Modeling the Effects of Sulfate Loading and Methylmercury Production in the Everglades
William Orem1, David Krabbenhoft2, Cindy Gilmour3, George Aiken4, Mark Schafer5, and G. Ronnie Best6 1U.S. Geological Survey, Reston, VA 2U.S. Geological Survey, Middleton, WI 3Smithsonian Environmental Research Center, Edgewater, MD 4U.S. Geological Survey, Boulder, CO 5U.S. Army Corps of Engineers, Jacksonville, FL 6U.S. Geological Survey, Ft. Lauderdale, FL Acknowledgments Funding: ●USGS Priority Ecosystems Studies Program for South Florida (G. Ronnie Best, Coordinator) ●US Army Corps of Engineers ●Florida Department of Environmental Protecti (Donald Axelrad) Sulfur Loading to the Everglades
●Excess sulfate has been observed in the freshwater Everglades by USGS, EPA, SFWMD, and others
●Background levels of sulfate in the ecosystem are estimated as < 1 mg/L and some areas far removed from anthropogenic sources have sulfate levels as low as 0.05 mg/L
●60% of the freshwater Everglades has sulfate concentrations exceeding background; average sulfate concentrations at marsh sites adjacent to canal or STA discharges are >60 mg/L
●There is a general decrease in sulfate from north to south across the Everglades; highest concentrations occur in canals within the Everglades Agricultural Area (EAA)
●Changes in the movement of canal water accompanying restoration is changing where sulfate loading to the ecosystem occurs – however, levels of sulfate in canals has not changed appreciably in the past 15 years
●Distributions of sulfide and organic S in soil parallel those for sulfate Lake West Sulfate Distributions Okeechobee Palm In Surface Water Beach > 50 mg/L EAA WCA 1 ~10-50 mg/L WCA 2 WCA ~1-10 mg/L 3 ? Big <1.0 mg/L Cypress Miami National Preserve Sulfate moves from the EAA and Lake Okeechobee down canals and is discharged into Gulf the Everglades through water of control structures and Mexico Everglades breaches in levees National Park Impacts of Sulfur Loading Why is This Important to Everglades Restoration
●Sulfate stimulates microbial sulfate reduction; fundamentally alters microbial community structure in impacted areas
●Sulfate promotes methylation of mercury to its most toxic and bioaccumulative form: methylmercury
●Sulfide is toxic to plants and animals
● Sulfate promotes release of nutrients from sediments (internal eutrophication)
● Sulfide binds metal ions and sequesters them in soils as metal sulfides
● Sulfate enhances biodegradation of organic soils Agricultural Fields and Canals Linking Sulfate and Methylmercury Everglades Agricultural Area in the Florida Everglades
Hg2+ mercury from sulfate in runoff distant sources from agricultural fields rainfall
Hg2+ sulfate Everglades SURFACE from canals Marsh WATER SOIL sulfate-reducing bacteria sulfate Hg2+ bioaccumulation of methylmercury sulfide methylmercury Sulfate/Sulfide and MeHg – Mesocosm Studies
Data from: Gilmour, Krabbenhoft, Orem, Aiken Day 57 0.6 0.5 0.4 0.3 y = 0.023x + 0.0572 2 0.2 R = 0.5854
0.1 Me202Hg,ng/gdw 0.0 0 5 10 15 20 SO4, mg/L
-Add sulfate to Everglades soil and MeHg production increases; confirmed at 5 different fieldsites and lab experiments
-Linear relationship between sulfate and MeHg production through 20 mg/L
-Sulfide inhibits methylation at 10-100 µM (0.3-3 mg/L); Field distributions, experimental studies in cores and in cultures are all consistent
Trends in Sulfur and MeHg Across the Ecosystem -Highest sulfate/sulfide in north near source (agricultural runoff)
-MeHg peaks in middle “Goldilocks Area” where sulfate and sulfide levels are just right (sulfate stimulation vs. sulfide inhibition)
1000 0.05 Sulfate
M methylation rate
Sulfide 0.04 100
0.03 -1
10 , d
0.02 meth
K
1 0.01
surface water sulfatesurface water
or pore water sulfide,or pore water
0.1 0.00 North ENR F1 U3 2BS 3A15 TS7 TS9 WCA1South
Research Needs Because of the level of sulfate contamination and its myriad impacts (especially methylmercury production and bioaccumulation) there is a need for land and water managers to understand how changes in water management accompanying restoration will impact sulfate loading to the ecosystem and resulting impacts.
Research needs include:
●Accurate mass balance models of sulfate sources, including agricultural, soil oxidation, atmospheric, ASR, groundwater, and other sources. ●Determine if sulfate is the major form of S entering the ecosystem, or if dissolved organic S and particulate S are also important
●Modeling of sulfate as it moves along canals or in sheet flow across the wetland landscape is needed to understand sulfate loading at various locations; especially as more sulfate-contaminated water is moved to sensitive areas like Everglades National Park
●Modeling of the impacts of various levels of sulfate loading on methylmercury production taking into account variations from location to location Sulfur Mass Balance of EAA Corrales J., Naja G.M., Dziuba C., Rivero R.G., and Orem W., 2011, Sulfate threshold target to control methylmercury levels in wetland ecosystems. Sci. Tot. Environ. 409: 2156-2162.
TS in from TS in from TS out by atmospheric agriculture harvest deposition applications 4,229 (4%) 11,775 (10%) 23,182 (17%)
TS in from 4% 10% 17% Lake Okeechobee 35,217 (31%) 31% TS out to EAA WCAs TS in from 83% levees 116,360 (83%) 5% 43% water 5,858 (5%) TS in from soil 8% oxidation
49,169 (43%) sediments TS in from groundwater 8,973 (8%) bedrock Another Variable – Sulfate Loading is Changing at Different Locations as Restoration Proceeds
WCA 2B WCA 2A – F1
WCA 3A-15 WCA 2A – U3 What Happens to MeHg Production in the Everglades if Sulfate Contamination is Reduced??
6 1 Sulfate, MeHg, and HgT in Surface Water 0.9 WCA 3A Site 15 Sulfate 5 0.8 HgT 0.7 4 MeHg Changes in 0.6 sulfate, MeHg, 3 0.5 and HgT at a
0.4 site in the MeHg (ng/L)MeHg 2 0.3 central
Sulfate (mg/L),Sulfate HgT (ng/L) Everglades 0.2 1 (WCA 3A-15) 0.1 from 1995 to 0 0 2003
08/18/03 11/17/03 3/15/1995 7-Jun-964-Dec-9611-Jul-97 19-Jul-99 10-Jul-00 1-Aug-01 4-Dec-019-Jan-026-Feb-026/10/20039/15/2003 13-Dec-9527-Mar-96 21-Apr-9714-Jan-9829-Jun-9810-May-0025-Sep-0029-Nov-01 12/15/2003 Date of Sample -Former MeHg hotspot showed dramatic decline in MeHg over time -Decline not correlated to declines in Hg deposition at this site -Decline closely linked to decline in sulfate at site -Illustrates fast response of ecosystem to declines in sulfate w/r to MeHg production and biopaccumulation What Happens When Sulfate Levels Increase in A Region of the Everglades?
Everglades National Park Sulfate Loading to Northern ENP Correlates with Increased Methylmercury Production
ENP west Sulfate is Removed and Sequestered By the Marsh But Much More Slowly Than Phosphorus Rates of Sulfate Removal by Overland Sheet Flow mg L-1 km-1
-Sulfate reduction and sequestration -dilution
Northern Everglades
Everglades National Park Theoretical Changes in Sulfate with Reductions in 60 mg/L EAA Canal Source Loading and Sheet Flow BMPs, Other Mitigation
50 mg/L STA
48 mg/L
35 km Marsh WCA 3A
<1 to 3 mg/L
Everglades National Park Conclusions ●Sulfate contamination is a key water quality issue facing Everglades restoration
●Sulfate is one of the principal drivers of methylmercury production within the ecosystem, but the biogeochemistry of the sulfur/methylmercury linkage is complex
●Both the loading and distribution of sulfate are important in determining where methylmercury production is highest
●Accurate sulfur mass balance models are needed to determine sulfur sources and design mitigation strategies
●Models addressing where sulfate loading occurs, and how sulfate- laden canal water moves across the marshes are needed to examine where methylmercury production will occur, and protect sensitive areas
●Slow sheet flow over extensive areas of marsh can effectively remove sulfate and sequester it in organic soils