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STA-1W Recovery Plan

By Anita Duncan,2014-08-12 10:00
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STA-1W Recovery Plan ...

    Internal Draft for Management Review Do Not Circulate or Cite

    Stormwater Treatment Area 1-West Lessons Learned

Introduction

    The 1994 Everglades Forever Act (EFA; ξ 373.4592, F.S.) required that all water released

    from the Everglades Agricultural Area into the Everglades Protection Area achieve compliance with applicable State and Federal water quality standards, including criteria for phosphorus (P). The use of large constructed wetlands, i.e., the Stormwater Treatment Areas (STAs), to reduce total P (TP) concentrations in agricultural runoff is a key component of the South Florida Water Management District’s Everglades Restoration Program. Six STAs with a combined treatment area of 16,154 ha (39,317 acres) have been constructed to date. Current plans call for adding ap-proximately 8,000 ha (~19,700 acres) of new treatment area to the STAs in the near future.

    The size of each STA was determined during design using a mass-balance model whose P removal coefficient was derived from water quality and sediment data collected along the nu-trient gradient in Water Conservation Area 2A, a portion of the remnant Everglades impacted by agricultural stormwater runoff (Walker, 1995). The mean design hydraulic loading rate for the -1-1-1STAs was 2.5 cm d (1.0 in d; Burns & McDonnell, 1994), but varied from 1.6 to 3.0 cm d -1(0.6 to 1.2 in d) depending on estimates of annual basin runoff into each STA. The design min-imum and maximum water depths for the STAs were 0.2 and 1.4 m (0.5 to 4.5 ft), respectively.

Design Envelope vs. Operational Data for STA-1W

    STA-1W (Figure 1) was constructed in two phases and encompasses 2,699 ha (6,670 acres). The first phase, known as the Everglades Nutrient Removal Project (ENRP; 1,545 ha [3,818 acres]), comprised Cells 1, 2, 3 and 4 and become operational in August 1994. The second phase, which nearly doubled the original ENRP treatment area, added Cells 5A and 5B and began flow-through operations in July 2000. The revised design parameters for STA-1W are summarized in

    Table 1 and are discussed below in relation to the operational data.

    Figure 1. Site map for STA-1W showing flow-ways and water control structures.

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    Internal Draft for Management Review Do Not Circulate or Cite

    Flow: STA-1W receives water from the S-5A Pump Station via G-302 (Fig. 1). Inflow is then distributed to the three flow-ways that comprise the wetland (i.e., Cells 1?3; Cells 2?4;

    Cells 5A?5B) and eventually discharged to Water Conservation Area 1A via the G-251 and G-310 Pump Stations. The annual hydraulic load anticipated for STA-1W during design (i.e., de-83sign HL) was 1.97 x 10 m (159,985 acre-feet) (Table 1). This assumed that STA-1E was fully operational and receiving a portion of the S-5A basin runoff.

    Table 1. Revised design parameters for STA-1W.

    PeakAverageTP ConcAveragePeakAverage

    FlowFlowppbHydraulicHydraulicNutrient

    cfsacre-feet/yrLoadingLoadingLoading

    RateRateRate

    2/yrg/mcm/dcm/d

    Inflow3,250159,9851392.0029.461.01

    Outflow3,490188,10024-30

    During the past year, the 365-day cumulative inflow volume to STA-1W generally exceeded the design HL. Additionally, the 30-day cumulative HL increased sharply beginning in August 832004. The HL in September 2004 alone was 1.36 x 10 m (109,912 acre-feet), which equated to

    69% of the design HL and reflected the large volume of rainfall the drainage basin received dur-ing Hurricanes Frances and Jeanne.

    Water depth: The long-term average water depth throughout STA-1W has been 0.58 m (1.9 ft) which is close to the design depth of 0.6 m (2.0 ft). However, water depth has reached 1.2 m (4 ft) during storm events. Because extended periods of deep water causes physiological stress on the emergent vegetation community and promotes floating cattail islands, the target water depth between storm events was reduced to 0.4 to 0.5 m (1.25 to 1.75 ft) in the various treatment cells of STA-1W.

    Total phosphorus: The annual TP load to STA-1W used for design (i.e., design P load) was 27,430 kg (60,473 lbs), which is based on the design HL ; a mean inflow TP concentration of -1139 µg L (= ppb) (Table 1). From May 1994 through September 2004, the flow-weighted -1mean inflow TP concentration at G-250 and G-302 was 151 µg L. Beginning in June 2004, -1weekly TP concentrations at G-302 often exceeded 150 µg L, with a peak monthly mean TP -1concentration during September 2004 of 296 µg L. Elevated inflow TP concentrations, com-

    bined with high inflow water volumes, resulted in 365-day cumulative TP loads that were often greater than the design P load. Conversely, the 30-day cumulative TP loads between November 2003 and late August 2004 were generally at or below the anticipated average monthly load, but experienced a sharp increase beginning September 2004 in response to increased runoff from Hurricanes Francis and Jeanne (Fig. 2).

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    Internal Draft for Management Review Do Not Circulate or Cite

Treatment performance

    Data presented in the 2004 Everglades Consolidated Report indicated that treatment perfor-mance, i.e., TP removal, in STA-1W was related to inflow TP concentrations and the aerial TP loading rate. More recent data suggest that if TP loading is reduced, the TP removal perfor-mance of STA-1W (expressed as percent TP load reduction) will improve (Fig. 2); TP load re-duction is moderately correlated with TP load. The monthly TP load into STA-1W has been at or below the design envelope since November 2003. Correspondingly, TP load reduction gener-ally increased from December 2003 through September 2004. The decline in TP removal per-formance that began in September 2004 (and the corresponding increase in TP load) can be attri-buted to the effects of Hurricanes Francis and Jeanne. In addition, the high winds and inflows experienced during the hurricanes may have reduced plant coverage in some treatment cells; field surveys indicate that much submersed aquatic vegetation was lost from Cell 5B during these storms.

    TP LoadLoad Reduction 90140

    r = 0.718585120

    8080100

    757580

    707060

    TP Load (T)656540

    606020TP Load Reduction (%)TP Load Reduction (%)

    55020406080100120

    Jun-00TP Load (T)Aug-00Oct-00Dec-00Figure 2. Time series of total phosphorus inflow load and total phosphorus load reduction in STA-1W Feb-01Apr-01Jun-01Cell 5B Berm Aug-01Oct-01A berm was constructed across the width of Cell 5B in anticipation that it would improve cell Dec-01hydraulics and thereby improve treatment performance. However, hydraulic modeling per-Feb-02Apr-02formed during design predicted at best only slight changes in flow patterns throughout the cell Jun-02after the berm was installed compared to pre-berm conditions (Piccone 2003). In addition, a Aug-02Oct-02post-berm tracer study could not determine the exact contribution, if any, that the berm made to Dec-02the hydraulic performance of this cell (DBEL 2004). A tracer study was not conducted prior to Feb-03constructing the berm to establish the need to improve cell hydraulics. Apr-03Jun-03 Aug-03Lessons Learned Oct-03Dec-03Loading: The treatment performance of STA-1W is sensitive to both the hydraulic and P Feb-04loads to this wetland. Prolonged loading above the design envelope can result in reduced TP re-Apr-04Jun-04moval as measured on a percent basis. When this STA has been operated for flood control, Aug-04treatment performance has suffered. However, the data also indicate that the performance of Oct-04

    STA-1W can recover when loading is subsequently reduced. Total P load reduction is moderate-ly correlated with TP load.

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    Internal Draft for Management Review Do Not Circulate or Cite

    Water depth: It is well known that the composition of the emergent vegetation community in wetlands (i.e., cattail) is influenced by water depth. Prolonged periods of deep water have been implicated in the formation of floating cattail islands in Cells 1 and 2 of STA-1W. The depth threshold for this process appears to be in the range of 60 to 90 cm. However, we suspect that soil conditions and high wind events also have a significant role in cattail island formation. The interaction of these factors complicates the development of a simple maximum depth rule to

    prevent cattail from floating in the future.

    Cell 5B berm: Cell hydraulics should be fully characterized before installing additional berms in the STA treatment cells in the future.

References

     Burns & McDonnell. 1994. Everglades Protection Project - Conceptual Design. 92-166-1-002.

    Report prepared for the South Florida Water Management District, West Palm Beach, FL.

    DBEL. 2004. Evaluation of Full Scale Stormwater Treatment Area Enhancements: Tracer

    Project. Contract ML040332. DB Environmental Laboratories, Inc., 365 Gus Hipp Blvd.,

    Rockledge, FL.

    Piccone, T.T. 2003. Two-dimensional Hydraulic Analysis for Cell 5 of STA-1W. Prepared for

    DEP Agreement No. G0040 Subtask 1.1 Hydraulic Analysis. South Florida Water Manage-

    ment District, West Palm Beach, FL.

    Walker, W.W. Jr., 1995. Design basis for Everglades stormwater treatment areas. Water Resources Bulletin 31, 671-685.

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