A Radioisotope Tracer Study of Natural Groundwater Discharge From St. Catherines Island, Georgia
The fairly pristine nature of St. Catherine’s Island, a 90.1 km2 barrier island on Georgia’s coast, makes it an ideal place to conduct research. The barrier island has a surficial aquifer that discharges fresh to saline groundwater into nearby coastal estuaries. This process is called submarine groundwater discharge (SGD). Little SGD research has been conducted in coastal regions along Georgia’s barrier islands, including St. Catherine’s Island. SGD plays an important role in the health of coastal ecosystems as this discharge is typically a large source of nutrients, bacteria, and dissolved organic carbon, which can have substantial impacts on coastal estuaries. For example, eutrophication events such as algal blooms, which can cause large fish kills or decimate native plant species, can be caused by SGD. In some areas, SGD has been shown to be more important than riverine inputs in terms of nutrient fluxes. We conducted an open water survey, via boat, of surface waters around the northern half of St. Catherine’s Island to determine SGD locations and fluxes to the ocean. 222-Rn is a naturally occurring radioisoptope, that exists in high concentrations in groundwater and low concentrations in seawater. We collected a 222-Rn data set, that was integrated at five-minutes intervals during the three hours surrounding low tide. 222-Rn is best measured during outgoing tides, as the tide draws the groundwater out of the aquifer. The boat traveled less than 5 kph, giving our data set a 9 to 446 meter spatial resolution, depending on boat speed. We also collected discrete groundwater samples from six surficial wells, four Upper Floridan wells, and eleven beach samples along the north and south beach of the island to determine 222-Rn groundwater end members for use in our groundwater flux model. This and future surveys will allow us to find areas where groundwater fluxes and nutrient concentrations are high, find out if there are seasonal changes in the quality and quantity of SGD, and have a better understanding of the overall health of the estuary. This research will be valuable to other researchers studying coastal ecology or future researchers studying aquifers and groundwater discharge, especially along Georgia’s barrier islands.
Georgia Undergraduate Research Conference (GURC)
Mobley, Robert K., Jacque L. Kelly.
"A Radioisotope Tracer Study of Natural Groundwater Discharge From St. Catherines Island, Georgia."
Geology and Geography Faculty Presentations.