Stormwater Treatment Area Optimization Research: The Result of Pulsed loading and Depth Changes on Half-acre Research Treatment Wetlands in South Florida.

Jana Majer Newman, and Kimberleigh Cayse

South Florida Water Management District, West Palm Beach, FL, USA

The Everglades ecosystem is known to be extremely sensitive to phosphorus (P) loading, and excess P has had negative impacts on Everglades flora and fauna. The Everglades Forever Act (EFA) requires the South Florida Water Management District (District) to construct a series of large treatment wetlands (ca. 17,000 ha) called Stormwater Treatment Areas (STAs) to reduce nutrients in runoff to levels that will have no negative impact on the Everglades. The STA Optimization research and monitoring program is mandated by the EFA to assist the District in developing an operational strategy that maximizes performance of the STAs. One part of this program involves conducting hydrologic research in the STA-1W test cells that are located at the inflow and outflow regions of the treatment wetland. This research examined how hydrologic conditions would influence STA performance; i.e., what water management scenarios would promote maximum TP removal efficiency in these systems and conversely, under what hydrologic conditions would TP removal efficiency fail to meet mandated requirements.

The test cells are shallow, fully lined wetlands, about 0.2 ha in size, located within the boundaries of the ENRP, a prototype STA built and operated by the District. Six test cells located at the northern end of STA-1W are dedicated to STA Optimization experiments. Two test cells are being used as controls and operated at a mean hydraulic loading rate (HLR) of 2.65-cm/d and nominal depth of 0.6 m, which approximates the average design conditions for the STAs. Two of the remaining four test cells (NTC-7 and NTC-8) were used to document the effect that depth has on nutrient removal, while two test cells were operated at a constant depth (NTC-6 and NTC-9) but with widely varied inflow volumes (pulsed).

During the depth experiments, the HLR was held constant while the depth was reduced from 0.6 m to 0.15 m for 180 days, and then increased to 1.2 m for the following 180 days. This effectively decreased the nominal hydraulic residence time (HRT) in these systems from about 20 days to 5.5 days during the low-depth experiments, while the HRT increased to 45.7 days during the high-depth study. The pulsing scheme consisted of changing the HLR biweekly, ranging between 0.05 to 15.27 cm/d, while the depth was held at 0.6 m. Holding the depth constant while pulsing the HLR results in varied HRTs, which, in part, simulated operation of an STA. The pulsed inflow pattern developed for this experiment was based on a 10-year period of record (1978 to 1988) for the STA-2 basin. The pulsing experiments were conducted for one calendar year that extended from October 2000 through September 2001 to include both wet and dry seasons.

Lowering water depth to a nominal 0.15 m resulted in a slight improvement in TP removal at the north site, but resulted in markedly poorer TP removal performance in the south site compared to the respective controls. The median outflow TP concentrations at the north control and low-depth test cells (20 versus 15 µg/L, respectively) were significantly different (Fig. 1). In the south, median outflow TP concentration for the low-depth cell (32 µg/L) was significantly greater than for the control cells (18 µg/L).


Increasing water depth in the north site had no significant effect on TP reduction; median outflow TP concentrations were 26 and 25 µg/L for the control and high-depth test cells, respectively. At the south site, the median outflow TP concentration from the control test cells (31 µg/L) was significantly less than outflow from the high-depth test cells (49 µg/L). Outflow TP concentrations from the controls and high depth cells often exceeded inflow concentrations during this experiment.

During the pulsed experiment, differences between median outflow TP concentrations for the control and pulsed-HLR cells were significant for both the dry season (20 vs. 29 µg/L, respectively) and the wet season (32 vs. 39 µg/L, respectively) in the north. At the south site, mean outflow TP concentration from the pulsed cell was 20 µg/L; slightly lower than the control cell mean (26 µg/L). As at the north site, the mean outflow TP concentration was higher during the wet season than the dry season although the percent reduction increased due to increased inflow TP concentrations.

Decreasing the depth of the system, while maintaining the HLR had a slight positive effect on the TP removal performance in the wetlands at the north site, while increasing the depth had no significant effect. Additionally, at the south site, both alterations in depth had negative effects, with increased depths having the greater effect. However, even at the constant depth the emergent systems showed little or no TP removal capabilities, probably due to extremely low inflow TP concentrations at the south site.

While pulsing in both the north and south site wetland systems during the dry season resulted in slightly higher degradation of TP removal compared to controls during the wet season, overall pulsing did not have a significant negative effect on TP removal performance from these wetland systems.

Jana, Newman, South Florida Water Management District, 3301 Gun Club Road, West Palm Beach, FL, 33406,

Phone: 561-682-2820, Fax: 561-682-0100, ,

Oral, Water Quality and Water Treatment Technologies