Figure Legends Fig. 1. Map of Apalachicola Bay, Florida showing the Apalachicola River and the four passes to the Gulf of Mexico. Our sampling stations are marked by diamonds. The groundwater study site at the St. George Island State Park is also indicated. Fig. 2. Monthly historical discharge of Apalachicola River measured near Sumatra, Florida since 1980. The gauge (USGS 02359170) is about 33 km upstream from the river mouth. The circles on the expanded part of the plot indicate sampling time and discharge during our seasonal study. Fig. 3. Submarine groundwater discharge assessment via automated seepage meter deployed on St. George Island. The figure shows very low advection rates during dry periods, the discharge rates increase for a duration of several hours after rain events (indicated by bars). Fig. 4. Ratios of 224Ra/223Ra measured in surface (full circles) and near bottom (open circles) waters in Apalachicola Bay and River during the four sampling periods. The ratios are plotted against the surface to bottom salinity ratios which are a measure of water stratification. Uncertainties were derived by error propagation of the individual radium measurements (± 1 σ based on counting statistics). The Apalachicola River discharge rates are also indicated. Fig. 5. Calculated water ages estimated for surface waters during our four sampling periods in August 2003, March and August 2004, and January 2005. The indicated river discharge is a five-day average before and during the sampling. Fig. 6. Salinity contour plots in Apalachicola Bay created based on spot measurements during sample deployments in August 2003 and March 2004. The river discharge was very similar during both samplings (1016 and 996 m3 s-1) but the salinity contours are very different due to the unsimilar wind conditions in these months as indicated in Table 1 and Fig. 5. Fig. 7. Wind speed and directions measured at NOAA CO-OPS station #8728690 located in Apalachicola, Florida. The horizontal line in each panel represents the sampling interval. Fig. 8. The variation of flushing rate with river discharge: Solid line - theoretical calculation based on dividing the bay volume (5.7 x 109 m3) with the river discharge, circles - total bay water turnover time based on Mortazavi et al. (2000) model, squares - flushing rates calculated from radium isotopes. 29.9 29.8 4 ANERR St. George Sound 16 17 Apalachicola 12 East Bay 15 River 3 St. George Island 6 13 14 29.7 5 12 7 Indian Pass 11 State Park study site 8 10 West Pass 9 10 km 29.6 Sike's Cut -85.2 -85 -84.8 -84.6 Dulaiova and Burnett, Fig. 1 4000 1500 ) -1 1000 s -1 3 s 3 3000 m 500 0 Jul Oct Jan Apr Jul Oct Jan 2005 2000 2004 1000 Apalachicola River Discharge Discharge (m River Apalachicola 0 1980 1985 1990 1995 2000 2005 Year Dulaiova and Burnett, Fig. 2 Dulaiova and Burnett, Fig. 3 50 August, 2003 Surface March, 2004 3 -1 1016 m s Bottom 996 m3 s-1 40 more stratified less stratified 30 Ra 223 Ra/ 20 224 10 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 50 August, 2004 January, 2005 338 m3 s-1 809 m3 s-1 40 30 Ra 223 Ra/ 20 224 10 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Surface Salinity/Bottom Salinity ratio Surface Salinity/Bottom Salinity ratio Dulaiova and Burnett, Fig. 4 1016 m3 s-1 996 m3 s-1 338 m3 s-1 809 m3 s-1 Dulaiova andBurnett, Fig. 5 August 2003 March 2004 Dulaiova and Burnett, Fig. 6 Dulaiova and Burnett, Fig. 7 18 Model 16 Theoretical calc. Radium isotopes 14 12 10 8 6 Flushing rate (days) 4 2 0 0 500 1000 1500 2000 2500 3000 3 -1 River discharge (m s ) Dulaiova and Burnett, Fig. 8 .