TEMPORAL VARIATIONS IN RECHARGE TO A COASTAL AQUIFER AND LINKED CHANGES IN FE CONCENTRATIONS OF THE SUBTERRANEAN ESTUARY

Iron (Fe) cycling in subterranean estuaries (e.g. Fe-oxide reduction and precipitation of Fe-sulfides) can influence organic carbon (OC) remineralization, and fluxes of Fe-oxide bound nutrients (phosphate) and contaminants (cadmium) to surface waters. Iron cycling is enhanced at the freshwater-saltwater boundaries of subterranean estuaries, which commonly coincide with steep gradients in redox conditions. In addition to the control by redox reactions, Fe concentrations are elevated at the freshwater-saltwater boundary by slow flow, which allows accumulation of dissolved Fe produced from Fe-oxide dissolution. A redox gradient and flow distribution in the subterranean estuary of Indian River Lagoon, Florida increases dissolved Fe concentrations by three orders of magnitude, from less than 1 µM at the shoreline to about 300 µM at the freshwater-saltwater boundary. To observe temporal variations in Fe cycling, pore water was sampled across this gradient eight times over a three-year period (November-2004, February-2005, May-2005, September-2005, May-2006, October-2006, April-2007, and September-2007). From November 2004 to October 2006, the freshwater-saltwater boundary (defined by the 300 mM chloride concentration contour) was more than 30 m offshore, but by April 2007 the boundary had shifted landward to about 22.5 m offshore. Simultaneous with this shift, Fe concentrations of between 200 and 300 µM moved from 30 m offshore in 2006 to about 22.5 m offshore. Reactive transport models show reaction rates of Fe-oxide dissolution were similar across the seepage face, at about 0.1 mg/cm²/year, and did not vary with time. Changes in reaction rate thus cannot cause the observed changes in Fe concentrations, which instead are likely caused by a decrease in recharge to the Surficial aquifer from tens of cm/year in 2004 to 2006 to about -10 cm/year in 2007. The decrease in recharge would narrow the width of the seepage face and move the freshwater-saltwater boundary landward. These results suggest that during drought conditions the width of the seepage face would decrease, and the freshwater-saltwater boundary would move landward. Because the width of the subterranean estuary changes temporally, the total amount of Fe-oxide dissolution and associated OC remineralization also changes temporally.