DRIVERS OF SPATIAL AND TEMPORAL VARIABILITY OF MERCURY AND METHYLMERCURY ACROSS THE EVERGLADES NATIONAL PARK (USA)
Elevated mercury (Hg) levels in the aquatic food web of the Everglades has been a public concern since the late 1980s, and remains a persistent issue confronting ecosystem restoration efforts. The formation of methylmercury (MeHg) in the system is driven by the bioavailability of inorganic Hg(II) and the metabolic activity of sulfate-reducing bacteria. These in turn are influenced by the abundance and character of dissolved organic carbon (DOC) and sulfate concentrations. Here, we report on an eight-year collaboration between the U.S. Geological Survey and the National Park Service to examine the drivers of spatial and temporal variability of Hg and MeHg within the Everglades National Park. Surface water and forage fish (Gambusia Holbrooki and Jewel Cichlid) were collected annually at 76 sites from 2008 to 2016. Water samples were analyzed for total mercury (HgT), MeHg, DOC, specific UV absorbance at 254 nm (SUVA), and major ions. Fish were analyzed for HgT, MeHg, and stable isotopes of carbon, and nitrogen. In addition, archived fish samples are currently being analyzed for stable isotopes of mercury to provide greater insight into trophic transfer.
Methylmercury concentrations in water and fish exhibited distinct regional patterns with peak levels in the Shark River Slough (SRS) a receiving area for sulfate and DOC-rich canal water. Throughout the SRS, HgT was a strong predictor of MeHg concentrations in filtered water (R-squared = 0.70), whereas in marsh sites unaffected by the canal this result does not hold true (R-squared = 0.15). Regression analysis using a gradient boosting algorithm revealed several additional drivers of MeHg production which included: dry days preceding sampling, sulfate concentration, and SUVA levels (all of which were greatly enhanced in the marsh relative to the SRS). Annual hydrologic conditions were an important driver of mercury levels in water each year. High water during an El Nio event (2009-10) corresponded to a decrease in HgT and MeHg concentrations, while low water levels during a strong La Nia event (2011-12) corresponded to a dramatic increase in concentrations due to drying and rewetting cycles.
Overall, trends of Hg in fish generally followed those of MeHg in filtered surface water with the exception of elevated Hg in fish from the marsh in 2010 (likely due to fish escaping the SRS in high water). In addition, the prevalence of exotic Jewel Fish in the Everglades has greatly expanded over the course of this study; recent data (2010-2013) show that Jewel Fish mercury concentrations (measured as HgT) are 15-20% higher than mercury concentrations in native Gambusia. Mercury sources and diet differences between Jewel Fish and Gambusia will be assessed using stable isotope techniques (Hg, C, N) to better understand their mercury uptake dynamics.
TRACKING SOURCES OF MERCURY IN THE IDRIJA MINING AREA – THE RELATIONSHIP BETWEEN MERCURY ISOTOPES AND SPECIES IN SOILS, SUSPENDED MATTER AND SEDIMENTS
Mercury (Hg) released by mining activities can be highly dispersed in the environment and may undergo species transformations processes, which are essential to evaluate environmental risk. Hg from roaster emissions or mining-residues is accumulated in soils and sediments. In aquatic systems Hg can be transported over great distances as dissolved organic matter complexes or bound to suspended matter (SM). Hg-isotope analysis have been used to track Hg-sources in Hg-mining areas although it is mostly unclear to which extent changes in Hg-isotope ratios are attributed to different sources or Hg-species transformations. In Idrija-Slovenia, Hg was mined as Hg-sulphide (cinnabar-ore, α-HgS) for centuries. Dumping of mining-residues caused accumulation of cinnabar in river sediments, whereas atmospheric Hg deposition led to contamination of soils and enrichment of organically-bound Hg, which, in contrast to cinnabar, can potentially be methylated in aquatic systems. Hg released from soils and mine dumps is mainly transported via SM in Idrijca river to the Gulf of Trieste (GT) in Italy. So far, high δ202Hg values in the marine environment were related to cinnabar from the Idrija mine, and different values from that of cinnabar where assigned to the Adriatic background, but organically-bound Hg derived from soils have been neglected. Here, we determine Hg-species and Hg-isotopes in SM, soils and sediments from the Idrijca-catchment to decipher variability of Hg-isotope ratios related to distribution of Hg-species. Hg-isotopes were determined by means of MC-ICP-MS while Hg-species were analyzed using Hg-thermo-desorption.
In general, δ202Hg values in SM correspond to those found in soils ranging from -2.81 to 0.73‰ and from -2.45 to 0.15‰, respectively. Speciation measurements in different grain sizes reveal that smaller grain sizes (0.45-20 μm) are dominated by organically-bound Hg, while larger grain sizes mainly contain cinnabar (>20 μm). Samples dominated by cinnabar show relatively higher δ202Hg values (~50% cinnabar, δ202Hg > -1.01‰) while organically-bound Hg shows lower δ202Hg values.
Hg-isotopic ratios combined with speciation analyses reveal that most Hg in SM transported to GT is soil derived organically-bound Hg. During rain events SM-loads in the river drastically increase, and also the portion of cinnabar in SM increases due to erosion of larger particles from soils and mining-residues. Our results indicate that the polluted soils in the wider Idrija area are the most important source of Hg transported to GT, and that this Hg organically-bound Hg forms have a large potential for methylation and uptake in the marine food web.
ASSESSMENT OF MERCURY POLLUTION IN SOUTHWEST BALTIC SEA AS A RESULT OF HISTORICAL USE OF AIR DEFENCE AMMUNITION COMBINING HG SPECIATION AND ISOTOPIC COMPOSITION
In the region of Kiel Bay (Dnisch-Niendorf), Southwest Baltic Sea, more than 1.2 million Hg-containing anti-aircraft grenades were shot from World War II artillery training grounds. The majority of these shells exploded in the air, spreading their Hg content. Meanwhile, an unknown proportion of these shells did not explode and ended up on the seafloor, which bear the potential of future Hg-releases into the marine environment. The condition of the unexploded grenades and their potential for Hg release are still unknown, and the current distribution of Hg and its species in waters, sediments and organisms in this sector of the Southwest Baltic Sea is unclear.
The quantification of Hg and its species is an essential tool to evaluate regional environmental pollution. More recently Hg isotopic composition has been widely applied in tracing Hg contamination sources, biogeochemical processes and metabolic fates in environment and living organisms. Sediments act as an important sink for anthropogenic Hg pollution. The polluted sediments exhibited Hg isotopic compositions of anthropogenic and background contributions, whose isotopic composition can be differentiated. Furthermore, despite sediments represent the main Hg source to the surrounding food web, the last one displays a different Hg isotopic signature according to the trophic position and foraging depth of the organism. Such variation of Hg isotopic composition values between biota and their nearby sediments are suggested to be related to biogeochemical processes (e.g. methylation/demethylation) and other environmental factors in each specific system. Therefore, a better comprehension of the biogeochemical fate of the Hg species is needed in such Hg impacted ecosystem.
In this work, Hg concentration, speciation (GC-ICP-MS) and isotopic composition (CVG-MC-ICP-MS) is determined in marine surface sediments and aquatic organisms along the food web, including benthic organisms, plankton and fish that were collected in the ammunition affected area. The main objective of this work is to estimate the Hg contamination extent from the anti-aircraft munitions site to the target living organisms and potential Hg biogeochemical pathways before its introducing into the food web.
INSIGHTS FROM THE SPATIAL AND TEMPORAL DISTRIBUTIONS OF MERCURY SPECIES AND ISOTOPES IN BIVALVES FROM THE FRENCH COASTLINE
Mercury cycles between atmosphere, terrestrial and aquatic reservoirs through a complex combination of transport and transformation processes, until it is buried in oceanic or terrestrial sediments. Monomethylmercury is the main species bioaccumulated and bioamplified in the food chain. Knowledge of the origin (continental, oceanic or local sediment) of this species for the littoral environnment can still be improved.
This study focuses on ~.200 samples of bivalves (mussels: Mytilus edulis and Mytilus galloprovincialis and oysters: Crassostrea gigas and Isognomon alatus) from ~80 stations along the French coast. The goals of this study were i) to describe Hg levels, speciation and isotopic composition in various bioindicator bivalves from the French coastline, and ii) to identify possible geographic, taxonomic or temporal variations in these properties.
In the bivalves, we observed that the variations in methylmercury concentrations follow linearly those of total mercury (HgT). Preliminary results of isotopic fractionation of HgT in some of the samples do show regional effects and allow to differentiate English channel,. Atlantic, and Mediterranean coastlines. These differences may come from the various trophic regimes revealed by NC stable isotopes, from different fractionnation by the three biological species of bivalves, from different mercury source isotopic signatures, or both.
At most of the study sites, HgT concentrations have not decreased since 1987, despite regulations to abate or ban mercury used by anthropic activities.
This study is part of the Trococo project, funded by Ifremer, Rgion Pays de la Loire (Pollusols), and INSU/EC2CO/Dril.
MERCURY ISOTOPE FRACTIONATION DURING DARK REDUCTION IN SATURATED EAST FORK POPLAR CREEK SOIL
Riparian soils and sediments of the East Fork Poplar Creek (EFPC) floodplain (Oak Ridge, TN) contain large quantities of mercury from up-gradient contamination at the Y-12 National Security Complex. Important questions remain, however, on the long-term fate of legacy mercury in soils and sediments at this location. A recent study by our group observed substantial dissolved gaseous mercury (Hg(0)) evolution in saturated soil from the EFPC floodplain that contains mercury predominantly as nanoparticulate mercuric sulfide. This finding challenges the notion that authigenic mercuric sulfide is an environmental sink for mercury under all conditions. Periodic soil flooding could lead to the recycling of legacy mercury to the atmosphere. We use stable mercury isotope measurements to (1) provide insights on the process(es) responsible for the formation of Hg(0) in laboratory saturation experiments and (2) quantify the isotopic signature of mercury emitted from the EFPC floodplain to the atmosphere. A comparison of laboratory and field measurements allowed us to evaluate the environmental significance of dark Hg(II) reduction on the fate of legacy mercury in soils.
The stable mercury isotope composition of pore water samples (n = 22) from microcosm experiments was quantified over 36 days of flooding; previous work showed that the reduction of divalent mercury (Hg(II)) to Hg(0) controls mercury release dynamics. We also quantified the stable mercury isotope composition of the soil prior to the start of the experiment (t = 0 day; n = 3) and throughout the progression of the 36 day experiment (n = 6). As flooding proceeded, substantial mass-dependent fractionation (MDF) of stable mercury isotopes was observed resulting in pore water signatures becoming progressively lighter (δ202Hg = -0.52 to -1.70‰) and soil signatures progressively heavier (δ202Hg = +0.15 to +0.75‰). Mass-independent fractionation (MIF) of odd-mass isotopes in pore waters was also observed (Δ199Hg = +0.03 to +0.23). We employed a binary mixing model to explain the processes responsible for MDF and MIF of pore water and soil mercury using two endmembers: (1) the soil mercury at t = 0 day and (2) Hg(0) formed through dark Hg(II) reduction. In the field, stable mercury isotope compositions were quantified on atmospheric samples collected by gold amalgamation above the EFPC floodplain (n = 6). Stable mercury isotope measurements of atmospheric samples are compared with results from the laboratory experiments and previous investigations of the contamination site, and are discussed in context of the fate of mercury in the EFPC floodplain.
TRACKING MERCURY AND ITS STABLE ISOTOPES IN SEDIMENTS OF THE HACKENSACK RIVER ESTUARY
Estuarine sediments accumulate mercury (Hg) from watershed runoff and industrial point sources. However, the resuspension and transport of sediments by estuarine, tidal, and storm surge associated mixing can redistribute Hg far from where it was originally deposited. As a result, identifying the sources of Hg in complex urbanized estuaries, is challenging. Here we use Hg stable isotopes to identify the sources of Hg in sediments within the Hackensack River estuary, a tributary to the lower Hudson River estuary in New York and New Jersey, USA. Concentrations of total Hg (>99% inorganic Hg) in sediment from the Hackensack River estuary varied from 0.06 to 154 ug g-1 with the highest concentrations measured in sediment from upper Berrys Creek estuary just below the West Riser tide gate (86 ug g-1) and upper Peach Island Creek (55 ug g-1). The mean d202Hg value of mercury from Berrys Creek estuary and two small tributaries, Peach Island Creek and Ackermans Creek, was significantly higher (p < 0.001) than that from the main stem Hackensack River and its tributary Bellmans Creek. The spatial distribution of mercury and its stable isotopes indicates that mercury from Berrys Creek and its tributaries mixes with, and is partially diluted by mercury from a lower concentration source that is more depleted in 202Hg (lower d202Hg values) as it enters the Hackensack River. Mixing plots of d202Hg vs. inverse total Hg concentration in sediment for the entire system show that d202Hg values decline linearly with decreasing Hg concentrations. Using a binary mixing model, it is estimated that most (>75%) of the mercury in Bellmans Creek and perhaps Mill Creek originated from upper Berrys Creek. Surface sediments from Peach Island Creek had very high mercury concentrations indicating that they could be an important source of mercury to the system.
USING MERCURY STABLE ISOTOPES IN FISH TO IDENTIFY BIOAVAILABLE MERCURY IN A CONTAMINATED ESTUARY
Tracking the sources and accumulation of mercury in coastal marine fish remains a challenge. While the isotopic composition of mercury in fish varies among species and ecosystems reflecting differences in diet as well as the extents and mechanisms of mercury transformations in the environment, it is not easily connected to feeding habitats or bioavailable pools of mercury within a single ecosystem. In order to build on previous work that explored the utility of mercury stable isotopes in the identification of bioavailable mercury in aquatic ecosystems, we estimated δ202Hg values of bioavailable total mercury (THg) and methylmercury (MeHg) prior to their accumulation in the food webs that lead to killifish (Fundulus heteroclitus) and white perch (Morone americana) in the mercury-contaminated Hackensack River estuary, New Jersey, USA. Among four spatially distinct and two developmentally defined populations of these fish, MeHg concentrations were not correlated with those in sediment or suspended particulate matter. Average mass-dependent mercury isotope ratios (δ202Hg) in the fish varied from -0.38‰ to +0.55‰, and fish from all six populations accumulated mercury with positive Δ199Hg and Δ201Hg values, which are produced by the mass-independent fractionation (MIF) of mercury stable isotopes. δ202Hg values of bioavailable THg and MeHg were estimated using the extents of MIF in fish to account for photochemical transformations of inorganic mercury and MeHg, and were strongly dependent on the percent of THg in fish present as MeHg (%MeHg), which varied from 30% to 100%. Estimated δ202Hg values of bioavailable MeHg for two spatially distinct populations of killifish were -0.28‰ and +0.30‰, which are similar to directly measured δ202Hg values for MeHg in sediments from the environments in which each population lived (-0.19‰ and +0.40‰, respectively). However, estimated δ202Hg values of bioavailable MeHg for adult and juvenile white perch (0.48 to 0.87‰) were higher than that for MeHg in sediments throughout the Hackensack River estuary (-0.4 to +0.4‰). This likely reflects the diverse diet and wide feeding range of migratory white perch which may consume prey with isotopically heavier MeHg within or outside of the Hackensack River. These results demonstrate that mercury stable isotopes can be used to identify local scale sources of mercury in fish living in complex environments and have the potential to be used to identify distinct populations of estuarine forage fish and as a proxy for their feeding locations.
SOURCE ASSESSMENT OF MERCURY CONTAMINATION ALONG THE GRAND PORTAGE TRAIL, MN (USA)
The 14-km Grand Portage Trail (GRPO), located in extreme northeastern Minnesota, was a primary route of transport between Lake Superior and inland waters for the French-Canadian Voyageur fur trade during the 17th-19th centuries. Recent work has indicated elevated levels of total mercury, methylmercury, and % methylmercury in components of the lower food web (water, sediment, soil, benthic invertebrates), compared to other national parks in the western Laurentian Great Lakes region. Surficial soil and sediment samples at GRPO yielded 3 to 4-fold enrichment in total Hg relative to organic matter content when compared to other regional forested sites. We sampled soil cores at five locations along the GRPO trail in June 2015 to define the spatial extent of total Hg contamination, and to determine whether the Hg stable isotopic signature differed between contaminated and non-contaminated sites. Two locations (Fort Charlotte and the Stockade) were located at each terminus of the trail, and were known historic locations of vermilion (a bright red HgS-containing pigment) storage and use. These terminal sites exhibited total Hg concentrations (Ft. Charlotte: 6.5±5.0 µg/g dw; Stockade: 0.58±0.36 µg/g) far exceeding locations between them along the trail (0.052±0.021 µg/g). Analysis of the Hg stable isotopes 199Hg and 202Hg by multi-collector ICP-MS indicated that the contaminated sites were indeed isotopically similar to each other (Ft. Charlotte and Stockade data combined: δ202Hg(II): -0.49±0.14‰, ∆199Hg(II): 0.00±0.03 ‰), but different from both the low-total Hg locations mid-trail (δ202Hg(II): -1.24±0.17 ‰, ∆199Hg(II): -0.15±0.07 ‰) and reference soils collected at the Experimental Lakes Area (ELA) in northwestern Ontario, Canada (δ202Hg(II): -1.67±0.17 ‰, ∆199Hg(II): -0.24±0.07 ‰). Samples formed a mixing line between the contaminated and non-contaminated sites, with the mid-trail sites falling between the GRPO contaminated sites and that of ELA. Two archived vermilion samples from the fur trade era were analyzed isotopically for comparison, one each from GRPO and Fort Michilimackinac (Mackinaw City, MI). The GRPO vermilion exhibited an isotopic signature that was markedly similar to the GRPO contaminated sites, while the Michilimackinac sample did not. Our results suggest that anthropogenic contamination associated with historic fur trade commerce is likely responsible for elevated total and methylmercury concentrations observed in the contemporary GRPO food web. We plan to analyze additional regional archived vermilion samples to aid in source attribution.
TRACING ATMOSPHERIC MERCURY FROM MARINE BOUNDARY LAYER IN THE YELLOW SEA AND BOHAI SEA USING STABLE MERCURY ISOTOPES
Source apportionments of mercury (Hg) using stable Hg isotopes have been applied in many previous studies, providing unique clues of typical emission sources and processes in the environmental circulation of Hg. However the fingerprint of atmospheric Hg isotopes, especially in the background area, has been rarely investigated limited by the extremely low concentrations and complicated transformation among three species of atmospheric Hg (gaseous elemental Hg, gaseous oxidized Hg and particle-bound Hg). In this study, total gaseous Hg (TGM, composed by gaseous elemental Hg and gaseous oxidized Hg) samples were collected using chlorized active carbon trap in marine boundary layer of the Yellow Sea and Bohai Sea in summer, 2016. Negative δ202Hg (-1.26±0.69‰, 1σ, n = 7) and Δ199Hg (-0.09±0.05‰, 1σ, n = 7) signatures were observed in TGM samples collected in the Yellow Sea, and samples collected in the Bohai Sea shows more negative δ202Hg (-2.02±0.69‰, 1σ, n = 9) and near-zero Δ199Hg (-0.02±0.04‰, 1σ, n = 9) signatures. The isotopic compositions and ratio of Δ199Hg to Δ201Hg (~1.0) suggesting different mixing ratio of gaseous Hg from both natural background pool and anthropogenic emissions transported from continent, with influences from environmental processes, i.e. the photochemical reactions and evaporation of Hg in the marine surface. In addition, negative Δ200Hg signatures (-0.04±0.03‰, 1σ, n = 7) observed in TGM samples collected in the Yellow Sea implying the active oxidation of elemental Hg and adsorption/desorption on the particle surface.