Stormwater Treatment Areas Research Articles

Substrate manipulation near the outflow of a constructed wetland reduced internal phosphorus loading from sediments and macrophytes

Submerged aquatic vegetation (SAV) has been successfully used in wetland-based water treatment systems due to its capacity for rapid nutrient uptake, water-column oxygenation, and dampening of sediment resuspension. For Everglades restoration, large engineered wetlands known as Stormwater Treatment Areas (STAs) rely heavily on SAV-dominated treatment cells, since they have been highly efficient at reducing phosphorus (P) in surface waters to mesotrophic levels (12–25 µg/L). However, Everglades STAs have strict water quality effluent limits (∼13 µg/L), and many SAV cells are unable to regularly achieve such levels. In shallow aquatic systems, SAV can be a potential source of internal P loading from sediment to the water column via “nutrient pumping” through root uptake and leaf turnover. This mechanism may be an important obstacle to achieving lower wetland outflow P concentrations, but little is known of the interactions between substrate and SAV nutrition under these low-nutrient conditions.Soil core microcosms were incubated with or without macrophytes to estimate internal P loading potential in an SAV wetland and test the hypothesis that wetland substrate manipulation in the downstream (outflow) region might have reduced such loads and enabled more reliable delivery of low-P discharges. During six sequential two-week batch incubations, water-column P concentrations above muck soil overlain by accrued marl (typical of substrates in existing SAV-dominated STAs) indicated significantly higher internal P loading and higher macrophyte biomass growth than above either a bare limerock (LR) substrate (representative of a 40-ha pilot substrate manipulation study, where the muck soil was removed to expose the underlying limestone bedrock, i.e., “limerock”) or LR with an accrued marl layer (representing post-startup conditions in the pilot, after muck removal and subsequent sediment accumulation under treatment wetland conditions).The results suggest that by removing muck soils to expose a LR substrate in the pilot wetland, SAV biomass and tissue P were diminished, thus reducing the potential for internal loading from SAV, which supports our hypothesis. For each of the three substrate types, internal P loading was higher in the presence of a vascular rooted plant, Potamogeton illinoensis, than the non-vascular macro-alga, Chara sp., (both taxa are prevalent in full-scale STAs), while internal loading was not different between Chara and the non-vegetated control. This indicates the role of P mining by rooted SAV. Overall, this study furthers our understanding of the wetland P cycle at near-oligotrophic levels, and is relevant to STA optimization for Everglades restoration.

The polluter pays principle and Everglades restoration

Florida’s Agricultural Privilege Tax (APT) is a unique example of the “polluter pays principle” applied to reduce nonpoint source pollution. It has been more than 20 years since the APT was enacted as a building block for restoration of the Everglades ecosystem, the most extensive environmental restoration project in the world. This article provides a historical perspective on the environmental, socio-political, and institutional factors that led to the enactment and evolution of the APT. The efficiency and equity of the tax as part of a broader program to achieve water quality goals for the Everglades are also evaluated. A key result of this evaluation is that the APT has encouraged reductions in nutrient loads from agricultural areas, but these contributions have been limited. Dramatic increases in abatement costs to treat nonpoint outflows with stormwater treatment areas have occurred, and the timeline to achieve water quality objectives has been pushed forward by decades beyond the original goal. Contrary to the polluter pays principle, political compromises that have shaped the APT since its inception have shifted an increasing share of the burden to the public to reduce the flow of nutrients into the Everglades. An inconsistent regulatory approach to reduce nonpoint sources throughout the Everglades watershed may be the most important impediment to Everglades restoration.

Spatiotemporal changes in soil phosphorus characteristics in a submerged aquatic vegetation-dominated treatment wetland

In South Florida, stormwater treatment areas (STAs) are used to reduce phosphorus (P) in runoff from agricultural areas before water is discharged into the Everglades Protection Area. The Everglades STAs retain a significant amount of P and play an important role in Everglades restoration. Wetland soils generally are long-term sinks for P; therefore, the sustainability of STA treatment performance can be assessed by tracking changes in soil characteristics. This study evaluated the spatiotemporal changes in soil P and related physicochemical characteristics in the unconsolidated floc and underlying surface soil layer (0–10 cm) of a 920-ha submerged aquatic vegetation (SAV)-dominated wetland (STA-2 Cell 3). Physicochemical properties in soil cores collected in 2003, 2007, 2009, and 2015 were evaluated and compared using geostatistical methods. Results indicated a gradual increase in floc depth over time. Total P (TP) concentrations in the floc were significantly higher than in the surface soil. Slight but statistically nonsignificant increases of mean TP in floc were observed. There was a significant increase in P storage in the floc layer between 2003 and 2007, with more P stored in the surface soil layer. Interpolated maps showed consistently higher TP and P storage values in the floc and surface soil near inflow areas of the cell during all sampling events. Furthermore, the 2003 to 2015 change maps showed TP and P storage decreasing from inflow to outflow. Bulk density (BD) in floc was approximately half of surface soil BD. Significant decline in the percentage of ash-free dry weight (AFDW) in floc from 2003 to 2007 indicated an increase in mineral content. This is consistent with increases in total calcium (TCa) in the floc, which was up to four times higher than in the surface soil layer. This indicates that TCa plays a central role in defining the characteristics of SAV cells. Overall, despite the heterogeneity of sediment attributes in the system, temporal trends and spatial patterns were observed in the physicochemical characteristics of soils. These trends and patterns can be used to understand long-term changes in large-scale treatment wetlands. Such insights are useful for optimizing and sustaining the treatment performance of STAs.

Effects of short-duration hydraulic pulses on the treatment performance of a periphyton-based treatment wetland

The effect of high flows on total phosphorus (TP) removal in a 40-ha periphyton-based stormwater treatment area wetland (PSTA Cell) located in Stormwater Treatment Area (STA)-3/4 was evaluated by subjecting the PSTA Cell to three short-duration (3-day) hydraulic (high-inflow) pulses. The intent was to mimic hydraulic conditions that would be experienced during large storm events should the PSTA concept be implemented throughout the complex of treatment wetlands (STAs) operated by the South Florida Water Management District for Everglades restoration. The average inflow rate to the PSTA Cell during the pulses (1.3–2.0m3s−1) was approximately three to four times higher than under normal operation. The hydraulic loading rates (HLRs) during the pulses (28–43cmd−1) matched the peak 3-day HLRs observed in STA-3/4 during high inflows associated with storm runoff. Correspondingly, the phosphorus loading rates (PLRs) and hydraulic retention times (HRTs) during the pulses (2.3–6.3mgm−2d−1 and 1.4–1.8 days, respectively) were substantially higher and lower, respectively, compared to PLRs and HRTs in the PSTA Cell under normal operation. No reduction in TP removal performance was observed in the PSTA Cell during the pulses, and there was no indication that post-pulse outflow TP concentrations increased compared to pre-pulse levels in response to the pulse events. The short-term increase in hydraulic and TP loads during this study did not compromise the treatment efficacy of the PSTA Cell. If implementable in a cost-effective manner in the STAs, the PSTA concept could be part of a treatment strategy that achieves TP effluent limits mandated for these wetlands.

Mapping Vegetation-Resistance Parameters in Wetlands Using Generated Waves

AbstractConstructed wetlands, referred to as stormwater treatment areas (STAs), are important components of the water management infrastructure in south Florida. These systems have complex internal...

An assessment of iron and calcium amendments for managing phosphorus release from impacted Everglades soils

The recent implementation of agricultural best management practices (BMPs) and treatment wetlands called stormwater treatment areas (STAs) have reduced phosphorus (P) concentrations and loadings to the Everglades Protection Area (EPA) in Florida (USA). There is a concern that despite reductions in external P loadings, internal loading from the legacy P enrichment of the EPA wetland soils will continue to elevate water column P concentrations, and may impede restoration outcomes. In an effort to explore ways to reduce soil P efflux, we retrieved intact, vegetated (cattail, Typha domingensis) soil monoliths from two P-enriched areas of the EPA and deployed them at a location where they received pre-treated (low P) surface water as ex situ flow-through mesocosms for 21 months with a mid-study 7-week dry down to mimic natural hydroperiod conditions. Two treatments were tested for soils from both sites, using triplicate mesocosms for each treatment. After applying a herbicide (glyphosate) to eliminate the cattail vegetation, iron (Fe as liquid FeCl3) amendments provided no P retention benefits in the organic soils from the two sites, and did not yield significantly (P > 0.05; n = 43) lower flux rates (6.1 and 3.5 mg Pm−2 d−1) than the herbicide/no soil amendment control (3.9 and 2.1 mg Pm−2 d−1), as was expected. A combination of low oxidation–reduction potential, heightened organic matter P mineralization, high pH, and sulfide production acted interactively to enhance Fe and P mobilization in the Fe-amended mesocosms. The herbicide/limerock (CaCO3)-amended soils exhibited significantly lower (P ≤ 0.05; n = 43) P flux (1.3 and 1.1 mg Pm−2 d−1) than the herbicide/no soil amendment control soils, but it remains unknown whether the observed reduction in P efflux (ranging from 48 to 67%) would justify the expense and potential environmental impacts of applying a surficial limerock amendment to large regions of the P-enriched wetlands.

A datamining approach to identifying spatial patterns of phosphorus forms in the Stormwater Treatment Areas in the Everglades, US

The Everglades ecosystem in Florida, USA, is naturally phosphorus (P) limited, and faces threats of ecosystem change and associated losses to habitat, biodiversity, and ecosystem function if subjected to high inflows of P and other nutrients. In addition to changes in historic hydropattern, upstream agriculture (sugar cane, vegetable, citrus) and urbanization has placed the Everglades at risk due to nutrient-rich runoff. In response to this threat, the Stormwater Treatment Areas (STAs) were constructed along the northern boundary of the Everglades as engineered ecological systems designed to retain P from water flowing into the Everglades. This research investigated data collected over a period from 2002 to 2014 from the interior of the STAs using data mining and analysis techniques including (a) exploratory methods such as Principal Component Analysis to test for patterns and groupings in the data, and (b) modelling approaches to test for predictive relationships between environmental variables. The purpose of this research was to reveal and compare spatial trends and relationships between environmental variables across the various treatment cells, flow-ways, and STAs. Common spatial patterns and their drivers indicated that the flow-ways do not function along simple linear gradients; instead forming zonal patterns of P distribution that may increasingly align with the predominant flow path over time. Findings also indicate that the primary drivers of the spatial distribution of P in many of these systems relate to soil characteristics. The results suggest that coupled cycles may be a key component of these systems; i.e. the movement and transformation of P is coupled to that of nitrogen (N).

Commentary on “Mitsch et al., 2015, Protecting the Florida Everglades wetlands with wetlands: Can stormwater phosphorus be reduced to oligotrophic conditions?”

Constructed wetlands have been utilized with varying levels of success in an effort to reduce nutrient concentrations entering waterways. In south Florida, stormwater treatment areas (STAs) have been utilized to remove excessive phosphorus (P) from surface waters for more than two decades with remarkable results. Even though the STAs have been operational for more than two decade, further improvement of P removal performance is necessary to achieve the desired outflow concentrations. This commentary critically reviews a recently published manuscript concerning the ability of alternative vegetative communities’ ability to improve STA treatment efficiency. The commentary provides clarification of Everglades STA effluent limit and the total P water quality criterion established for the Everglades Protection Area. The commentary also addresses fundamental issues with the authors’ experimental design and interpretability in light of the studies treatment fidelity, controls and scalability.

Spatially Distributed Hydrodynamic Modeling of Phosphorus Transport and Transformation in a Cell-Network Treatment Wetland

AbstractUnderstanding the spatio-temporal distribution of treatment wetland functions is critical for optimizing contaminant removal performance. This study presents the development of a spatially distributed, coupled hydrodynamic-biogeochemical model to simulate phosphorus (P) dynamics in Stormwater Treatment Area 1 West (STA1W), a large cell-network treatment wetland in South Florida. The STAs receive most of their inflow from P-enriched agricultural drainage waters. The model dynamically simulates internal hydrology, and P transport and transformation within treatment cells that are connected by hydraulic structures. The modeled biogeochemical transformations are based on primary mechanisms regulating P behavior in soils and the water column. The model was compared to measured water levels (percent model error, PME=14.4%), discharge (PME=7.3%), chloride and P concentrations in the water column (PME=13% and 18.6%), and soil P at multiple locations of each treatment cell. Then, the model was used to char...

An integrated environmental model for a surface flow constructed wetland: Water quality processes

It is a challenge to apply coupled hydrodynamic, sediment, water quality, submerged aquatic vegetation (SAV), and emergent aquatic vegetation (EAV) models to the studies of constructed wetlands (CWs). So far, there are few published studies on CWs that included comprehensive SAV, EAV and nutrient cycling processes, had detailed model-data comparisons for multiple years, and were applied to support CWs management. Stormwater Treatment Areas (STAs) in south Florida are CWs that have been built to reduce phosphorus (P) concentrations from agricultural drainage waters and Lake Okeechobee discharges. The scale of this CWs project is unprecedented in terms of size, cost, and scientific challenges. The STA management needs models/tools to provide detailed spatial and temporal information to optimize the P removal efficiency and to predict the dynamic response of STAs under a variety of management conditions. Based on the Lake Okeechobee Environment Model (LOEM) developed in the past 15 years, the LOEM-CW water quality model is developed for simulating water quality processes in CWs. The coupled interactions of SAV and EAV with P variables are included in the LOEM-CW to ensure that the P cycling processes are represented realistically. The LOEM-CW is calibrated and verified with 6 years of measured data (2008–2013) at 6 locations in STA-3/4 Cells 3A/3B. Through graphic and statistical comparisons, it is shown that the model simulated P, nitrogen, and dissolved oxygen processes in the STA well. The model results illustrate that increasing water depth increases the retention time and decreases effluent TP concentration. By adjusting the existing outflow rates, the STA can be more efficient in removing P. It has been shown that the LOEM-CW can serve as a useful tool in wetland management.

Persistence of legacy soil P and elevated background water P concentrations in Water Conservation Area 2A, a northern Everglades wetland

AbstractUpstream source control and Stormwater Treatment Areas (STAs) have reduced phosphorus (P) loads to Water Conservation Area 2A (WCA‐2A), a northern Everglades wetland, by three quarters since year 2000. Nevertheless, large storages of P remain in enriched peat soils and it is unclear how legacy stores will impact spatial and temporal scales of recovery. We remeasured soil P enrichment along a well‐studied eutrophication gradient in WCA‐2A and applied a profile modeling approach with uncertainty analysis to assess changes in longitudinal soil P gradients 13 years after load reductions. We then analyzed existing internal water P data, using a novel data screening approach, for evidence of lowest possible water P concentrations independent from inflows. We interpret such water P limits as evidence of the strength of internal loading at a location. Results indicate that soil P enrichment persists in the ∼7.5 km long “impacted” zone, with no significant evidence of net advancement or recession, while a large pool of labile P in the flocculent layer consolidated and diminished. There is indeed evidence, both spatial and temporal, that this extensive zone of enriched soil P continues to elevate lowest achievable water P concentrations. The corresponding gradient of elevated water P limits is both receding and diminishing since load reductions, thus providing further evidence toward recovery. However, results also suggest that these “transitory P limits” due to internal loading are likely to persist for decades above water quality targets. These results advance our understanding of recovery in impacted wetlands and are relevant to Everglades restoration.

An integrated environment model for a constructed wetland – Hydrodynamics and transport processes

Constructed wetlands (CW) have become a popular technology for treating urban and agricultural stormwater runoff. In South Florida, Stormwater Treatment Areas (STAs) have been built to reduce phosphorus (P) concentrations in runoff from agriculture and other sources, including Lake Okeechobee discharges, prior to delivery to the Everglades Protection Area. The scale of this constructed wetland project is unprecedented in terms of size, cost, and scientific challenges. Models/tools are needed to provide detailed spatial and temporal information to optimize the P removal efficiency and to predict the dynamic response of STAs under a variety of management conditions. The Lake Okeechobee Environment Model (LOEM) developed for Lake Okeechobee has been enhanced to simulate hydrodynamics and transport processes in the wetland environment. The flow resistance caused by Submerged Aquatic Vegetation (SAV) and Emergent Aquatic Vegetation (EAV) is included in the LOEM-CW. The LOEM-CW is calibrated and validated with 6 years of measured data (2008–2013) at different locations in STA-3/4 Cells 3A and 3B. Through graphic and statistical comparisons, it is shown that the model simulated stage, flow velocity, water temperature, and total suspended solid (TSS) in the study area reasonably well. The LOEM-CW is poised to serve as a powerful tool in wetland management and STA operation.

Soil and phosphorus accretion rates in sub-tropical wetlands: Everglades Stormwater Treatment Areas as a case example

Wetlands are known to serve as sinks for particulate matter and associated nutrients and contaminants. Consequently rate of soil accretion is critical for continued performance of wetlands to provide ecosystem services including water quality improvement and reduce excess contaminant loads into downstream waters. Here we demonstrate a new technique to determine rate of soil accretion in selected subtropical treatment wetlands located in southern USA. We also report changes in soil accretion rates and subsequent phosphorus (P) removal efficiency with increasing operational history of these treatment wetlands.Utilizing discernible signatures preserved within the soil depth profiles, ‘change points’ (CP) that corresponded to specific events in the life history of a wetland were determined. The CP was observed as an abrupt transition in the physico-chemical properties of soil as a manifestation of prevailing historical conditions (e.g. startup of treatment wetlands in this case). Vertical depth of CP from the soil surface was equivalent to the depth of recently accreted soil (RAS) and used for soil accretion rate calculations. Annual soil and P accretion rates determined using CP technique (CPT) in studied wetlands ranged from 1.0±0.3 to 1.7±0.8cmyr−1 and 1.3±0.6 to 3.3±2gm−2yr−1, respectively. There was no difference in RAS depth between emergent and submerged aquatic vegetation communities found at the study location. Our results showed that soil and P accretion rates leveled off after 10yr of treatment wetlands' operation. On comparison, soil accretion rates and RAS depth determined by CPT were commensurate with that measured by other techniques. CPT can be easily used where a reliable record of wetland establishment date or some significant alteration/perturbation is available. This technique offers a relatively simple alternative to determine vertical accretion rates in free-water surface wetlands.

Diagnosis of the artificial intelligence-based predictions of flow regime in a constructed wetland for stormwater pollution control

Monitoring the velocity field and stage variations in heterogeneous aquatic environments, such as constructed wetlands, is critical for understanding hydrodynamic patterns, nutrient removal capacity, and hydrographic impact on the wetland ecosystem. Obtaining low velocity measurements representative of the entire wetland system may be challenging, expensive, and even infeasible in some cases. Data-driven modeling techniques in the computational intelligence regime may provide fast predictions of the velocity field based on a handful of local measurements. They can be a convenient tool to visualize the general spatial and temporal distribution of flow magnitude and direction with reasonable accurancy in case regular hydraulic models suffer from insufficient baseline information and longer run time. In this paper, a comparison between two types of bio-inspired computational intelligence models including genetic programming (GP) and artificial neural network (ANN) models was implemented to estimate the velocity field within a constructed wetland (i.e., the Stormwater Treatment Area in South Florida) in the Everglades, Florida. Two different ANN-based models, including back propagation algorithm and extreme learning machine, were used. Model calibration and validation were driven by data collected from a local sensor network of Acoustic Doppler Velocimeters (ADVs) and weather stations. In general, the two ANN-based models outperformed the GP model in terms of several indices. Findings may improve the design and operation strategies for similar wetland systems.

The use of discharge perturbations to understand in situ vegetation resistance in wetlands

AbstractThe ability to better quantify resistance to water flow exerted by vegetation is receiving increased attention due to ongoing worldwide efforts to restore natural vegetation communities in the wetlands and use of vegetation for environmental benefits in streams and wetlands. In south Florida, vegetation resistance affects discharge through shallow wetlands of the Everglades and projects under way in the system to restore remaining natural systems. A more detailed knowledge of the flow dynamics in these wetlands is required to improve modeling of these systems that supports restoration and management efforts. The goal of this investigation is to understand the flow dynamics and the vegetation resistance within a 3 km by 7 km area in the Everglades referred to as STA‐3/4 Cell 3A. Methods are developed to demonstrate the use of analytical solutions of partial differential equations (PDEs) and inverse methods to obtain bulk and spatially varying resistance parameters. To achieve this goal, a field test was conducted using sinusoidal discharge disturbances capable of creating water waves in the storm water treatment area (STAs). The discharges, wave speeds, and the wave attenuation rates from the test are used to develop graphical and empirical functions expressing discharge in terms of water depth and energy slope. The empirical functions developed are power law type, and different functions are developed for different depths. The results show that the Manning's equation is not applicable for wetlands with thick emergent vegetation, as well as the difficulty of applying a single power law‐type expression for vegetation resistance over a wide range of depths and energy slopes without errors. This is partly due to the existence of multiple flow regimes and different power exponents over depth and energy slopes in these regimes. Results show that the flow regime at low depths is similar to porous media flow, and the flow regime at higher depths is more turbulent.

Comparison of long-term phosphorus removal performance of two large-scale constructed wetlands in South Florida, U.S.A

The Everglades Stormwater Treatment Areas (STAs) have been constructed as a component of the Everglades restoration in southern Florida, U.S.A. These large-scale treatment wetlands function to remove excess phosphorus (P) from agricultural and urban runoff. The STAs can receive variable inflow hydraulic and P loading in response to storm events or regional droughts and are operated under state and federal permits. An evaluation of the long-term P removal performance for two of the best performing STAs, STA-2 and STA-3/4 is presented along with the challenges in managing these large wetlands.The trends in the inflow loadings and P retention over the period of record for STA treatment flow-ways were examined on an annual and wet/dry seasonal basis. Daily water depths and weekly outflow P concentrations were also examined. The period of record average annual inflow hydraulic loading rates were relatively low (2.0–3.8cmd−1) and the average annual P loading rates were low to moderate (0.6–1.6gm2yr−1) compared to the other STAs, although variable seasonal and annual loadings were observed. Despite receiving variable hydraulic and P loads, these STAs have reduced surface water TP concentrations from 100 and 87μgPL−1 at inflow structures to 23 and 18μgPL−1 at outflow structures in STA-2 and STA-3/4, respectively. The outflow concentrations did not consistently respond to changes in the inflow loading rates, suggesting that the P assimilative capacity of the wetlands may not be exceeded on a long-term basis. The occasional elevated outflow TP concentrations indicate that internal physical processes, such as sediment transport or resuspension, may have occurred or the short-term phosphorus assimilative capacity of the wetland was temporarily exceeded, but it appears as though the long-term mechanisms related to biogeochemical processes were not affected.

Long-term phosphorus removal in the Everglades stormwater treatment areas of South Florida in the United States

Annual phosphorus (P) removal in the six Everglades stormwater treatment areas (STAs) was examined during the period of record water years 1995–2011. The STAs have received approximately 13.6billionm3 of stormwater inflow containing an annual total P (TP) concentration (flow-weighted mean) of 143±62μgPL−1, with annual P loading rate (PLR) of 1.56±0.91gPm−2year−1 and annual hydraulic loading rate (HLR) of 3.1±1.36cmday−1, and retained 1500 metric tons of TP. The STAs produced discharges with an annual TP concentration of 41±31μgPL−1 and have annual load-based, TP retention of 76±11%. Annual areal removal rate was 1.1±0.5gPm−2year−1, and annual k value (settling rate for TP removal) was 16.1±7.4myear−1. Regression analysis indicates that outflow TP concentrations increased with increasing inflow TP concentrations (R2=0.705, p<0.05, n=59) and PLRs (R2=0.435, p<0.05, n=59). The k value increased with increasing HLRs (R2=0.663, p<0.05, n=59) and PLR (R2=0.289, p<0.05, n=59), and decreased with increasing inflow TP concentration (R2=0.105, p<0.05, n=59). STA performance, in terms of outflow TP concentration and/or k value, depended on three primary variables: HLR, inflow TP concentration, and PLR. To improve and sustain P removal in the STAs, it is operationally necessary to maintain these important variables at optimal ranges. Additional data on internal P removal mechanisms, such as P flux and microorganism activities, along the flow-ways should be collected to further understand the controlling variables and processes in the STAs. Challenges for STA management and operation were also discussed.

Protecting the Florida Everglades wetlands with wetlands: Can stormwater phosphorus be reduced to oligotrophic conditions?

The Florida Everglades is being threatened by high-nutrient stormwater coming from agricultural runoff. The main nutrient problem is phosphorus, which causes the highly oligotrophic sawgrass (Cladium jamaicense) communities in the northern Everglades to become eutrophic Typha latifolia/T. domingensis communities. Current government directives require that the total phosphorus concentration of storm water drainage into the Everglades be limited to approximately 10ppb (μg-P/L). Over 23,000ha treatment wetlands, referred to locally as stormwater treatment areas (STAs), have been created from farmland to treat the stormwater. They are generally effective in removing 60–80% of the total phosphorus; however, the 10ppb goal has rarely been achieved. A three-year experiment, involving mesocosms planted with Everglades-native wetland plants was conducted in the Florida Everglades from March 2010 to March 2013. Eighteen flow-through mesocosms (6m×1m×1m with 40-cm water depth) received about 2.6cm/day inflow. The eighteen mesocosms were randomly assigned with six different plant communities with three replicates of each treatment, consisting of sawgrass (C. jamaicense); waterlily (Nymphaea odorata); cattail (Typha domingensis); submerged aquatic vegetation (SAV) including Najas guadalupensis, and Chara sp. and a Nymphaea–Eleocharis sp. mixed community; and soil without vegetation as a control. Total phosphorus (TP) in the inflow water was 25±1μg-P/L (n=55) over the 3 years. Through 2012 the average outflow of all of the treatments was 34±1μg-P/L, a 51% decrease from the average outflow of 69±6μg-P/L for 2011. Outflows began to be routinely lower than the inflow in the 3rd year of the study. The average total phosphorus concentration decreased overall to 19±1 (n=5) at the end of the study in 2013 suggesting that the suspected phosphorus reflux from the mesocosm soils into the water column slowed after 2–2.5 years of mesocosm operation. Comparing outflows of the individual treatments for 2013, the Nymphaea, control/Chara, and Typha treatments were lower (p<0.05) than the inflow with average outflow concentration of 11±1, 15±3 and 16±1μg-P/L respectively. When the 2013 data are isolated, 4 of the 6 vegetation treatments showed total phosphorus removal, ranging from mixed community (17% removal), to Typha (28% removal), to the control (34% removal) and to Nymphaea (51% removal). We conclude that any treatment wetland constructed with local Florida soils and designed to achieve low (∼10–15ppb P) concentrations would probably take a minimum of 2 years to become sinks of phosphorus. We also conclude that wetlands can be created to achieve these low thresholds if low TP loading and self-design strategies are incorporated into the project design.

Modeling sulfate transport and distribution and methylmercury production associated with Aquifer Storage and Recovery implementation in the Everglades Protection Area

Abstract Aquifer Storage and Recovery (ASR) technology has been proposed to meet the competing ecological and water-supply needs of south Florida and the Everglades Protection Area (EPA). The water recovered from ASR, however, may have altered water quality. Of particular concern is the enrichment of ASR recovery water in sulfate, which can stimulate microbial sulfate reduction and methylmercury (MeHg) production within the EPA. MeHg is already a serious issue with regard to wildlife and human health, and there is concern that ASR might exacerbate the problem. In order to address these concerns, the Lake Okeechobee Environmental Model (LOEM) and the Everglades Landscape Model (ELM) were adapted with sulfur modules to predict concentrations and distributions of sulfate within Lake Okeechobee and the EPA resulting from the release of ASR water. In addition, equations were developed relating the biogeochemistry of sulfate and MeHg production to produce a MeHg production risk assessment from the modeled ASR sulfate loading. Baseline runs (no ASR discharge water), and three different ASR release scenarios with varying sulfate loading were evaluated. Results show that ASR release will temporarily elevate sulfate concentrations in Lake Okeechobee from the present level of about 30 mg/L to as high as 50 mg/L in a worst case scenario, but that this will have little impact on MeHg production in the lake. The model indicates that ASR release will have minimal impacts on sulfate loading to the EPA, primarily because of the already large sulfate loading from other sources within the Everglades Agricultural Area (EAA). Maps of sulfate distributions show that certain locations in the EPA, especially those near canal or Stormwater Treatment Area (STA) discharges, may experience significantly higher sulfate loading from ASR. Overall impacts with regard to increased MeHg production risk are predicted to be low based on this model, although sites with increased ASR sulfate loading (e.g. canal and STA discharge sites) may experience some change in MeHg risk.

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