Floc Sediment as a Nutrient Sink in Constructed and Natural Wetland Systems, South Florida.

Floc sediment is a layer of loosely aggregated fine detritus present in wetland systems that is readily re-suspended into the overlying water column. Due to its colloidal nature, floc is considered the most active part of the sediment layer, and is involved in sequestering or releasing nutrients and contaminants in an aquatic ecosystem. In this study, floc samples were collected from two constructed wetlands, i.e., the Stormwater Treatment Areas (STAs): STA 1 West (STA-1W) and STA-2, which began operation in 1994 and 2000, respectively, and from Water Conservation Area 1 (WCA-1), in south Florida. Covering over 50,000 acres, the STAs were constructed to remove excess nutrients from agricultural runoff. Bulk water samples were also collected from STA-1W Cell 4. Porewater was extracted from floc samples and analyzed for total phosphorus (TP) and total Kjeldahl nitrogen (TKN). Subsamples of the porewater were filtered through a 0.45-µm membrane filter and analyzed for dissolved total P (DTP), ammonium (NH₄-N), and nitrate (NO₃-N). Water samples were also analyzed for the same parameters. The mean concentrations of TP and DTP in water samples from STA-1W Cell 4 were 0.135, and 0.072 mg L^-1, respectively, whereas the corresponding values in porewater were 1.12 and 0.211 mg L^-1, respectively, which were 8.3 and 2.9 times higher than in the water column. Of the three sampling locations, the rank order of TP and DTP concentrations in porewater was STA-1W Cell 4 > STA-2 Cell 3> WCA-1. The concentrations of NO₃-N were generally less than 0.24 mg L^-1, but NH₄-N concentrations were mostly higher than 1.0 mg L^-1, with the highest value of 8.3 mg L^-1. The concentrations of NO₃-N were lower but NH₄-N was much higher in porewater than in the overlying water column, indicating that the floc materials may be negatively charged and tend to retain NH₄-N but repulse NO₃-N through electrostatic force. There was no apparent difference in NO₃-N concentration in porewater among the three sampling locations. However, NH₄-N and TKN in porewater decreased in the order of STA-1W Cell 4> WCA-1> STA-2 Cell 3. In WCA-1, the TN/TP molar ratio in porewater was > 70, which was greater than ratios in STA-1W Cell 4 (<45) and STA-2 Cell 3 (<40). These results indicate that both P and NH₄-N tend to accumulate in the floc layer and the lower TN/TP ratio in the STA floc porewater is indicative of higher P input from the agricultural runoff.