Tracer Studies in a Treatment Wetland with a 10AU

The aerial photo shows the front half of a 147-ha wetland four hours after receiving a pulse of fluorogenic rhodamine-WT dye, injected along the levee on the left of the picture. Tracer studies are frequently performed to identify hydraulic characteristics such as short-circuits, residence time, and dispersion, which are critical to understanding the contaminant removal performance by a treatment wetland. Although some of the short-circuit flow paths are readily visible at the inflow region of the wetland, the concentration of the fluorescent dye decreases rapidly with distance downstream from the injection areas and over time.

With support provided by the South Florida Water Management District, scientists at DB Environmental used a Turner Designs Model 10-AU-005-CE field fluorometer to measure the dye to concentrations less than 0.10 ppb. The fluorometer was situated in a nearby field lab, and water samples were analyzed for dye concentration on a daily basis. This near real-time data acquisition was critical in deciding when to vary frequency of sample collection.

The high accuracy and precision of the Model 10-AU-005-CE fluorometer in the sub-ppb range facilitated an accurate assessment of the hydraulic retention time and dispersion within the wetland. This enabled DB Environmental to determine the degree of hydraulic efficiency of the existing wetland, and to provide suggestions for structurally altering the wetland to improve efficiency as well as to offer design enhancements in constructing future treatment wetlands.