ground-water level, and water-use data collected since the completion of the older models will be incorporated. A new regional hydrogeologic synthesis at the North Carolina - South Carolina State line will be produced. The model will be calibrated to ground-water levels and stream discharges for both steady-state (pre-development) and transient (post-development) conditions. Calibration to transient conditions will be completed for at least two time periods, including calibration to data collected during a synoptic ground-water level and stream-dicharge data collection effort planned for November 2004. The completed transient model will result from an integration of the most relevant, complete, and current hydrogeologic and water-use data available in Federal, State, and local agency databases. An essential part of the modeling effort will be collaboration with cooperators and stakeholders in both States and the formation of several project liaison committees early in the project.