2009 Portland GSA Annual Meeting (18-21 October 2009)
Paper No. 149-6
Presentation Time: 9:25 AM-9:40 AM

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

ROY, Moutusi, Department of Geological Sciences, University of Florida, P.O box 112120, Gainesville, FL 32611, moutusi@ufl.edu, MARTIN, Jonathan B., Department of Geological Sciences, University of Florida, 241 Williamson Hall, PO Box 112120, Gainesville, FL 32611-2120, SMITH, Christopher G., St. Petersburg Coastal and Marine Science Center, U.S. Geological Survey, 600 Fourth Street South, St. Petersburg, FL 33701, and CABLE, Jaye E., Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803

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.

2009 Portland GSA Annual Meeting (18-21 October 2009)
General Information for this Meeting
Session No. 149
The Subterranean Estuary—Examining the Geochemical and Physical Processes of These Subsurface Mixing Zones
Oregon Convention Center: D137/138
8:00 AM-12:00 PM, Tuesday, 20 October 2009

Geological Society of America Abstracts with Programs, Vol. 41, No. 7, p. 397

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