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projects > linking land, air and water management in the southern everglades and coastal zone to water quality and ecosystem restoration: task 1, mercury cycling, fate and bioaccumulation
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Project Work Plan
Department of Interior USGS GE PES
Fiscal Year 2008 Study Work Plan
Study Title: Linking Land, Air and Water Management in the Southern Everglades and Coastal Zone to Water Quality and Ecosystem Restoration: Task 1, Mercury Cycling, Fate and Bioaccumulation
Overview & Objective(s): Water quality remains one of the biggest issues facing restoration of the Everglades. However, a complete understanding of all the factors (external and internal) the regulate past and present water quality in the Everglades, and even more challenging to anticipate future water quality conditions that will occur in response to the restoration effort, is a significant challenge to both scientists and resource managers. Water quality studies in the Everglades over the past 10-20 years have largely focused on the phosphorus contamination and its ecological impacts. Concerns over phosphorus contamination have resulted in one of the most expensive aspects of the Everglades Restoration program, the establishment of over 45,000 acres of storm water treatment area (STAs). Because so much attention has been focused on phosphorus in south Florida, however, in some ways water quality and phosphorus have become synonymous, and thus a complete understanding of all the potential linkages between various pollutant sources, physical and hydrologic changes resulting from the restoration, and water quality is not being realized. While we recognize that excessive phosphorus loading has deleterious effects on the Everglades, other contaminants also warrant attention so that the overall restoration goal of "...to improve the quantity, quality, timing, and distribution of clean fresh water needed to restore the South Florida Ecosystem" can be achieved. The USGS has been assessing two other important contaminants, mercury (Hg) and sulfate, which are present at concentrations sufficient to pose a threat in larger portions of the ecosystem than phosphorus. Mercury, an atmospherically transported contaminant, affects the entire ecosystem. Sulfate, on the other hand, is dominantly derived from the same contamination source as phosphorus (runoff from agricultural uses), but affects a larger fraction of the Everglades presently (about 30 to 60 percent) due to its greater mobility in the environment and because the STAs in their current configuration due little or nothing to abate sulfate transport to the downstream Everglades. Sulfate alone can have profound impacts on natural redox conditions in wetlands, which can stress and/or kill native vegetation. In addition, the co-contamination of the environment with Hg and sulfate has an extremely important synergistic effect on the toxicity of Hg through the conversion of inorganic Hg (form atmospheric deposition) to methylmercury (MeHg), the most toxic and bioaccumulative form of Hg. Methylmercury comprises >95% of all the Hg in predator-level species and thus is responsible for essentially all of the toxicological concern for this widespread contaminant. Wildlife toxicologists are only now determining many of the important ways that MeHg may be affecting fish and wildlife, including the new observations on White Ibis from the Everglades that population level effects may be occurring through toxicity to the unborn, or through substantial hormone disruption (Dr. Peter Frederick, U. of Florida).
Mercury methylation in the environment is dominantly the result of respiration of sulfate by reducing bacteria (SRB). These organisms utilize sulfate and organic carbon for their normal function. However, if inorganic Hg is present, MeHg can be produced as an accidental byproduct. Thus any actions that increase sulfate reduction (such as sulfate loading) or increase Hg availability (such as increases in Hg deposition) may serve to exacerbate Hg toxicity on ecosystems by yielding more MeHg. In addition, seemingly unrelated activities like alterations to hydrologic cycles (wetting and drying periods), flushing rates, other water quality constituents (especially dissolved organic carbon [DOC], iron and pH), can have pronounced effects on Hg methylation. For example, oxidation of soils leads to conversion of organic sulfur to sulfate, and thus subsequent stimulation of sulfate reduction and methylation upon re-inundation. In addition, substantial amounts of DOC are also derived from EAA runoff and shows about the same aerial extent as sulfate. We know DOC plays an important role in facilitating Hg availability to methylating microbes, but other important processes that may be affected by DOC increases include light penetration limitation, nutrient uptake, and cycling of other exogenous metals. Lastly, another byproduct of sulfate reduction, sulfide, is deleterious to many freshwater wetland plants and infauna that are indigenous to the Everglades through oxygen deprivation and suffocation, limiting nutrient uptake, and or direct toxicity.
To this point, the USGS studies on Hg and sulfate contamination in south Florida have largely focused on the water conservation areas (WCAs). Our studies have served as a template worldwide on how to conduct studies of Hg in the environment and why wetland-rich ecosystems are areas of heightened concern for MeHg exposure. In addition, our study demonstrated for the first time, the important linkages that exist between an air derived contaminant (Hg) and another from land-based sources (sulfate). During our period of study, we have documented a substantial (>90%) decline of MeHg concentrations in water, sediment and mosquito fish at our study site in central WCA3A-15, but that our other sites in WCA2A, WCA2B, and WCA1 showed no apparent change. Our data clearly show that the MeHg declines in WCA3A are directly related to declines in sulfate concentration at this site. Mesocosm dosing tests at this site confirm that MeHg abundance is strongly controlled by sulfate additions, without any additional Hg added. This observation poses the question whether there have been large declines in sulfur uses in the EAA, or changes in water routing internal to the Everglades. Since sulfate levels over time at our northern canal and WCA2 sites show similar or modestly lower levels of sulfate, we hypothesize that changes to water routing within the Everglades are responsible for the dramatically reduced sulfate levels at WCA3A that in turn lead to near-detection level concentrations of MeHg. If this is true, then we might expect that the sulfate-rich waters that previously flowed through our study site are now discharging elsewhere, likely south to Everglades National Park (ENP) or west to Big Cypress National Preserve (BCNP), where increased water delivery is a priority for the Restoration program. Indeed, evidence for steadily increasing fish Hg concentrations in the ENP over the past 5 years is available for at least one monitoring site, North Prong Creek (Ted Lange, Florida Fish and Wildlife Conservation Commission). However, since our research has focused primarily north of the ENP, we do not have contemporaneous water quality data to support or refute the conclusion that the increasing fish Hg levels are due to increasing sulfate loads due to increasing water delivery from canals to the Shark River Sough. We hypothesize that the delivery of larger volumes of water to ENP will result in a greater load of sulfate, and increases in MeHg production and bioaccumulation. The overall objective of this next phase of our research is to extend our understanding of the interactions of Hg, sulfate, DOC contamination to areas of the Everglades that are anticipated to receive increasing water delivery from sulfate rich EAA runoff or ASR waters, including: ENP, BCNP, and Loxahatchee National Wildlife Refuge (LNWR).
Specific Relevance to Major Unanswered Questions and Information Needs Identified: This study supports several of the projects and overall goals listed in the DOI science plan. The DOI science plan lists three overarching restoration questions (page 9) that this study has direct relevance and provides information toward answering, including: (1) What actions will improve the quantity, timing, and distribution of clean fresh water needed to restore the South Florida ecosystem? (2) What actions will restore, protect, and manage natural resources on DOI lands in South Florida? (3) What actions will recover South Florida's threatened and endangered species? Aquifer Storage and Recovery (ASR) has substantial potential to affect water quality everywhere recovered water is released to the south Florida ecosystem, and is an area of concern in the DOI Science Plan (page 27). This study has demonstrated links between water quality characteristics of waters to be injected (sulfate, DOC, DO, and pH), the water quality characteristics of water recovered, and the water quality characteristics of water within the receiving surface and ground waters. In addition, the Comprehensive Integrated Water Quality Feasibility Study ( CIWQFS; page 84) identifies degraded water bodies, types and sources of waterborne pollution, establishing load reduction targets for pollutants, and the need to improve water quality. Findings from this study will assist the DOI in providing needed information to multiagency CIWQFS Project Delivery Team in identifying the linkages between water quality targets and ecosystem restoration. The need to understand the sources, cycling and fate of critical chemical constituents like mercury, and to quantify the types and sources of pollution is stated on page 85. Linked to cycling and fate, the Science Plan cites the need for water quality performance targets (page 85) that can be used to evaluate the progress of restoration, and to identify areas in need of adaptive management. This project has shown clear linkages between water quality, land management (siting and operation of STAs; pange 86), and restoration plans, which will be critical for evaluating the overall success of the Restoration effort. Finally, the Science Plan specifically identifies the need to predict the effects of hydroperiod alterations and soil and water chemistry on the bioavailability of mercury to methylation (Page 89). This project not only discovered these hydro-cycle mercury-methylation linkages, but continues to unravel its complexities. The intent of these studies is to help land managers to make decisions that reduce the effects of hydroperiod alterations on mercury methylation.
Status and Plans: Mercury research in the Everglades was originally initiated under Phase I and Phase II of the Aquatic Cycling of Mercury in the Everglades (ACME) project, which has concluded. This project focused on deriving a complete understanding of the ecosystem scale factors regulating MeHg generation in the Everglades. The second project "Integrated Biogeochemical Studies in the Everglades", which focused on the execution of in-field experimentation (dosing experiments) to confirm the inferred effects of Hg , carbon and sulfate on MeHg production ad bioaccumulation. Finally, we have proposed to extend our findings to relatively less well studied portions of the Everglades, especially those that may receive greater amounts of canal water discharge as a result of the restoration efforts (e.g., ENP, BCNP, and LNWR). This three year project will seek to extend our knowledge of the controlling factors of mercury toxicity in the Everglades, with specific attention to geographical areas and land use and changes related to the restoration that may affect methylmercury production and bioaccumulation. Because work under ACME was largely conducted in the WCA's, we propose to direct our current and future efforts on the federally managed lands. Three specific areas of work will be conducted in FY07: (1) sampling surveys on the federally managed lands, particularly those that are receiving canal water discharge; (2) sampling in the newly operating periphyton/limerock STAs and previously existing cattail/peat STAs; (3) sampling of the McCormick canal water dosing mesocosms in LNWR; (4) participation on the planning and execution of the Water Quality of the Greater Everglades: Fate and Transport of Nutrients and Other Contaminants symposium at the Spring 2008 GEER conference; and, (5) continued efforts toward publication of past and current results.
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 04 September, 2013 @ 02:09 PM(KP)
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