Can Stormwater Treatment Areas be Managed to Optimize Total Phosphorus Concentration Reductions for Everglades Restoration?
Martha K. Nungesser
South Florida Water Management District, West Palm Beach, FL, U.S.A.
For the last eight years, the South Florida Water Management District has been required to meet a 12-month rolling average total phosphorus (TP) outflow concentration of 50 ppb from the Stormwater Treatment Areas (STAs). The prototype STA, originally called the Everglades Nutrient Removal Project (ENRP), has succeeded in meeting this standard throughout its period of record (1994 through 2002). A stricter TP outflow concentration, expected to be 10 ppb, requires determination of whether and how these wetlands can be managed to further reduce TP concentrations to this new standard.
The ENRP consists of four treatment wetlands. Cells 1 and 3 form one flow path, and cells 2 and 4 form the second. Inflow, outflow, and internal hydrologic and water quality parameters have been collected for the ENRP and for the internal cells for eight years, allowing performance tracking for TP reduction. After operating as the ENRP for four years, an additional large treatment cell was added in 1999, but monitoring continued largely uninterrupted. These data provide a unique opportunity to analyze the performance of an STA under two management regimes, one as a prototype experimental wetland and the other as a fully operational stormwater treatment area. Inflow and outflow concentrations of TP have varied over time, as have various biotic and abiotic factors. Reducing outflow TP concentrations is a major objective of the STAs, and identifying management options to minimize TP concentrations is a significant goal.
During the 4-year period the STA was operated as the ENRP, the project reduced flow-weighted TP inflow concentrations from a mean of 89 ppb to 25 ppb at the outflow. For the four-year period during and following construction of the STA, mean inflow flow-weighted concentrations of 125 ppb were reduced to a mean outflow of 40 ppb.
To identify factors that can be manipulated to produce lower outflow concentrations of total phosphorus, monthly flow-weighted concentrations were calculated using measured TP concentrations and flows. Monthly means or sums of water and total phosphorus concentrations, hydraulic retention time, water depth, vegetation types and cover, wind, net radiation, air temperature, and other biotic and abiotic variables were analyzed. The entire ENRP and each internal treatment cell (1 through 4) were analyzed separately. Correlations between outflow TP flow-weighted concentrations and inflow TP flow-weighted concentrations, flow rates, depths, residence time, vegetation type and extent, and climatic variables identified possible linear relationships between each of these variables and outflow concentrations. Stepwise multiple regressions were used to identify combinations of variables that could lead to operational strategies that would consistently reduce outflow TP concentrations.
No statistically significant relationships were identified between operational variables and outflow flow-weighted TP concentrations for either the ENRP and STA periods, nor for treatment cells 1, 2, or 4. Only treatment cell 3, a mixed emergent marsh, showed a significant positive relationship between inflow and outflow TP concentrations. Treatment cell 4, managed for submerged aquatic vegetation, achieved very low outflow concentrations for a two-year period in the late 1990’s. While no significant relationships have been discerned between cell 4 operational parameters and outflow TP concentrations for this period, relationships to vegetation and seasonality were noted.
Except for cell 3, there were no clear relationships between operational factors and outflow TP flow-weighted concentrations for these treatment wetlands. These results suggest that managing operational variables alone may not achieve flow-weighted TP concentrations of 10 ppb. Possible explanations include 1) that relevant processes involve feedbacks or processes that do not show up in linear representations; 2) that the responsible processes reside in vegetation and sediments and cannot be readily measured or controlled operationally; or 3) that the system is working optimally and has not been pushed hard enough to reveal controlling factors.
South Florida Water Management District, Lower East Coast Division, P.O. Box 24680, West Palm Beach, FL 33416-4680
Phone: 561-682-6614, Fax: 561-697-7219,