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).
500
North
South
North
South
400
Total Phosphorus (µg/L)
300
200
100
0
Inflow
Contr
ol
Low
Depth
Inflow
Contr
ol
D
Low
epth
Inflow
Contr
ol
Depth
High
Inflow
ol
th
Contr High Dep
Fig. 1 Total Phosphorus Outflow Concentrations during low and high depth studies
for North and South Sites
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 highdepth 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, jmnewman@sfwmd.gov,
Oral, Water Quality and Water Treatment Technologies