Temporal and Spatial Patterns of Internal Phosphorus Recycling in a South Florida (USA) Stormwater Treatment Area

Abstract

Large constructed wetlands, known as stormwater treatment areas (STAs), have been deployed to remove phosphorus (P) in drainage waters before discharge into the Everglades in South Florida, USA. Their P removal performance depends on internal P cycling under typically hydrated, but with occasionally desiccated, conditions. We examined the spatial and temporal P removal capacity under different hydrologic conditions along a STA flow path. While inflow soils are P enriched, the outflow region of the wetland contained P-unsaturated soils with minimal net recycling of bound soil P to the water column as plant-available P. The outflow-region soils were characterized by low porewater soluble reactive P (SRP) (≤40 μg L) and high total sulfide (TS) (2-9 mg L) concentrations, and total ammoniacal nitrogen (TAN) and SRP flux rates that averaged 1.51 and 0.002 mg m d, respectively. Pronounced increases in porewater and surface-water concentrations of SRP, dissolved organic P (DOP), and TAN were observed immediately after rehydration of the cell after an extended drought. Elevated total P concentrations persisted at the outfall of the cell for several months thereafter, resulting in an annual outflow total P concentration nearly threefold higher than the long-term mean. Relative to processes that can occur during extended periods of inundation, such as sulfate-enhanced P release from organic matter mineralization or iron sulfide formation, aerobic oxidation of organic matter during prolonged dryout periods is a more significant biogeochemical process in compromising soil P retention in STAs.

Characterization of biogenic phosphorus in outflow water from constructed wetlands

“…However, the composition of organic P in the outflows from several of the wetlands are similar to the biogenic P composition in the upper floc layer of some of the wetlands (Turner and Newman, 2005;Turner et al, 2006). Hence, we expect the discharged particles to resemble the upper benthic floc layer, which may still be relatively labile (Dierberg et al, 2012;Fisher and Reddy, 2010). In the wetlands, under flooded and reducing conditions, the particulate orthophosphate diesters and their degradation products seem to be stable in the benthic floc (Turner et al, 2006) due to the anoxic conditions which suppress the decomposition of organic matter (DeBusk and Reddy, 1998).…”

Comment on “Spatial and temporal phosphorus distribution changes in a large wetland ecosystem” by X. Zapata‐Rios et al.

“…This is further supported by the most recently compiled STA performance data, which remains convincing that in this range of discharge P, no systematic relationships exist between wetland size (e.g., P mass loading rate and/or hydraulic residence time) and outflow concentrations [see Ivanoff et al ., , Figure 5‐4]. Only one full‐scale STA treatment train continues to regularly exceed this discharge P expectation (STA‐2 Cell 1), but its unique qualities are fundamentally different than the 19 flow paths referenced above, and unfortunately are not easily replicated [ Juston and DeBusk , ; Dierberg et al ., ]. There also remain optimization issues in the STAs that should be considered in this light, particularly in regard to emergent vegetation and water depth management [ Pietro , ], that must be resolved before the treatment wetlands can consistently realize this level of performance.…”

Large Constructed Wetlands for Phosphorus Control: A Review

“…In contrast, STA56 flow way 7 received no water in the dry season, resulting in complete dry-out to negative depths (below ground). Such complete dry-out has performance implications, because stored organic sediments can oxidize under dry conditions, releasing stored phosphorus [39]. Emergent wetland plants can, in general, withstand dry conditions, but submerged aquatic vegetation (SAV) cannot.…”