STA & Reservoir Performance Measures for the Everglades Restudy
prepared for
U.S. Army Corps of Engineers & U.S. Department of the Interior
by
William W. Walker, Jr., Ph.D, Environmental Engineer
July 1998
Introduction
This site is being developed to assist in evaluating alternatives under the Everglades Restudy being conducted by the U.S. Army Corps of Engineers. It focuses on performance of Stormwater Treatment Areas (STA's) & Storage Reservoirs (SR's) for phosphorus removal. The STA's have been designed under the Everglades Construction Project (ECP) to reduce phosphorus concentrations in agricultural runoff and releases from Lake Okeechobee prior to discharge into the Everglades Water Conservation Areas (Burns & McDonnell, "Everglades Construction Project, Conceptual Design", prepared for South Florida Water Management District, February 1994). Agricultural Best Management Practices (BMP's) and STA's will be used to meet interim treatment requirements established in the State/Federal Settlement Agreement (1991) and Everglades Forever Act (1994). BMP's have been fully implemented in the EAA since ~1995. The Everglades Nutrient Removal Project (a 3700-acre pilot-scale STA) has been in operation since August 1994. The first full-scale STA (STA-6) started operation in December 1997. The last STA (STA-34) is scheduled for startup in October 2003. Hydrologic and water-quality monitoring programs being conducted by South Florida Water Management District (SFWMD)provide bases for tracking long-term responses to these control measures.
Water-management alternatives being considered under the Restudy can influence water loads, nutrient loads, and depth regimes in STA's and Storage Reservoirs. These changes, in turn, can influence performance, as measured by outflow phosphorus concentrations or load reductions. The following analyses have been conducted to evaluate the potential effects of Restudy alternatives on the performance of currently-planned treatment facilities. Results could be used as a partial basis to rank Restudy alternatives and/or to identify additional treatment measures required to provide desired water-quality benefits for a Restudy alternative selected based upon hydrologic criteria.
Mass Balance Framework
Results are based upon monthly water budgets generated by the South Florida Water Management Model (SFWMM) for each Restudy alternative (95Base, 50Base, Alternatives A, B, C, & D). The SFWMM predicts flows and water levels for 1965-1995 hydrologic conditions. For each Restudy alternative, flow and phosphorus are routed through the Everglades Agricultural Area (EAA) and treatment areas according to pathways specified in Figure 3.3.7 of SFWMM documentation (modified to include EAA storage reservoirs). A glossary of flow terms identified in the above figure and used in formulating water and phosphorus balances is attached. Required flow and stage values are extracted from model output files supplied by SFWMD for each model run (daily stages in STAs & reservoirs, monthly water balances (monbud), and structure daily flows (str2x2.dss)).
The spatial scope of the analysis is currently limited to watersheds contributing to the Everglades Construction Project (EAA, EAA 298 Districts, C-139, and C-15W) and to releases from Lake Okeechobee to the STA's or Water Conservation Areas. Future efforts may involve expansion of the scope to include inflows from other basins west and east of the WCA's.
Restudy Alternatives
The analysis considers two base conditions (95Base, 50Base) and five Restudy Alternatives (A,B,C,D,D13R). Detailed descriptions are given elsewhere. The 95Base run represents 1995 land use conditions with agricultural BMP's in place, but without the ECP. The 50Base run represents 2050 land use conditions with agricultural BMP's and the ECP. The Alternatives reflect different water-management scenarios imposed on the 50Base run. One major difference between the 50Base and Alternatives is the addition of storage reservoirs in the Everglades Agricultural Area. These facilities would store runoff and lake releases for meeting EAA irrigation demand and for discharge to the WCA's through STA-34.
Periods for Record
Flow and phosphorus balances are developed for two time periods:
Calendar Years 1965-1995: entire SFWMM period of record; used for evaluating Restudy alternatives
Water Years 1979-1988 (Oct 1978 - Sept 1988): hydrologic period used for designing interim control measures under the State/Federal Settlement Agreement(SA) and Everglades Forever Act (EFA). STA's have been designed to treat all runoff during this period and produce outflow with a long-term, flow-weighted concentration less than or equal to 50 ppb.
The first period is consistent with that used for evaluating Restudy alternatives based upon hydrologic criteria. The second period permits comparisons of predicted results for each alternative with the design basis for the Everglades Construction Project and with interim treatment requirements specified in the SA and EFA. The current analysis produces only 31-year or 10-year average mass balances. Future refinements will consider shorter time steps (daily, monthly, annual).
Phosphorus Removal Models
Equations used in predicting phosphorus removal in STA and storage reservoirs are attached. These equations are applied in the context of 31-year or 10-year average water and mass balances. Outflow concentrations and loads are predicted based upon inflow concentration, mean depth, and hydraulic residence time (or water load). Secondary terms considered in each model include atmospheric deposition, seepage inflow, and seepage outflow.
STA performance is predicted with the mass-balance model used in STA design (Walker, W.W., "Design Basis for Everglades Stormwater Treatment Areas", Water Resources Bulletin, Vol. 31, No. 4, pp. 671-685, August 1995, Click here to download stadesign.pdf (1.1 mb). The STA's have been designed to treat all runoff and achieve a long-term flow-weighted mean concentration of 50 ppb under Water Year 1979-1988 hydrologic conditions. The model assumes that the rate of phosphorus removal per unit area is proportional to the long-term-average water-column phosphorus concentration. Accretion of new soil is the sustainable removal mechanism. Performance is governed by a "settling rate" coefficient of 10.2 m/yr, which has been calibrated to data from impacted areas of WCA-2A and is consistent with performance data from other constructed wetland treatment systems (Kadlec & Knight, Treatment Wetlands, Lewis Publishers, 1996). The sensitivity of predicted STA outflow concentrations to alternative settling rate estimates is demonstrated for the 2050 Base Run and Alternative D13R.
Storage Reservoirs are included in the alternatives primarily for hydrologic purposes. Water quality benefits may also be derived as a consequence of phosphorus retention in these systems. Reservoir performance is predicted using an empirical model which has been tested against detention pond data (Walker, W.W., "Phosphorus Removal by Urban Runoff Detention Basins", Lake and Reservoir Management, Volume 3, North American Lake Management Society, pp. 314-238, 1987, Click here to download dbasins.pdf (.8 mb). The current analysis considers EAA storage reservoirs discharging into STA-34. The EAA reservoirs consist of 2 or 3 cells in series, depending upon alternative. Future analyses will consider storage reservoirs located elsewhere in the planning area. Generally, the reservoir model predicts lower phosphorus removal rates than the STA design model for a system with the same inflow concentration, depth, and water residence time. Results may be conservative (i.e. under-predict actual P uptake) in reservoir cells.
The equations partially consider the effects of dry periods by shutting down removal mechanisms. Net phosphorus removal is assumed to be zero during periods when the average water depth in an STA or Reservoir (average water surface elevation - average ground elevation) is less than 0.1 feet (1.2 inches). The model does not explicitly account for recycling of phosphorus resulting from oxidation of peat during extended dry periods. The effect is implicit in the settling rate estimate (10.2 m/yr) developed from a 26-year period of record in WCA-2A that included extreme droughts.
Effects of inflow and outflow seepage are considered in formulating phosphorus balances for STA's and storage reservoirs. Seepage magnitudes are determined from groundwater inflows and outflows calculated by the SFWMM. Based upon monitoring data from the Everglades Nutrient Removal Project, inflow seepage is assumed to have a flow-weighted-mean concentration of 20 ppb. Outflow seepage is assumed to be recycled to the treatment area inflow (either directly via seepage collection canals or indirectly via drainage canals in the watershed) and is not considered part of the STA or reservoir outflow for evaluating downstream impacts. Seepage terms are generally small and have little influence on the overall mass balances (e.g., seepage rates tabulated for Alternative D13R). Since the SFWMM does not simulate seepage collection and recycling within treatment areas, actual magnitudes and effects of seepage may be larger than represented in these calculations.
Inflow Concentrations
STA and Reservoir inflows consist of runoff from various watersheds (4 major EAA basins, EAA 298 Districts, C139, C51W, L8), releases from Lake Okeechobee (structures S354, S351, and S352), and outflow from upstream storage areas (from EAA reservoirs to STA-34). Concentrations in EAA runoff (the largest phosphorus source) are influenced by Best Management Practices (BMP's). The EAA Regulatory Rule enacted by SFWMD in 1992 requires a 25% reduction in phosphorus loads from the 523,000-acre EAA basin relative to loads measured in 1979-1988. Phosphorus balances are developed for two assumptions regarding future BMP performance:
25% Load Reduction. This is required under the EAA Regulatory Rule. It is the same assumption used in sizing STA's under the Everglades Construction Project to achieve an outflow concentration of 50 ppb. Runoff concentrations for various basins and structures are equal to those used in STA design. These were originally developed by adjusting historical concentrations (1979-1988) to account for BMP implementation (Burns & McDonnell, Inc., "Historical Discharge Data for the Everglades Agricultural Area", TM 3021-A1-002, Sept 1992).
51% Load Reduction. This is the observed load reduction for the 3 year period between May 1994 and April 1997, as measured using the methodology specified in the EAA Regulatory Rule. Runoff concentrations for various basins and structures are set equal to those measured during this three year period.
Results from the first assumption are emphasized. There is no law or regulation to guarantee continued BMP performance at the 51% level. The second assumption is included primarily to evaluate sensitivity of project outflow concentrations and alternative ranking to BMP performance. In both concentration scenarios, lake release concentrations at structures S351, S352, and S354 are set equal to long-term, flow-weighted-mean concentrations in releases to the Water Conservation Areas (flow-thru terms in EAA water balance) between October 1978 and September 1997. An attached table lists concentrations used for each source and BMP scenario.
Because SR's generally have lower hydraulic loads than STA's, predicted outflow concentrations are more sensitive to atmospheric P deposition. A concentration of 30 ppb is assumed for bulk precipitation. This is applied to the monthly rainfall volumes provided by the SFWMM for each water-budget unit. This value is somewhat lower than that used in STA design (50 ppb), but is supported by recent atmospheric deposition data from the Everglades Nutrient Removal Project and WCA-1.
STA & Reservoir Configurations
The areas, elevations, and design inflows for each STA and EAA reservoir are listed in an attached table. STA areas and inflows are taken from the latest Everglades Program Management Plan (SFWMD, Rev 3 Draft, December 1997). Differences in STA configuration between the Conceptual Design and Restudy model runs are discussed below. Detailed comparisons of mass balance results are attached.
All Restudy model runs (50Base and alternatives) assume that Section 2 of STA-6 is not constructed. Runoff from the C-139 Annex is discharged to L-4/L-28 instead of STA-6. All C-139 runoff is discharged to STA-5. Accordingly, a treatment area of 870 acres (Section 1) has been assumed for STA-6 in Restudy simulations, as compared with the full design of 2,280 acres.
EAA storage reservoirs (not part of ECP) consist of 2 (Alt-A,B) or 3 (Alt-C,D,D13R) cells in series. Inflow to the first cell consists of runoff from the EAA S7 and S8 basins. Outflows from the first cell are used primarily to meet EAA irrigation demand. Inflows to the second cell consist of outflow from the first cell (relatively minor) and releases from Lake Okeechobee (S354 and S351). Reservoir outflows are discharged to STA-34 or used to satisfy EAA irrigation demand. Water levels in the first cell are relatively deep (averaging ~ 6 feet). The second and third cells are relatively shallow and dry out frequently. Most of the phosphorus removal is accomplished in the first cell, which has much more stable water levels. Mass-balance results for Alt-D13R indicate indicate negative phosphorus retention in the third cell (outflow structure load exceeds inflow structure load). Structure phosphorus loads to this cell are only slightly above atmospheric loads. Given the wide variations in water levels, modeling at shorter time steps may provide better predictions of phosphorus dynamics in the reservoir cells.
Results
Bottom-line results are contained in a matrix of Key Performance Indicators, which is also supplied in an Excel workbook form. Results of the analysis can be accessed from the following links:
- Key Performance Indicators.
- Detailed Results for 1965-1995.
- Detailed Results for Water Years 1979-1988.
- Detailed Results for Alternative D13R.
- Comparisons with ECP Design.
- Sensitivity of STA Performance to P Settling Rate, Alternative D13R
- General Table Index. All tables at site.
- Equations Used to Estimate P Removal in STA's and Reservoirs
- Links to STA & Reservoir Daily Stage Plots.
The above links are also accessible from the site index located in the left screen frame.
Tables are in PDF format and require Adobe Acrobat Reader, Version 3.0. Hint: To view a PDF document online, left-click on link. To download a copy of the document & save on disk, right-click on link and select "Save Target As" (IE4.0).
Made minor corrections & adjustments to calculations.
Added further discussion of differences in key performance indicators among Restudy alternatives.
Added analysis & discussion of differences between ECP & Restudy alternatives.
Added sensitivity analysis of STA performance & treatment area requirements vs. settling rate for each BMP assumption.
Added Alternative D13.
Replaced Alternative D13 with Alternative D13R
Click here to download the entire site in a compressed file.
Map of Everglades Construction Project
Everglades Restudy - Home Page
Everglades Restudy - Hydrologic Performance Measures
Thanks to Siaka Kone, Luis Cadavit, and Lehar Brion of SFMWD for help in interpreting SFWMM output and for providing valuable feedback.