Particulate phosphorus transformations in south Florida stormwater treatment areas used for Everglades protection

Abstract

Six large treatment wetlands (352–6698 ha), designated Stormwater Treatment Areas (STAs), have been constructed to sequester phosphorus (P) in drainage waters before entering the Everglades. We performed oxic and anoxic laboratory incubations to assess the stability of P in suspended particles from inflow and outflow waters for selected flow-paths of STA-2, a “well-performing” wetland, and STA-1 West (W), an “under-performing” wetland. Particles were concentrated by filtration prior to incubation. STA-2 flow-paths exhibited a reduction in the particulate P (PP) concentration (e.g., 28 to 3–8 μg/L), as well as an increase in particle stability (i.e., reduced conversion of PP to soluble reactive P [SRP]) with passage through the wetland. By contrast, while the STA-1W flow-path exhibited a PP reduction of 68–46 μg/L under anoxic incubations, the particles exiting the wetland were more labile than those entering. For PP collected from both STAs, anoxic incubations produced higher conversion rates to SRP than did oxic conditions. This redox influence on SRP release was more pronounced for outflow PP from STA-1W than for PP exiting the STA-2 flow-paths.

STA-2 is comprised of flow-paths dominated either by emergent vegetation (Typha domingensis and Cladium jamaicense) or submerged (Najas guadalupensis, Potamogeton illinoensis, periphyton) macrophytes. Internal sampling of water column P species in the submerged macrophyte wetland revealed a sharp decline in SRP levels near the wetland inflow, and more gradual declines in PP and dissolved organic P (DOP) concentrations, primarily within the first half of the wetland. Mean outflow TP concentrations (15 μg/L) during these monitoring events were comprised of 13% SRP, 27% PP and 60% DOP. Particles exiting the emergent macrophyte wetlands were higher in organic matter content, but lower in total suspended solids, algal cell, chlorophyll a, and carbonate concentrations than particles in the inflow waters. By contrast, particles exiting the submerged macrophyte wetland were more similar to inflow waters with respect to all of the above characteristics except total suspended solids.

This study demonstrates that the behavior and transformation of PP constituents will have a marked impact on the utility and cost-effectiveness of watershed and STA phosphorus control efforts. Observed differences between STA-2 and STA-1W suggest that the STA P loading rate history is a prominent factor in dictating both the total mass of PP, as well as bioavailable fraction of PP, discharged from the STAs.

Introduction

Considerable effort has been devoted during the past decade to reduce phosphorus (P) exports from Lake Okeechobee and the 2830-km² Everglades Agricultural Area (EAA) to the downstream Everglades Protection Area (EPA) in south Florida, USA. These efforts have included the implementation of Best Management Practices (BMPs) in the EAA, which have served to reduce P export from farms into drainage canals (Daroub et al., 2002), and the construction of Stormwater Treatment Areas (STAs) during the past 15 years (Chimney and Goforth, 2001). The STAs are six large wetlands, ranging in size from 352 to 6698 ha, located along the major South Florida Water Management District (SFWMD) conveyance canals (Fig. 1). The massive size of the STAs, and their low-targeted interim outflow total P (TP) concentration (<50 μg/L, with proposed targets lower than 20 μg/L), make them one of the world's most ambitious ecological engineering projects.

Because of varying P loading rates and previous land uses, the outflow TP concentrations can differ substantially among the STAs. A recent study of initial operational data from four of the six STAs demonstrated that two of the treatment wetlands produced outflow TP concentrations below 20 μg/L, whereas two STAs are “under-performing”, producing outflows well above 40 μg/L (Juston and DeBusk, 2006).

To date, both BMP- and STA-related efforts throughout the south Florida watershed have focused on the reduction of TP levels (Chimney and Goforth, 2001). Prior studies, however, suggest that the different P species, broadly characterized as particulate P (PP), dissolved organic P (DOP), and soluble reactive P (SRP), have varying impacts to biota because of differing bioavailabilty and transport characteristics. Soluble reactive P is, for the most part, immediately available for biological uptake (Dorich et al., 1980, Peters, 1981, Sharpley, 1993, Uusi-Kämppä et al., 2000, Dierberg et al., 2002). While it is well documented that DOP and PP can provide a long-term source of P for aquatic plant growth (Wildung et al., 1974, Carignan and Kalff, 1980, Cooper et al., 1999, Pant et al., 2002), consistent background water column concentrations of PP and DOP in the ultra-oligotrophic environment of the Everglades (Noe et al., 2007) suggest, however, that some of these P compounds are resistant to degradation and biological uptake.

Most field studies on the water quality impacts of agricultural runoff have been performed in temperate watersheds characterized by high topographic relief and mineral soils (Gburek et al., 2000, Uusitalo et al., 2001, McDowell and Sharpley, 2003), with runoff directly entering the water body of interest. By contrast, conveyance of agricultural drainage water (ADW) from the EAA and surface water from Lake Okeechobee is highly managed. Due to the minimal elevation gradients in south Florida, on-farm canals may trap and hold ADW for long periods before releasing it to a regional canal system that is operated by pumps and water control structures under the management of the SFWMD. During the on-farm retention period, biological materials such as plankton, filamentous algae, and macrophyte particles, rather than soil particles, dominate P export (Stuck et al., 2001). In south Florida, Lake Okeechobee water, which also contains phytoplankton and suspended solids, is also periodically released for flood control and water supply purposes. Water management activities on the farms and regional canals may affect the amount and heterogeneous nature (density, size, and P content) of the particles reaching the STAs. For example, the relative contributions of the blended source waters (ADW and Lake Okeechobee discharge) to the STAs can dictate how much of the suspended particle load consists of plant detritus, algae, and soil particles. Additionally, the length of the distribution canals (as long as 45 km) and the intermittent flow schedules create ample opportunity for P species redistribution (e.g., particle settling and resuspension; SRP uptake and release) to occur within the farm canals, and in the larger regional distribution canals (Stuck et al., 2001, Daroub et al., 2003).

It also is likely that P transformations occur within the STAs, which are subject to dynamic operational conditions that can affect the character and extent of P transformations. The hydraulic loadings and retention times within an STA can vary widely (from days to months). Depending on the hydrologic regime, conditions that promote particle and detrital resuspension, vegetation displacement and shearing, and phytoplankton growth may occur within STAs.

Understanding the bioavailability and fate of PP from STA discharges is important due to the low TP threshold concentration (10 μg/L) adopted by the State of Florida as protective of the Everglades (Sklar et al., 2005), and because PP can be a prominent component of the wetland outflow waters. In a small-scale (10 m²) periphyton dominated raceway, the only man-made wetland treatment system to achieve consistent long-term outflow TP concentrations as low as 10 μg/L, outflow PP concentrations averaged 4 μg/L (40% of TP) over an 18-month study (DeBusk et al., 2004). However, neither the characteristics nor the bioavailability of the P in the particles discharged from this system were evaluated.

In the present study, we collected inflow and outflow waters from separate flow-paths within two STAs and characterized the stability of the water-borne particles with respect to P release. We selected both well-performing (STA-2) and under-performing (STA-1W) STAs to characterize PP, since these wetlands are likely to exhibit differences in the manner in which inflow P forms are processed. Moreover, STA-2 contains a recently farmed, submerged aquatic vegetation (SAV)-dominated flow-path as well as two historical, uncultivated, emergent aquatic vegetation (EAV)-dominated flow-paths. This enabled us to characterize and assess the stability of particles being exported from two distinct STA vegetation community types. Within the easily accessible SAV-dominated wetland, we also were able to assess spatial changes in water column P speciation within its long (4.6 km) flow-path.

The objective of this paper is to characterize the stability of PP entering and leaving wetland cells within two STAs that exhibited different P removal characteristics. Concentrated PP was exposed to both oxic and anoxic conditions in the lab to test whether differing reduction–oxidation (redox) environments would have an affect on the extent of PP transformation. For one of the wetlands (STA-2), the nature of the particulates (algal abundance and chlorophyll a, organic matter, and carbonate contents) is also described to further characterize the particulate processing within the treatment cells.