Reactive Transport Modeling of Fe Flux in the Seepage Face of Indian River Lagoon, Florida

The seepage face is a dynamic reactive transport system where both fluid sources (terrestrial freshwater and recirculated marine water) and flow paths of submarine groundwater discharge control redox conditions. Utilizing porewater Cl concentration versus depth profiles and seepage meter measurements at the seepage face of Indian River Lagoon, FL, seepage rates have been found to range between 0.02 to 0.9 m³/day per meter of shoreline and decrease to no seepage approximately 25 meter offshore. The porewater contains no sulfide at the shoreline and sulfide concentrations increase from 170.3 to 512.5 µM from 5 meter to 20 meter offshore. The depth of sulfide maxima increases from 20 cmbsf (centimeter below seafloor) to 40 cmbsf from 5 meter to 20 meter offshore. Solid phase S percent have distributions similar to porewater sulfide, decreasing from 0.16% in the upper 40 cmbsf to 0.04% below this depth. Dissolved Fe is provided to porewater from the reduction of Fe-oxyhydroxide coatings on sediments at depths below 40 cmbsf. Iron is removed from the porewater above these depths by precipitation of Fe-sulfide, thereby trapping Fe and decreasing Fe fluxes to the seepage face. We have estimated fluxes of Fe from porewater using a fully implicit two-point boundary model. Preliminary results from our model show the fitted parameter estimating the Fe source to the porewater, 10⁵ µmols/L per year, is two orders of magnitude lower near the shoreline than at the offshore edge of the seepage face. These preliminary results suggest the precipitation rate of Fe-sulfide is 15 orders of magnitude lower than reduction rate of Fe oxides. The model will eventually include the solid phase S% and porewater sulfide to constrain the Fe-sulfide precipitation and will estimate Fe-oxyhydroxide reduction rate.