STA-3/4 PSTA Cell. The STA-3/4 PSTA cell has provided effective treatment performance since its inception, with annual average inflow TP levels ranging from 14 – 27 µg/L and mean annual outflow levels ranging from 8 – 13 µg/L. Our ongoing work in the STA-3/4 PSTA Cell has included wet- and dry-season sampling of surface water, sediment, plants and periphyton along fixed internal transects; deployment of periphytometers at three internal locations, and monitoring water quality and periphyton before, during and after flow pulse events. This work is being complemented by additional research and monitoring efforts by District scientists. This work has offered insight on each of the five guiding research questions. For example, we observed that short (ca. 1 week) flow pulses, mimicking those provided to the full-scale STA flow paths (on a relative basis), had no detrimental effect on outflow TP concentrations (Key Question #1). Alkaline phosphatase activity (APA) is an indicator of P limitation and a measure of the ability for naturally occurring monoesterase enzymes to increase bioavailability of recalcitrant P compounds. While water column APA was depressed and/or diluted during high flow conditions, levels increased in the days following the pulses, indicating that a temporary increase in P loading during high flows does not curtail enzyme production by biota. Another concern, that flow pulses will create flow velocities high enough to disrupt the sediments and biomass, has proven unfounded within the current PSTA cell. In the largest pulse test, thought to be representative (on a relative basis) of high pulses in the full-scale STA flow paths, the biota and sediments remained intact and well-functioning. Indeed, direct measurements of enzyme activity in periphyton communities before and after the largest flow pulse demonstrated no reduction in activity as a result of the pulse. Other factors controlling enzyme activity, and the removal of DOP and PP within PSTA cells, are still under investigation. Sixteen internal water quality surveys have been performed in the PSTA cell since July 2011 and have typically shown declining or flat TP profiles, and gradually increasing phosphatase enzyme activities, within the wetland. Full-scale PSTA systems, therefore, can exhibit a reasonable ability to succeed under naturally variable flow conditions. With respect to Key Question #2, studies both within the PSTA cell and in mesocosms are yielding insight into the characteristics of sedimen...
STA-3/4 PSTA Cell. A “full-scale” PSTA demonstration area was constructed from 2004 to 2005 for the purpose of addressing uncertainties associated with large-scale implementation of periphyton-based treatment technology. The PSTA Project is located on a total of 400 acres in STA-3/4, and is comprised of a 200-acre Upper SAV Cell, a 100-acre Lower SAV Cell and a 100-acre PSTA Cell (Figure 1). The PSTA Cell is unique among existing STA treatment cells in that the peat layer was scraped and hauled away, exposing the underlying rock.
Figure 1. Location map of STA-3/4 and PSTA Project, including the Upper and Lower SAV Cells, the PSTA Cell, and related water control structures. Green arrows show flow direction. The PSTA Cell has successfully developed into an oligotrophic, periphyton-rich wetland treatment cell. Due to favorable growing conditions, several species of submerged aquatic vegetation (SAV) have also colonized the PSTA Cell. Over the years, visual observations suggest that the SAV community in the PSTA Cell has fluctuated slightly in terms of species composition, relative density, and coverage. Periphyton colonization of the benthic surface and surfaces of submerged and emergent macrophytes also appears to vary over time, and generally appears robust in most parts of the cell (Figure 2).
Figure 2. Periphyton growing on Chara sp. and Eleocharis stems in the STA-3/4 PSTA Cell. The STA-3/4 PSTA Cell has shown promising performance by discharging outflow waters with ultra-low TP concentrations (findings are described in the next section of this report). Replicating the success of the PSTA Cell across other STA flow ways may improve the SFWMD’s ability to meet the WQBEL established to protect downstream marshes. However, questions remain regarding the factors contributing to this wetland’s low outflow TP concentrations. Under certain stage conditions, seepage into the cell is thought to occur at a high rate. If groundwater inflows into the wetland contain low TP concentrations, then the wetland’s observed low-level TP discharge concentrations could be an artifact of dilution by seepage, rather than the actual removal of P by biogeochemical processes. Another concern is that the wetland was not challenged sufficiently during its initial five years of operation: inflow volumes instead were rather steady, and low, which differs from the periodic high flow pulses received by the full-scale STA flow paths. Such pulse conditions with high hydraulic loading rates create short wetl...
