Stormwater Treatment Areas Research Articles

Growth of Calcareous Epilithic Mats in the Margin of Natural and Polluted Hydrosystems: Phosphorus Removal Implications in the C–111 Basin, Florida Everglades, USA

ABSTRACT The Florida Everglades is a fragile wetland system that is naturally depleted in phosphorus (P). This hydrosystem has been heavily impacted by human activities, including the draining of wetlands to provide water for agricultural and urban use. The result is a highly compartmentalized system with altered hydropatterns; wetlands receive canal discharges from diffuse agricultural/urban runoff containing high levels of pollutants, including P. Excess loading of P has induced ecological changes, including dramatic effects on periphyton, the dominant producer community. An Everglades rehabilitation plan has been established to restore natural hydropatterns and decrease P loads. On the southern edge of a large canal (C-111) draining the southern Everglades, levees have been removed to rehabilitate hydrology in the adjacent marsh. Levee removal resulted in exposure of limestone bedrock that, when flooded with shallow water from the canal, favors the development of thick calcareous epilithic mats. When flooded approximately 6 months a year, this margin area between the polluted and the natural hydrosystem functions as a Periphyton-based Stormwater Treatment Area(PSTA), a biological tool developed at the Southeast Environmental Research Center (Miami, FL), being considered in the Everglades as a means of P removal from enriched waters. Here, we evaluate the harvesting rates of periphyton that promote the most efficient removal of TP from the water entering the marsh. Results indicate that harvesting of periphyton at 3 month intervals provides the greatest TP sequestration.

PHOSPHORUS RETENTION IN SOILS FROM A PROSPECTIVE CONSTRUCTED WETLAND SITE: ENVIRONMENTAL IMPLICATIONS 1

The phosphorus (P) retention characteristics of soil layers are critical for predicting the effectiveness of a constructed wetland, built in high water table soils, in reducing P transport to the receiving water body. Soil samples were collected from surface and subsurface horizons for P sorption and mineralogical studies directed to the assessment of a proposed site for stormwater treatment area (STA) construction in the Okeechobee Basin, Florida. The P sorption maxima were positively correlated with oxalate-extractable aluminum (ox-Al) and citrate dithionite bicarbonate-extractable aluminum (CDB-Al) under anaerobic conditions, but there was no significant correlation with either ox- or CDB-extractable iron (Fe). The ox/CDB ratio for extractable Fe was relatively high (>0.5), indicating that most of the Fe was in noncrystalline form, i.e., highly reactive and prone to rapid dissolution under reducing conditions. The X-ray Diffraction (XRD) patterns indicated that smectite was the dominant mineral in the clay-size fraction. The permanent negative charge of smectite limits contribution to P sorption. The equilibrium P concentrations (EPC0) of the soils were significantly correlated to CDB-Al but not to CDB-Fe, suggesting that Al-associated P is involved primarily in elevating EPC0 in these soils. Both P sorption capacity and EPC0 were fairly low for soils from most locations, indicating that the proposed STA cannot remove P from the rerouted water to a significant extent under ambient conditions and would not also cause significant P loading to the water table aquifer. Thus, alternatives for effective P sequestration, such as establishment of vegetative communities or chemical amendment, would be required.

Influence of flooding on phosphorus mobility in manure-impacted soil.

Agricultural lands are often used for constructing stormwater treatment areas (STAs) to abate nutrient loading to adjacent aquatic systems. Flooding agricultural lands to create STAs could stimulate a significant release of phosphorus (P) from soil to the water column. To assess the suitability of agricultural lands, specifically those impacted by animal operations, for the construction of STAs, soils from different components of the New Palm-Newcomer dairies (Nubbin Slough Basin, Okeechobee, Florida, USA) were collected by horizon and their P retention and release capacities estimated. In general, P released from A-horizon soil under flooded (anaerobic) conditions was greater than under drained (aerobic) conditions due to redox effect on iron (Fe) and consequent P releases. However, the P released from Bh-horizon soil was greater under aerobic conditions than under anaerobic conditions, possibly due to excessive aluminum (Al) content in the horizon. Double acid-extractable calcium (Ca), magnesium (Mg), Al, and P explained 87% of the variability in P release under aerobic conditions, and 80% of that under anaerobic conditions. The P release maxima indicated a high solubility of P in A-horizon soil from both active and abandoned dairies (13 and 8% of the total P, respectively), suggesting that these soils could function as potential sources of P to the overlying water column when used in STA construction. Preestablishment of vegetative communities or chemical amendment, however, could ameliorate high P flux from soil to the water column.

Submerged aquatic vegetation-based treatment wetlands for removing phosphorus from agricultural runoff: response to hydraulic and nutrient loading

Submerged aquatic vegetation (SAV) communities exhibit phosphorus (P) removal mechanisms not found in wetlands dominated by emergent macrophytes. This includes direct assimilation of water column P by the plants and pH-mediated P coprecipitation with calcium carbonate (CaCO3). Recognizing that SAV might be employed to increase the performance of treatment wetlands, we investigated P removal in mesocosms (3.7m2) stocked with a mixture of taxa common to the region: Najas guadalupensis, Ceratophyllum demersum, Chara spp. and Potamogeton illinoensis. Three sets of triplicate mesocosms received agricultural runoff from June 1998 to February 2000 at nominal hydraulic retention times (HRTs) of 1.5, 3.5 or 7.0 days. Mean total P (TP) loading rates were 19.7, 8.3 and 4.5 g/m2/yr. After eight months of operation, N. guadalupensis dominated the standing crop biomass and P storage, whereas C. demersum exhibited the highest tissue P content. Chara spp. was prominent only in the 7.0-day HRT treatments while P. illinoensis largely disappeared. Inflow soluble reactive phosphorus (SRP) (10–163μg/L) was reduced consistently to near the detection limit (2μg/L) in the 3.5- and 7.0-day HRT treatments, and to a mean of 9μg/L in the 1.5-day HRT treatment. The mean inflow TP concentration (107μg/L) was reduced to 52, 29 and 23μg/L in the 1.5-, 3.5- and 7.0-day HRT treatments, respectively. Total P concentrations in new sediment (mean=641, 408 and 459mg/kg in the 1.5-, 3.5-, and 7.0-day HRT mesocosms, respectively) were much higher than in the muck soil used to stock the mesocosms (236mg/kg). The calcium content of new sediment was twice that of the muck soil (16.5% vs. 7.6%), demonstrating that CaCO3 production and, perhaps, coprecipitation of P occurred. We observed no nocturnal remobilization of SRP despite diel fluctuations in pH and dissolved oxygen. Mean outflow TP (21μg/L) from a 147 ha SAV wetland (4-day nominal HRT) was similar to mean outflow TP in the 3.5-day and 7.0-day HRT treatments. The mesocosms adequately mimicked P removal and other important characteristics of the larger system and can be used to address research questions regarding treatment performance of full-scale SAV wetlands. Available data suggest that the incorporation of SAV communities into the stormwater treatment areas may benefit Everglades restoration.

Model prediction of the effects of changing phosphorus loads on the Everglades protection area

The Everglades Phosphorus and Hydrology (EPH) model was developed to simulate water movement and phosphorus transportin the Everglades Protection Area which is comprised of theEverglades National Park (ENP) and surrounding wetlands knownas the Water Conservation Areas (WCAs). Water flows from theEverglades Agricultural Area (EAA) through the WCAs intoEverglades National Park (ENP). The model is designed to represent the system as a series of cells in which water flowsfrom one cell to the next. The code allows for pumped inputsand pumped outputs of water as well as sorption and removal ofphosphorus through peat accretion. Model application involved dividing the system into twenty cellsrepresenting different segments of the WCAs. Inputs to each cellconsisted of water pumped from the EAA (where appropriate), flowfrom upgradient cells, and precipitation. Outputs included pumped outputs and flow out of each cell. Using data collectedby the South Florida Water Management District, the model wascalibrated by matching simulated and observed flows, water elevations, and phosphorus (P) concentrations for the period 1980–1988. The model was then validated for the 1988–1992 period using the same model parameters derived from the calibration process and comparing simulated and observed values.Reasonable agreement between simulated and observed values wasattained for both the calibration and validation periods. The calibrated and validated model was used to simulate the impacts on annual average total P concentrations in each cellresulting from the implementation of the management plan mandated by the Everglades Forever Act. This plan calls for the construction of six Stormwater Treatment Areas (STAs) to treat discharges from the EAA, hydrologic modifications of thesystem to promote sheet flow, and the implementation of BestManagement Practices to reduce P runoff from individual farms. In addition, the model was used to evaluate the impact of notbuilding one of the STAs (STA 3/4), and sensitivity analyseswere conducted to determine the effects of changing STA outletP concentrations throughout the system. Model results indicatethat phosphorus concentration reductions will occur in areas near EAA discharges in response to reductions in input P concentrations. However these measures will have little impacton phosphorus concentrations for 85% of the area of theWCAs and on the water entering Everglades National Park. The scenario analyses also indicate that phosphorus concentrationsthroughout most of the WCAs are similar with or without the construction of STA-3/4.

The Everglades Nutrient Removal Project test cells: STA optimization - status of the research at the north site

The Everglades is an oligotrophic ecosystem that is being adversely impacted by hydrologic changes and nutrient-rich runoff generated from urban and agricultural sources. The Stormwater Treatment Area (STA) Optimization Research and Monitoring program is mandated by the 1994 Everglades Forever Act and will assist the South Florida Water Management District in developing operational strategies that maximize performance of emergent macrophyte STAs. The primary objective of this research is to examine how hydrologic conditions may influence STA performance. The study was conducted in 0.2 ha, shallow, fully lined test cells located within the perimeter of the Everglades Nutrient Removal Project. Experiments were designed to examine the effect of increased and decreased hydraulic loading rate (HLR) on wetland performance and to determine, if possible, the HLR at which STA treatment fails to reduce outflow total phosphorus concentration to the interim target of 50 microg-P/L. To date, two HLR experiments have been completed at the north site. Preliminary data indicated at all HLRs tested that particulate phosphorus and dissolved organic phosphorus ratios remained virtually unchanged from inflow to outflow. The dissolved organic and particulate compounds within these test cells are extremely recalcitrant, and are not easily assimilated within the system. High HLRs may not result in detention times long enough to mineralize these forms into easily assimilated inorganic compounds, resulting in mean TP concentrations greater than 50 microg-P/L.

Evaluation of phosphorus retention in a South Florida treatment wetland

The Everglades Construction Project of the South Florida Water Management District (District) will employ large constructed wetlands known as Stormwater Treatment Areas (STAs) to reduce phosphorus concentrations in runoff entering the Everglades. The District built and operated a prototype STA, the 1,545 ha Everglades Nutrient Removal Project (ENRP), to determine the efficacy of subtropical wetlands for improving regional water quality with a focus on reducing total phosphorus (TP). In five years of operation, the ENRP has consistently exceeded its performance goals of TP outflow concentrations <50 microg P/L and a 75% TP load reduction. Since August 1994, the ENRP has retained 70.3 metric tons of TP that otherwise would have entered the Everglades. When corrected for surface area and inflow TP load, TP removal efficiency was highest in the inflow buffer cell and decreased generally in a downstream fashion through the wetland. High TP removal efficiency in treatment cell 4 was attributed to superior performance of its submerged aquatic vegetation community relative to the emergent and floating macrophyte community in the other cells. Controlled experiments in the District's STA Optimization Research Program will help clarify what effect vegetation and operational conditions may have on nutrient removal in the STAs.

Surmounting the engineering challenges of Everglades restoration

The South Florida Water Management District, in partnership with other agencies and stakeholders, is undertaking one of the world's largest ecosystem restoration programs. The foundation of the nutrient control program for the Everglades is a set of six large constructed wetlands, referred to as Stormwater Treatment Areas (STAs). The initial treatment goal is to reduce phosphorus entering the Everglades to 50 parts per billion. The STAs comprise almost 17,000 hectares, with a capital cost of approximately $700 million. Approximately 4,720 hectares are currently operational, another 2,600 hectares are in the start-up phase, and construction is just getting under way on the remaining areas. Throughout the design process, engineers and scientists collaborated to capture the best available information on wetland treatment systems, and to develop the most appropriate design criteria. Some of the more challenging issues included characterizing stormwater inflows and phosphorus loads, determining appropriate nutrient removal performance characteristics, and estimating hydraulic design parameters relating to densely vegetated systems. The design process combined in-house staff with engineering consultants, construction contractors, external review groups and independent peer-review. This paper summarizes major design aspects and key assumptions, and sets the stage for addressing future challenges associated with achieving long-term water quality goals of Everglades restoration.

Environmental impacts to the Everglades ecosystem: a historical perspective and restoration strategies

The Everglades is a vast subtropical wetland that dominates the landscape of south Florida and is widely recognized as an ecosystem of great ecological importance. As a result of anthropogenic disturbances over the past 100 years (i.e., agricultural and urban development, eutrophication resulting from stormwater runoff, changes in hydrology and invasion of exotic species), the biotic integrity of the entire Everglades is now threatened. To protect this valuable resource, the state of Florida and the Federal Government, in cooperation with other interested parties, have developed a comprehensive restoration strategy that addresses controlling excess nutrient loading and reestablishment of a more natural hydrology. These efforts include building approximately 17,000 ha of treatment wetlands, referred to as Stormwater Treatment Areas, to treat surface runoff before it is discharged into the Everglades. We briefly discuss the history of the Everglades in the context of environmental disturbance and outline the steps being taken to ensure its survival for future generations.

Progress in the research and demonstration of Everglades periphyton-based stormwater treatment areas

The South Florida Water Management District (District) is conducting research focused on potential advanced treatment technologies to support reduction of phosphorus (P) loads in surface water entering the remaining Everglades. Periphyton-based stormwater treatment areas (PSTA) are one of the advanced treatment technologies being researched by the District. This detailed research and demonstration project is being conducted in two phases. Basic research in field-based mesocosm experiments was conducted during the first phase within the District's Everglades Nutrient Removal Project (ENR). Studies were conducted in 24 portable PSTA mesocosms and three of the south ENR test cells. Phase 1 studies addressed the effects of system substrate (shellrock, organic peat, or sand), water depth, hydraulic loading rate, vegetation presence, depth:width ratio, and inhibition of algal growth on total phosphorus removal performance of the PSTA mesocosms. A second phase of research is currently under way, during which PSTA feasibility will be evaluated further in four field-scale constructed mesocosms totaling about 2 ha, and follow up studies within the ENR test cells and portable mesocosms will be conducted to further investigate the effects of other inorganic substrates, shallow water depth, and velocity on treatment performance. Phase 1 monitoring has determined that periphyton-dominated communities can be established in constructed wetlands within 5 months. The algal component of these periphyton plant communities is characteristic of natural Everglades periphyton. High macrophyte densities resulted from use of peat soils in PSTA mesocosms, while shellrock and sand soils promoted more desirable sparse macrophyte stands. P removal rates under the conditions of this research were relatively high considering the low influent total P concentrations tested (average 23 microg/L). PSTA mesocosms on shellrock soils were able to attain long-term average outflow total P concentrations as low as 11 microg/L. The maximum one-parameter TP first-order removal rate constant (k1) measured was 27 m/y. Minimum attainable outflow total P concentrations and mass removals appear to be the result of a balance between internal P loading from antecedent soils, uptake and burial processes in new sediments, and rainfall inputs. A different soil type (limerock) will be tested for effectiveness during Phase 2. Selected existing treatments will also be continued to look for trends over a second growing season.

IDENTIFICATION OF AN OPTIMAL SAMPLING STRATEGY FOR A CONSTRUCTED WETLAND1

ABSTRACT: The objective of this investigation was to determine the effect of sampling frequency and sampling type on estimates of monthly nutrient loads and flow‐weighted nutrient concentrations in a constructed wetland. Phosphorus and nitrogen loads and concentrations entering and leaving a subtropical wetland (the Everglades Nutrient Removal Project, ENRP) were calculated on the basis of three sampling frequencies. The first frequency included weekly composite samples (three daily samples composited for one week) and grab samples from August 1994 to July 1997, representing a base‐line condition for comparison with results using reduced sampling frequencies. The second and third sampling frequency included three and two composite samples per month, respectively, drawn from the weekly samples. Total phosphorus and nitrogen loads calculated using two and three samples per month were almost identical to results based on four samples per month (least‐squares regression coefficients ranged from 0.96 to 0.98). Results of monthly mean flow‐weighted nutrient concentrations, obtained using reduced sampling frequencies, also were strongly correlated to concentrations calculated using the base‐line sampling frequency (r2ranged from 0.82 to 0.93). Grab samples did not always provide good estimates of loads or concentrations, particularly at the inflow when data were highly variable. From the results of this study, we can recommend that bi‐weekly composite sampling be used to monitor nutrient concentrations and loads discharged from larger‐scale Everglades Stormwater Treatment Areas (STAs) now under construction. Because there are high costs associated with water sample collection and processing, studies to identify optimal sampling frequencies should be a key feature in the design of any comprehensive wetland‐monitoring program.

Phosphorus storage and release in response to flooding: implications for Everglades stormwater treatment areas

As part of the Everglades restoration program, 16 000 ha of constructed wetlands will be reestablished on land presently in agricultural production. These wetlands will be used to remove Phosphorus (P) from agricultural runoff before it enters the Everglades. Histosols, organic soils, are the predominant soil type in the Everglades Agricultural Area (EAA), and the conversion of these soils from drained to flooded conditions has important implications for P storage. Phosphorus storage in organic soils has been shown to be both positively and negatively affected by anaerobic conditions. In this study, P storage and release was followed in a 146 ha area during its conversion from farmland to wetland. The development of a productive biological community, as evidenced by strong diel dissolved O 2 and pH cycles, occurred within 3 weeks of flooding at one site and 2 months at a second site. This biological community was considered influential in maintaining the low concentrations of both N and P in the water column relative to soil porewater concentrations. Maximum total P (TP) and total Kjeldahl N (TKN) concentrations of 0.3 and 5 mg l −1, respectively, were recorded in the water column following flooding. These concentrations declined to background levels within 2–3 months. Soil porewater TP and total dissolved Kjeldahl N (TDKN) concentrations increased to maxima of 4 and 24 mg l −1, respectively, 2 months following flooding. Nutrient profiles across the soil–water interface were used to estimate flux rates. Calculated NH 4-N flux rates ranged between 0.18 and 0.74 μg cm −2 d −1 and P fluxes ranged between 0.03 and 0.15 μg cm −2 d −1. Phosphorus fluxes from the soil to the overlying water are a function of the mobility of different P fractions. Phosphorus fractions within soil cores, collected immediately upon flooding and again 1 year after flooding, were identified using bicarbonate (labile), sodium hydroxide (Fe- and Al-bound) and hydrochloric acid (Ca- and Mg-bound) extractions. Labile inorganic P increased, while labile organic P decreased in response to flooding. Phosphorus associated with Ca and Mg increased in the surface 0–45 cm soil profile in response to flooding. These data suggest that immediately following flooding the reestablished wetlands will act as a source rather than a sink for P, and P concentrations in the water column will not meet discharge requirements. Although this only occurs for a short time period, steps need to be taken to contain or recycle the water during this initial start-up. Soil fractionation data indicate that while organic P is the primary means of P retention within these soils, Ca-phosphates may play a significant role in P storage. Therefore, the reestablished wetlands should be operated to enhance Ca phosphate formation in addition to biological P uptake.

SOUTH FLORIDA: THE REALITY OF CHANGE AND THE PROSPECTS FOR SUSTAINABILITY: The design of ecological landscape models for Everglades restoration

Restoration of the Everglades is a multi-objective, multi-scale, multi-agency program that requires numerous computer models to test alternatives, understand ecosystem processes, and evaluate restoration performance. Landscape models used for Everglades restoration include hydrologic models, transition probability models, gradient models, distributional mosaic models, and individual-based models. As tools for restoration feasibility and as the backbone of the policies that will drive Everglades restoration for the next 20 years, it is critical that a wide audience evaluate the strengths and weaknesses of six landscape models. Simulations of historic hydropatterns and current hydropatterns, based mostly upon sheet-flow equations and canal-flow equations, respectively, have been the realm of the Natural Systems Model (NSM) and the South Florida Water Management Model (SFWMM). Despite a lack of biology in these two models, a comparison of their spatial output became the basis for the Comprehensive Everglades Restoration Plan (CERP) approved by the US Congress in October, 2000. SAWCAT, a transitional probability model, was based upon an analysis of the patchiness of cattail (Typha) and sawgrass (Cladium) cells in association with levees, water depth, and phosphorus. This statistical approach was used to predict the amount of sawgrass that would be converted to less desirable cattail, if phosphorus runoff patterns to the Everglades remained constant. The Everglades Water Quality Model (EWQM), a mass-balance gradient approach used to track phosphorus according to a simple net phosphorus removal coefficient, was used to design Storm Water Treatment Areas (STA) and to evaluate where and when phosphorus ‘thresholds’ would be exceeded under various hydrologic restoration plans. The Everglades landscape Model (ELM), a complex distributional mosaic model, used site-specific biogeochemical mechanisms and mass-balance to control energy and material flows, and to predict changes in carbon and phosphorus structure of the soil, water, and plant communities as a result of modified water deliveries to the Everglades. The Across Trophic Level Spatial Simulation (ATLSS), also a distributional mosaic modeling approach, used individual-based rules of behavior to predict animal movement and abundance in relation to hydrologic restoration plans. When these landscape models are combined, they effectively contribute to water management and policy for Everglades restoration. To insure their effectiveness, an applied science strategy provides the framework for their integration into the restoration process.

ECOLOGICAL RESEARCH FOR AQUATIC SCIENCE AND ENVIRONMENTAL RESTORATION IN SOUTH FLORIDA

The theme of this feature—the land–water interface: science for a sustainable biosphere—provides a forum to highlight the relationship between science and resource management, using restoration of the Kissimmee-Okeechobee-Everglades (KOE) ecosystem of south Florida as a case study. This subtropical ecosystem encompasses 16 counties and 44 000 km2, from the Kissimmee Chain of Lakes in central Florida to the shallow estuarine waters of Florida Bay, and is within the jurisdiction of the South Florida Water Management District. During the next two decades, the floodplain and channel of the Kissimmee River will be re-coupled into a meandering river system with riparian wetlands and a more natural hydrology. An evaluation program on this restoration has been designed using ecological concepts and will provide opportunities to corroborate river/floodplain theory and document the varied responses of biotic communities to hydrological restoration. The evaluation program will provide the information needed for adaptive management of the river/floodplain ecosystem. Scientists and engineers are testing an array of ecological hypotheses on Lake Okeechobee, a central feature of the KOE ecosystem, to reduce uncertainty in predicting responses to nutrient loading, lake stage variation, and exotic species invasion. Research on the lake has clarified the linkage between physical factors, nutrient levels and biotic variables, and the frequency of algal blooms. This information has been used to support decisions and plans for managing the lake and its watershed. Restoration of the Florida Everglades is grounded in a diverse suite of scientific projects that are contributing to wetland science, ecosystem modeling, and restoration ecology. Studies on the effects of nutrients on wetland ecosystem structure and function have provided information at several spatial scales which is being applied directly to management issues. Findings from research and monitoring have been crucial in supporting decisions on the completion of six large stormwater treatment areas in the Everglades Construction Project. At the southern edge of the ecosystem, Florida Bay has been the focus of intensive research leading to changing paradigms on the relative effects of nutrients, turbidity, physical factors, and fresh water on the functions of this unique estuary. Scientific findings on the bay support the current direction of management actions to increase freshwater inputs from the southern Everglades, although much remains to be learned about this subtropical system. Management-oriented research has contributed to a large increase in the rate of scientific publication on the KOE ecosystem. Approximately 1500 articles have been published on the system in the 1990s, and these have contributed to advancing basic science as well as resource management. Ecological reports being published on the ecosystem use state-of-the-art approaches to time and space scales and appropriate blends of monitoring, experimentation, and modeling. Peer review and interagency cooperative planning have fostered relevant, timely, and objective science. The application of science to decision making in adaptive resource management plays an important role by clarifying those facts and concepts with the most significant predictive value for management and heuristic value for fundamental science. Improvement of the science–management linkage in south Florida and elsewhere awaits better interaction between scientists and managers in planning research and more effective communication of research findings. Relevant scientific information of high quality, particularly when published in the peer-reviewed literature, cannot be distorted easily and is vital to support informed and equitable social choices in the decision-making process.

CALIBRATION OF A STEADY-STATE SEEPAGE MODEL FROM TRANSIENT RECOVERY OF FIELD DATA

ABSTRACT: The South Florida Water Management District (District) constructed three experimental test cells (20 ± 50 feet = 6.1 ± 15.2 m) to determine the seepage influence of a major agricultural canal adjacent to the cells. These three cells represented the first stage of a series of experimental cells to determine the efficiency of algal-bacterial communities (periphyton) to successfully remove phosphorus (P) from the water. This treatment technology is known as Periphyton-based Stormwater Treatment Area (PSTA) and is being considered under the Supplemental Technology Program (Phase II) to comply with the Everglades Forever Act (EFA) of 1994. However, the efficiency of a PSTA in removing nutrients has not been documented yet. To determine the efficiency of PSTA technology to remove phosphorus (P) from the water, it is necessary to quantify all the components of the water budget. The purpose of the cells was to evaluate the potential seepage due to the operational range of stages into and out of the test cells. A steady-state, two-dimensional seepage (FastSEEP/SEEP2D) model was applied and successfully calibrated for that purpose. The model was calibrated using mean seepage flows obtained after drawdowns in each test cell, by pumping water out. These seepage flows as well as the boundary conditions (stages) were estimated by discretizing the recovery data. The recovery data represents the seepage from the agricultural canal into each of the test cells after the drawdowns, since the stages in the canal during the experiment did not change significantly and were high. Each of the three test cells was excavated to different bottom elevations to determine the effect of substrate differences on seepage rates. Values of hydraulic conductivity for five different layers in the models were based on field observations, lithologic descriptions, and slug tests. Modeled seepage produced reasonable results for the three test cells compared with estimated seepage from the recovery data, achieving a satisfactory calibration. The errors of the simulated seepage ranged from −6.1 to 16.8 percent (under and over estimation, respectively).

Nutrient retention dynamics of the Everglades Nutrient Removal Project

The Everglades Nutrient Removal (ENR) Project was constructed to reduce nutrient concentrations in stormwater runoff water from the Everglades Agricultural Area. Although nutrient concentrations of influent water ranged from 66 to 201 μg TP L and 140 to 541 μg TN L−1 and varied substantially over time, the outlet concentrations remained low, 9 to 39 μg TP L−1 and 99 to 286 μg TN L−1, during the first three years of operation (from August 1994 through August 1997). Nutrient removal efficiency was calculated in terms of decrease in both loads and nutrient concentrations. Nutrient loading rates averaged 1.17 g TP m−2 year−1 (±0.12 SE) and 31.56 g TN m−2 year−1 (±2.92 SE) at the inflow and 0.23 g TP m−2 year (±0.02 SE) and 20.71 g TN m−2 year−1 (±1.63 SE), at the outflow. TP load removal ranged from 66% to 91% and averaged 82% for the period of record. while TN load removal ranged from 11% to 76% and averaged 55%. Rainfall contribution to the ENR Project nutrient budgets was small, averaging only 4% and 3% for TP and TN, respectively. The ENR Project performance results during the first three years suggest that additional Stormwater Treatment Areas (STAs) will be effective in removing TP from stormwater runoff.

Hydrologic balance for a subtropical treatment wetland constructed for nutrient removal

This paper reports on an analysis of a water budget for the Everglades Nutrient Removal (ENR) Project in South Florida, USA, for the first 2 years of operation. Estimates of nominal hydraulic retention time (HRT) based on average monthly values are compared with HRT obtained from steady-state two-dimensional hydrodynamic simulations, and show good agreement. Statistical analysis is performed to develop stage- and depth-duration curves for the ENR Project. The ENR Project was constructed south of Lake Okeechobee by the South Florida Water Management District to begin the process of removing nutrients (especially phosphorus) from agricultural drainage and stormwater run-off before entering the Everglades. The State of Florida's Everglades Forever Act of 1994 mandates, among other things, completion of stormwater treatment areas (STAs), and research to optimize phosphorus retention capacity, and to define threshold phosphorus concentrations that lead to an imbalance of biota. The ENR Project, a 1544-ha wetland, was designed and constructed as a pilot project to gain experience on design, construction, and operation of the STAs. It began operation in August 1994. For the 732 days analyzed (19 August 1994–19 August 1996), the average water inputs into the project were as follows: 86.2% from the inflow pumps, 11.2% from rainfall, and 2.6% as emerging measured and estimated seepage from an adjacent area with higher stages (Water Conservation Area 1). The average water outputs from the project consisted of 85.1% from the outflow pumps, 8.9% as evapotranspiration, and 6.0% as a net seepage and groundwater component. This net component accounts for elements of the surface/subsurface water interaction, either entering or leaving the project, which are unknown at this time. Considering monthly average values, there were only 3 months within the study period with positive values of this net component. These months were June 1995, and March and July 1996. The two most important elements included in the net seepage and groundwater component are expected to be the surficial aquifer recharge (outflow), and the unmeasured seepage (inflow) from Water Conservation Area 1 (subsurface seepage). The unmeasured subsurface seepage has recently been determined from computer model simulations.

LONG‐TERM EQUILIBRIUM PHOSPHORUS CONCENTRATIONS IN THE EVERGLADES AS PREDICTED BY A VOLLENWEIDER‐TYPE MODEL1

ABSTRACT: Anthropogenic phosphorus loading, mainly from the Everglades Agricultural Area (EAA), is believed to be the primary cause of eutrophication in the Everglades. The state of Florida has adopted a plan for addressing Everglades eutrophication problems by reducing anthropogenic phosphorus loads through the implementation of Best Management Practices (BMPs) in agricultural watersheds and the construction of stormwater treatment areas (STAs). Optimizing the effectiveness of these STAs for reducing phosphorus concentrations from agricultural runoff is a critical component of the District's comprehensive Everglades protection effort. Therefore, the objective of this study was to develop a simple tool that can be used to estimate STAs’performance and evaluate management alternatives considered in the Everglades restoration efforts. The model was tested at two south Florida wetland sites and then was used to simulate several management alternatives and predict ecosystem responses to reduced external phosphorus (P) loadings. Good agreement between model predictions at the two wetland sites and actual observations indicated that the model can be used as a management tool to predict wetlands’response to reductions in external phosphorus load and long‐term P levels in aquatic ecosystems. Model results showed that lowering P content of the Everglades Protection Area (EPA) depends on reducing P loads originating from EAA discharges, not from rainfall. Assuming no action is taken (e.g., no BMPs or STAs implemented), the steady state model predicted that the average concentration within the modeled area of the marsh would reach 20 μg L−1 within five years. With an 85 percent reduction in P loading, the steady‐state model predicted that Water Conservation Area 2A (WCA‐2A) P concentration will equilibrate at approximately 10 μ L−1, while elimination of all loadings is projected to further reduce marsh P to values less than 10 μg L−1.

DESIGN BASIS FOR EVERGLADES STORMWATER TREATMENT AREAS1

ABSTRACT: The State of Florida (1994) has adopted a plan for addressing Everglades eutrophication problems by reducing anthropogenic phosphorus loads. The plan involves implementation of Best Management Practices in agricultural watersheds and construction of regional treatment marshes (Stormwater Treatment Areas or STA's). This paper describes the development, testing, and application of a mass‐balance model for sizing STA's to achieve treatment objectives. The model is calibrated and tested against peat and water‐column data collected in Water Conservation Area‐2A (WCA‐2A), where phosphorus dynamics and eutrophication impacts have been intensively studied. The 26‐year‐average rate of phosphorus accretion in peat is shown to be proportional to average water‐column phosphorus concentration, with a proportionality constant of 10.2 m/yr (90 percent Confidence Interval = 8.9 to 11.6 m/yr). Spatial and temporal variations in marsh water‐column data suggest that drought‐induced recycling of phosphorus was important during periods of low stage in WCA‐2A. Maintaining wet conditions will be important to promote phosphorus removal in STA's. Sensitivity analysis of STA performance is conducted over the range of uncertainty in model parameter estimates to assess the adequacy of the model as a basis for STA design.