Abstract
In Florida, large constructed treatment wetlands called Stormwater Treatment Areas (STAs) have been engineered south of Lake Okeechobee to treat runoff from the Everglades Agricultural Area (EAA). These STAs help to stabilize and trap particulate matter and reduce (although not eliminate) resuspension before the waters finally enter the Everglades system. Understanding sediment characteristics such as particle size is central to help predict what hydrodynamics are necessary to (re)suspend deposited particulate matter and what conditions would be required to sustain suspension or cause settling. This study focuses on characterizing sediment particles using different methods in STA 3/4 (treatment cell 3B), a submerged aquatic vegetation (SAV) and periphyton-based STA which aims to reduce total phosphorus (TP) to a level equal to or less than 10 µg/L. The various methods were then contrasted from one another. Three different instruments were used to measure the sediment particle sizes: the Malvern Mastersizer 2000 (Malvern) and the Laser In Situ Scattering and Transmissometer 100x (LISST), both of which use laser diffraction technology, as well as the digital floc camera (DFC) which uses camera imaging for particle size determination. In addition, sediment samples were sorted into 8 different size classes (fractions) using sieves and GFF filters to compare this method with the aforementioned methods above, as well as to investigate phosphorus concentrations, dry weight partitioning and bulk densities by location. From the collected filtrate, the TP concentrations were determined according to standard protocols established for Comprehensive Everglades Restoration Plan (CERP) projects (SFWMD-LAB-SOP-3210-002). It was found that the two laser diffraction instruments (LISST and Malvern) showed similar results when looking at % volumes of particles in different size classes, and the camera-based instrument (DFC) results were very different, which was expected due to the differences in the methods of particle measurement. The DFC provided a greater estimate of the smaller particle sizes while the LISST and Malvern showed more particles in the larger particle sizes. Because the LISST and Malvern showed such similar results, there is no additional benefit in using both laser diffraction instruments together, however, there appears to be value in pairing a laser diffraction instrument with a camera-based instrument. When the results of the phosphorus analysis were investigated, a significant difference was found between TP and size class, as well as TP and location. The smallest size fractions, notably 0.7 µm, contained the highest TP concentrations, while the sieved size fractions contained lower TP concentrations that were similar to each other. However, when the TP concentrations were adjusted for the mass of each size fraction, this trend was reversed. When the mass of each size fraction was considered, the larger size classes, 210, 600, 1000, and 2000 µm accounted for the highest TP, while the 0.7 µm size class had the smallest.
