Improved coastal hydrodynamic model offers insight into surface and groundwater flow and restoration objectives in Biscayne Bay, Florida, USA

Biscayne Bay, a marine lagoon along the southeast Florida coast, has been affected by altered freshwater input due to water management construction features and operations. The Comprehensive Everglades Restoration Plan aims to improve the quantity, quality, timing, and distribution of freshwater flow through central Florida to the southern coasts and to restore nearshore estuarine conditions in Biscayne Bay. The Biscayne Bay Simulation Model (BBSM) is used to evaluate the effects of proposed changes to freshwater flow on salinity in the bay. However, a recent version of the model has left out the groundwater component due to uncertainty in its quantity, a critical shortcoming considering the porous substrate connecting the bay to inland freshwater sources and historic evidence of fresh groundwater discharge. As a result, the BBSM had been unable to reproduce mean salinity and salinity variability along the shoreline. In the present effort, salinity data from an expanded set of monitoring stations were used to estimate the quantity of the fresh groundwater discharge component. Adjustments in friction in the shallow nearshore regions were also made to increase salinity variability to mimic natural conditions. The result, referred to as BBSM version 4, has improved salinity behavior in the critical nearshore region while maintaining previous salinity performance mid-bay. A substantial coastal freshwater component was required to simulate nearshore salinity during portions of the year. This is an interesting finding that suggests groundwater and other freshwater sources along the coast, including convection driven precipitation events, may have been underestimated and require further investigation.