Ground‐Water Flow Simulation for Management of a Regulated Aquifer System: A Case Study in the North Carolina Coastal Plain

Abstract The State of North Carolina designated a portion of the North Carolina Coastal Plain as its first and only Capacity Use Area in 1969 in response to large‐scale withdrawals from the Castle Hayne aquifer system for phosphate mining operations. Withdrawals from this regulated aquifer require water‐use permits. Applications for water‐use permits require information on the use of the water, including an assessment of the impacts on the source aquifer(s). The U.S. Geological Survey finite‐difference model MODFLOW was used to simulate ground‐water flow in a 1,300 square mile area in the Capacity Use Area to estimate the effects of a proposed mine advance anticipated to require more extensive pumping from the aquifer. In addition, the model was used to evaluate the effects of a proposed municipal wellfield also to be located in the Capacity Use Area. The model provided estimates of required pumping rates for the mining activities. Simulated potentiometric surfaces and hydrographs for the Castle Hayne aquifer resulting from both the mining and the wellfield operations indicated that the Castle Hayne aquifer achieves steady state relatively rapidly and is impacted over a large area as a result of the depressurization pumping. Wellfield pumping has a relatively minor effect. Impacts on ground‐water levels in the Castle Hayne aquifer at nearby communities are mainly dependent on the location of the active mining center. Regional‐scale ground‐water flow models can provide information on the potential effects on the flow system of significant hydrologic stresses and could therefore be useful in assessing the impacts of the stresses as required in the permitting process.