SEASONAL COMPARISON OF SUBMARINE GROUNDWATER DISCHARGE TO A NORTHERN GULF OF MEXICO ESTUARY

Submarine groundwater discharge is a coastal hydrological process through which fresh, terrestrial groundwater and recirculated seawater seep across the seafloor into surface waterbodies. Previous research has identified submarine groundwater discharge as an important source of water and dissolved materials, including contaminants, to the coastal ocean. Despite recognition of ecological and economic consequences associated with contaminated submarine groundwater discharge, regional studies of submarine groundwater discharge are sparse, due in part to the fact that groundwater seepage in coastal environments is diffuse and temporally variable, and as a result, notoriously difficult to quantify. In an effort to resolve the effects of seasonal hydrological changes (e.g. precipitation, evapotranspiration, etc.) on submarine groundwater discharge, measurements from Lee-type seepage meters were used to compare submarine groundwater fluxes during wet (July) and dry (October) seasons in Mobile Bay, AL. Four seepage meters were installed perpendicular to the shoreline at a spacing of 12.5 m, and fluxes were recorded throughout the diurnal tidal cycle (~ 24 hrs.). In addition, areas influenced by fresh submarine groundwater discharge were identified in the study area by plotting surface salinity data recorded using a water quality sonde affixed to an ocean kayak. Results indicate an average submarine groundwater flux from the four seepage meters of 2.4 cm d^-1 in July compared to an average of -0.3 cm d^-1 in October. Results from individual seepage meters reveal a decrease in submarine groundwater flux with increased distance from the shoreline in July. In contrast, a zone of negative flux (i.e. submarine groundwater recharge) 25 m from the shoreline with positive fluxes further offshore was observed for October. Surface salinity maps reveal a distinct zone of lower salinity (freshwater influenced) water directly adjacent to the shoreline during July, while there was no distinct freshwater influenced zone in October. These results suggest that submarine groundwater discharge in Mobile Bay is particularly sensitive to seasonal changes in precipitation and water table elevation, and highlight the need for multiple measurement periods when determining submarine groundwater discharge in other estuaries.