Using electrical resistivity tomography to calibrate seawater intrusion models along the Alabama Gulf Coast
Numerical models (e.g., MODFLOW/SEAWAT) are commonly used to quantify the extent and magnitude of seawater intrusion (SI). Since coastal communities must account for SI into local aquifers, numerical models can be an effective planning and management tool as coastal populations increase. While these models tend to be limited by the availability of hydrological data, uncertainty in these results can be substantially reduced through the integration of field geophysical measurements. Field measurements employing electrical resistivity tomography (ERT) were utilized to calibrate a density-dependent groundwater flow model for Gulf Shores, Alabama. Specifically, six independent ERT profiles were produced within a modeled area adjacent to a shallow lake and along the near-shore boundary. The ERT methods were employed to a depth of 95 meters, encapsulating two aquifer systems with previously identified SI concerns (Chandler et al, 1986; Murgulet and Tick, 2008). Results from the ERT deployments were compared to previously developed models' near shore density boundary constructed from local borehole data, then utilized to calibrate the model freshwater/saltwater mixing zone. Other geophysical investigations employing similar methodology (e.g. Time-Domain Electromagnetics) have been used to calibrate SI models and to explore the extent of SI in a number of coastal regions. This study expands upon previous research through the integration of a 3-D groundwater flow model with greater resolution ERT measurements to more accurately determine the extent of SI, and delineate the near shore freshwater/seawater boundary zone.