Rapid and Intense Phosphate Desorption Kinetics When Saltwater Intrudes into Carbonate Rock
It is important to understand how phosphate sorption dynamics of coastal carbonate aquifers are affected by seawater intrusion, because many coastal aquifers are composed of carbonate rocks and subject to an increase in saltwater intrusion during relative sea-level rise. Twelve carbonate rock and unconsolidated sediment specimens were acquired from a test corehole spanning the full thickness of the Biscayne aquifer in southeastern Florida. All 12 samples exhibit low phosphorus content but variable contents of iron. Column leaching experiments were conducted with two carbonate aquifer samples, alternating between freshwater and saltwater flow. With the first influx of saltwater, phosphate concentration in leachate increased rapidly from a freshwater value of approximately 0.2 μM to peaks of between 0.8 and 1.6 μM. The phosphate concentration began to diminish as saltwater continued to flow, but sustained desorption continued for over 2 h. Overall, seawater drove sorption behavior much more than chemical composition for the aquifer rocks and sediment from the seven rock samples for which we did isotherm sorption experiments. Our results indicate that an immediate and intense pulse of phosphate desorption from carbonate rock and sediment with low phosphorus content occurs in response to an influx of seawater and that the duration of desorption will vary by layer within a single aquifer.
KeywordsFlorida Everglades Groundwater Submarine groundwater discharge
Thanks to Dr. Kevin Cunningham of US Geological Survey for assisting in the laboratory with lithologic descriptions and selection of intact Tamiami Formation specimens and for providing access to G-3784 test corehole. Dr. René Price, Rafael Travieso, and Edward Linden of Florida International University provided for field and laboratory support, and Charles Fischer of National Oceanic and Atmospheric Administration/Atlantic Oceanographic and Meteorological Laboratory assisted in sample analysis. We also wish to thank the associate editor and anonymous reviewer who provided detailed feedback, vastly improving the manuscript, particularly with regard to stratigraphy. This material is based upon work supported by the National Science Foundation through the Florida Coastal Everglades Long-Term Ecological Research program under Cooperative Agreements #DEB-1237517, #DBI-0620409, and #DEB-9910514. This is contribution number 823 from the Southeast Environmental Research Center in the Institute of Water & Environment at Florida International University.
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