Estimation of Groundwater Loads into a Coastal Estuary Using SEAWAT

Abstract

The research of regional groundwater flow is of vital importance at the Indian River Lagoon (IRL) due to the potential contribution groundwater flow has on nutrients into the IRL. If too many nutrients are introduced into the IRL, algal blooms could occur and impact the health of IRL system. The IRL is a coastal estuary located on the east coast of Florida. Submarine groundwater seepages (SGD) and groundwater loads into the IRL were numerically estimated by using SEAWAT, a program designed to simulate variable-density groundwater flow coupled with multi-species solute transport. Models were completed at three different study transects that were set in the longitudinal direction, perpendicular to the estuary coastline. The modeling domain for each transect extended from the water table divide on the mainland to either the neighboring barrier island or to the Atlantic Ocean depending on the transect location. Groundwater seepages along with groundwater loads were determined after calibrating the models by comparing measured and predicted freshwater hydraulic head distributions in the unconfined aquifer. Three statistical analysis tests were performed in order to further validate the model calibrations, the root mean square error (RMSE), the two-sided test, and the Nash-Sutcliffe efficiency (NSE) index. A new approach was used for the statistical analysis of calibrated models where additional points from the measured equipotential head distributions were added to the statistical analysis in order to improve calibration results. The leakage from the Floridian Aquifer into the unconfined aquifer of the IRL, via the Hawthorn Formation, was estimated at different sites. A slug test was conducted at a study site in order to compare the measured lateral hydraulic conductivity (Kx) values to the previously calibrated model. This research presents the following results: a) introduces new statistical analysis methods, b) shows the impact of recharge on the SGD, groundwater loads, and direction of flow to the IRL through the unconfined aquifer, c) compares the slug test variable Kx value to the previous Kx and d) estimates the salinity distribution in the unconfined aquifer.