Numerical Modeling of Groundwater Flow below an Estuary Using MODFLOW and SEAWAT—Comparison of Results

Groundwater seepages, along a vertical transect, were estimated below the Indian River Lagoon (IRL), a coastal estuary located on the east coast of Florida, using MODFLOW, which is an uncoupled model, and SEAWAT, which is a coupled model. In the longitudinal direction, the modeling domain, perpendicular to the estuary coastline, extended from the water table divide on the mainland to the Atlantic Ocean (9.74 km). Vertically, the domain extended 33.5 m to the top of the Hawthorn Formation, considered an impermeable boundary. The flow in the unconfined aquifer below the estuary is density-dependent since the brackish water in the IRL and the ocean water mix with the freshwater in the aquifer. The hydraulic head boundary conditions for the brackish water of the estuary and the ocean water were converted to freshwater hydraulic heads when using MODFLOW. The following four sets of results were compared over three sampling events: freshwater hydraulic head contours in the unconfined aquifer, groundwater flow directions in the unconfined aquifer, spatial distribution of submarine groundwater discharge (SGD) from the unconfined aquifer into the IRL along the transect, and total SGD into the IRL from the aquifer. Visual comparisons showed that the hydraulic head distributions, spatial distributions of the SGD, and the velocity vectors obtained from the two models were quite similar. The differences in the total SGDs predicted by the two models ranged from 1.8% to 4%. The results indicate that MODFLOW may be a viable option to a coupled model even when the groundwater flow is density dependent.