COMPARISON OF ANALYTICAL AND NUMERICAL ESTIMATES OF THE RESPONSE OF A COASTAL AQUIFER SYSTEM TO DECREASING RECHARGE

The ground-water flow system in Kaloko-Honokohau National Historical Park consists of a body of brackish water overlying denser saltwater. Precipitation and fog drip that infiltrates the aquifer in mountainous inland areas is believed to flow seaward through the dense low-permeability dike complexes and the geologically young, highly permeable lava flows of Hualalai Volcano. Ground water discharges from the aquifer to fishponds, anchialine pools, and wetlands in the park. The response of the ground-water flow system to decreasing recharge was analyzed using a 1-D analytical model based upon the Ghyben-Herzberg and Glover relations, which allows for vertical flow at an outflow face through which water discharges to the sea, and a 2-D cross-sectional numerical model capable of simulating variable-density flow. Results were compared to a previously developed regional 2-D numerical ground-water flow model that uses a sharp-interface code. The models differ in their treatment of heterogeneity and the diffuse transition zone that exists between freshwater and saltwater in the aquifer. The results of the analyses provide insight into the behavior of the aquifer system in response to ground-water withdrawals and constrain the range over which ground-water salinity may increase due to seawater intrusion.