Mercury cycling and transformations across Louisiana estuarine and wetland gradients

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1 Mercury cycling and transformations across Louisiana estuarine and wetland gradients By Mark Marvin-DiPasquale 1 Lisamarie Windham-Myers 1 David P. Krabbenhoft 2 George Aiken 3 Chris Swarzenski 4 Britt Hall 5 1 USGS, Menlo Park, CA 2 USGS, Middleton, WI 3 USGS, Boulder, CO 4 USGS, Baton Rouge, LA 5 Univ. of Regina, Canada

2 What We ve Done To Date Study Focus Areas: Hg cycling and geochemistry in sediment Hg speciation & Hg- DOC interactions (sed & water) Hg-plant interactions (emergent marsh) 5 Field Trips ( ) Estuarine salinity Gradient: upper rivers Lake Pontchartrain salt marsh sloughs Wetland Gradient: cyprass swamp spartina saltmarsh Wetland sub-habitats: vegetated marsh, sloughs, ponds and mudflats

3 Lake Rivers Wetlands Bayou Lacombe Bayou Jean Lacombe Lake Laffitte Felicity Blind Skull (or River Junop?) Cr. Swamp Blind R. Rigolets Caernarvon Rigolets Robert s Marsh Caernarvon (Skull Creek) Lake Junop Lake Felicity Jean Laffitte

4 Sediment Biogeochemistry Microbial Processes Hg-methylation MeHg-degradation Sulfate reduction Methanogenesis Mercury Speciation pw THg & MeHg sed. THg & MeHg sed. REACTIVE Hg(II) Iron Chemistry sed. & pw Fe(II) sed. amorphous Fe(III) sed. crystalline Fe(III) Carbon Chemistry pw DOC pw acetate sed. organic content (LOI) sed. C/N sed. lignin content Sulfur Chemistry pw SO 4 2-, HS - sed. AVS (FeS) sed. CRS (S 0, FeS 2 ) Other Chemistry sed. ph sed. Redox pw Cl -

5 Water Chemistry & Plant-Hg Interactions Water Chemistry Particulate THg & MeHg Dissolved THg & MeHg DOC SUVA (aromaticity) DOC isolates (XAD) HPOA, HPON TPIA, TPIN LMW HPI ph major cations & anions redox metals (Fe, Mn) Wetland Plant Metrics Above and below ground plant biomass Root: density & depth Leaf: % lignin Leaf and root: THg & MeHg %C & %N 13 C & 15 N isotopes K:Na ratio (stress index)

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7 Source-Receptor Relations Industry Category Chlorine Production Gold Mining Hazardous Waste Incinerators Medical Waste Incinerators Utility Coal Combustion Municipal Waste Combustion SE Louisiana is in the hot zone for THg deposition

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10 Conceptual Model Mercury Cycling at the Marine-Continental Margin Mercury emission sources RGM sorbs to sea salt aerosols Ocean breeze returns With elevated RGM HgCl 2 Continental winds & mercury sources Cl 2, Br 2 O 3 + Hg 0 Volatilization of free halogens Land Ocean

11 What Controls MeHg Production? MeHg Production = k meth x Hg(II) bio Hg(II)-Methylating Bacteria Activity (k meth ) High Low Moderate MP Low MP (e.g. oxic water column) High MP (e.g. reflooded wetland soils) Moderate MP Low High Hg(II) bio Concentration 203 Hg(II) Incubations: method for assessing (k meth ) Reactive mercury (Hg(II) R ): surrogate for Hg(II) bio

12 Sediment: All Data ( ) Habitat Type and Salinity Range [Cl - ] pw (mmol / L) pore water chloride lake-river wetlands [SO 4 2- ]pw (µmol / L) pore water sulfate lake-river wetlands pore water salinity (ppt) pore water salinity (ppt) pore water salinity = [Cl - ] + [SO 4 2- ] (mg/l = ppt) [Sulfate]: drives microbial sulfate reduction (SR) SR: a key microbial process in MeHg formation

13 Sediment: All Data ( ) Habitat Type and Salinity Range SR (nmol / g dry sed / d) Microbial sulfate reduction lake-river wetlands TRS (µmol/g sed) dry wt Sediment: Total reduced sulfur pore water salinity (ppt) pore water salinity (ppt) SR an TRS are generally higher in wetland sites both increase from fresh (< 0.5 ppt) to low brackish (0.5-5 ppt)

14 Sediment: All Data ( ) Habitat Type and Salinity Range Sediment: Total mercury lake-river wetlands 80 Sediment: Grain size lake-river wetlands THg (ng/ g) dry wt < 63 µm (%) pore water salinity (ppt) pore water salinity (ppt) THg: lowest in upper freshwater rivers, highest in depositional areas tracts sediment grain size distribution

15 Sediment: All Data ( ) Habitat Type and Salinity Range lake-river, < 0.5 ppt lake-river, ppt Hg(II)-methylation rate const. lake-river wetlands 10-1 wetlands, < 0.5 ppt wetlands, ppt wetlands, 5-17 ppt k meth (1/d) k meth (1/d) pore water salinity (ppt) Microbial SR (nmol/g/d) dry wt. Microbial Hg(II)-methylation activity (k meth ) highest in wetlands Increases with microbial sulfate reduction

16 Sediment: All Data ( ) Habitat Type and Salinity Range Hg(II) R (ng/g) dry wt Sediment: reactive Hg(II) % Hg(II) R lake-river, < 0.5 ppt lake-river, ppt wetlands, < 0.5 ppt wetlands, ppt wetlands, 5-17 ppt 10-2 pore water salinity (ppt) Sediment TRS (µmol / g) dry wt. Microbially available reactive Hg(II): highest in wetlands %Hg(II) R decreases with with increasing TRS concentration

17 MPP = k meth x Hg(II) R : Predicts sediment MeHg concentration 10 sediment MeHg (ng/g) dry wt lake-river, < 0.5 ppt lake-river, ppt wetlands, < 0.5 ppt wetlands, ppt wetlands, 5-17 ppt Sediment MPP (pg/g/d) Much better than k meth or Hg(II) R individually (not shown)

18 Surface Water: THg and MeHg Sept data MeHg (ng/l) Bayou Lacombe Caernarvon JL - Treasure Island JL - Mamalo Blind River Total Hg (ng/l)

19 Sediment Pore water: THg and MeHg Sept data Bayou Lacombe Caernarvon Treasure Island Jean Laffitte MeHg (ng/l) Total Hg (ng/l)

20 S.E. Louisiana: August 03 Overlying Water DOC is very Aromatic Stuff! 5 Lake Upper R. Lower R. Wetland Everglades eutrophic Suwannee R 4 SUVA Everglades Snake R. Rio Grande 3 2 Williams Lk Lake Fryxell 1 0 SUVA = measure of DOC aromaticity

21 Surface Water: THg and MeHg data Hall et al. 2008, Environ. Pollut., vol. 154, p MeHg is NOT correlated with DOC, but with HPOA (aromatic DOC) THg is correlated with DOC

22 Wetlands: vegetated sites have higher sediment MeHg concentrations Sediment MeHg (ng/g) dry wt non-vegetated vegetated Sediment Total Hg (ng/g) dry wt. Sept data only

23 Root Biomass correlated with sediment [MeHg] Sediment MeHg (ng g -1 ) dry wt. Jean Lafitte /Treasure Island 8 Bayou Lacombe 7 Caernarvon R = Root Biomass (g m -2 in top 1cm) Sept data only

24 K:Na Ratio: Plant Stress Indicator Leaf THg (ng g-1) R 2 = 0.65 BL CA JL Sediment MeHg (ng / g) dry wt R 2 = 0.91 R 2 = 0.40 BL CA JL Root K:Na Ratio Root K:Na Molar Ratio Low ratio = stressed conditions Potential (cheap) indicator of wetland MeHg production hot spots?

25 Take Home Message The Gulf coast region is a hot spot for Hg deposition and elevated Hg blood levels in humans May be linked to diet and/or regional enhancement of MeHg in the local food chain MeHg production is a function of BOTH microbial activity (k meth ) and Hg(II) availability (Hg(II) R ) k meth increases with microbial SR (highest in saltmarshes) Hg(II) R highest in freshwater wetlands; decreases with TRS Habitat type is a key factor controlling MeHg production, with wetlands as hot spots Surface water MeHg transport is associated with aromatic DOC Emergent wetland plant stress index appears associated with regional MeHg production potential

26 Future Directions / Collaborative Opportunities More detailed synoptic freshwater marine transects Explore further the use of wetland plant type, stress indicators and seasonal life cycles (e.g. senescence) as a predictor of MeHg hot spots extrapolate with GIS mapping Add biota components to current Hg investigations Examine fundamental relationships between Hg and C cycling (large geographic relevance) Further explore coastal regions as zone of enhanced reactive Hg(II) deposition Managed vs non-managed wetlands