New Insight into Biomarkers of Human Mercury Exposure Using Naturally Occurring Mercury Stable Isotopes

Human exposure to methylmercury (MeHg) and elemental mercury vapor (Hg⁰_(g)) are often estimated using total Hg concentrations in hair and urine, respectively. We investigated whether Hg stable isotopes could be used to better distinguish between exposure to Hg⁰_(g) versus MeHg. We found that hair from North American dental professionals was characterized by high positive Δ¹⁹⁹Hg values (mean = 1.86‰, 1 SD = 0.12‰, <i>n</i> = 11). This confirms that among people who regularly consume fish, total Hg concentrations in hair reflect exposure to MeHg. In contrast, we found that urine from the same individuals was characterized by a range of Δ¹⁹⁹Hg values (0.29 to 1.77‰, 2 SD = 0.06‰, <i>n</i> = 12) that were significantly correlated to the number of dental amalgams in each individual’s mouth. We hypothesize that fish-derived MeHg is demethylated within the body, causing mass-dependent fractionation and the excretion of inorganic Hg in urine. Mercury in urine therefore represents a mixture of demethylated fish-derived MeHg and amalgam-derived inorganic Hg. We estimate that the majority (>70%) of Hg in urine from individuals with <10 dental amalgams is derived from ingestion of MeHg in fish. These data suggest that within populations that consume fish, urine total Hg concentrations may overestimate Hg exposure from personal dental amalgams

Isotopic Characterization of Mercury Downstream of Historic Industrial Contamination in the South River, Virginia

Historic point source mercury (Hg) contamination from industrial processes on the South River (Waynesboro, Virginia) ended decades ago, but elevated Hg concentrations persist in the river system. In an effort to better understand Hg sources, mobility, and transport in the South River, we analyzed total Hg (THg) concentrations and Hg stable isotope compositions of streambed sediments, stream bank soils, suspended particles, and filtered surface waters. Samples were collected along a longitudinal transect of the South River, starting upstream of the historic Hg contamination point-source and extending downstream to the confluence with the South Fork Shenandoah River. Analysis of the THg concentration and Hg isotopic composition of these environmental samples indicates that the regional background Hg source is isotopically distinct in both Δ¹⁹⁹Hg and δ²⁰²Hg from Hg derived from the original source of contamination, allowing the tracing of contamination-sourced Hg throughout the study reach. Three distinct end-members are required to explain the Hg isotopic and concentration variation observed in the South River. A consistent negative offset in δ²⁰²Hg values (∼0.28‰) was observed between Hg in the suspended particulate and dissolved phases, and this fractionation provides insight into the processes governing partitioning and transport of Hg in this contaminated river system

Investigation of Local Mercury Deposition from a Coal-Fired Power Plant Using Mercury Isotopes

Coal combustion accounts for approximately two-thirds of global anthropogenic mercury (Hg) emissions. Enhanced deposition of Hg can occur close to coal-fired utility boilers (CFUBs), but it is difficult to link specific point sources with local deposition. Measurement of Hg stable isotope ratios in precipitation holds promise as a tool to assist in the identification of local Hg deposition related to anthropogenic emissions. We collected daily event precipitation samples in close proximity to a large CFUB in Crystal River, Florida. Precipitation samples collected in Crystal River were isotopically distinct and displayed large negative δ²⁰²Hg values (mean = −2.56‰, 1 SD = 1.10‰, <i>n</i> = 28). In contrast, precipitation samples collected at other sites in FL that were not greatly impacted by local coal combustion were characterized by δ²⁰²Hg values close to 0‰ (mean = 0.07‰, 1 SD = 0.17‰, <i>n</i> = 13). These results indicate that, depending on factors such as powdered coal isotopic composition and efficiency of Hg removal from flue gas, Hg deposited near CFUBs can be isotopically distinct. As this tool is further refined through future studies, Hg stable isotopes may eventually be used to quantify local deposition of Hg emitted by large CFUBs

Isotopic Characterization of Mercury in Natural Gas via Analysis of Mercury Removal Unit Catalysts

Natural gas (NG) represents an important and rapidly growing global energy source, and some commercially relevant reserves of NG are reported to contain mercury (Hg) at concentrations between 0.01 and 5,000 μg/m³. The overall amount of Hg released to the atmosphere from gas production and combustion is largely unknown, but gaseous elemental Hg release is likely an increasing contribution to the global atmospheric Hg pool. However, no Hg isotopic compositions have been published for Hg entering the atmosphere from NG. In an effort to characterize the isotopic composition of Hg released from NG, we analyzed the stable isotopic compositions of mercury removal unit (MRU) catalysts that were loaded with Hg from NG production and supplied by Johnson Matthey Inc. We suggest that the bulk of Hg adsorbed to catalysts near the inlet of each MRU reactor is representative of the Hg isotopic composition of the NG source. In different gas fields, values of δ²⁰²Hg and Δ¹⁹⁹Hg range from −3.75 to −0.68‰ and −0.02 to 0.65‰, respectively. Analysis of four samples from different positions within a single MRU reactor demonstrates significant isotopic fractionation of a small fraction of Hg that is not removed at the entrance to the MRU. We suggest that this fractionation is due to sorption of Hg to the catalyst surface from the gas phase and that this process follows a Rayleigh fractionation model with ε ≈ −0.40‰. In total, these results suggest that Hg isotopic analysis may be a feasible monitoring tool for Hg emissions from NG production in some gas fields. With further analyses of NG from around the world, a global average isotopic composition of NG-hosted Hg could be estimated to characterize this input to atmospheric Hg isotope models

Identification of Multiple Mercury Sources to Stream Sediments near Oak Ridge, TN, USA

Sediments were analyzed for total Hg concentration (THg) and isotopic composition from streams and rivers in the vicinity of the Y-12 National Security Complex (Y12) in Oak Ridge, TN (USA). In the stream directly draining Y12, where industrial releases of mercury (Hg) have been documented, high THg (3.26 to 60.1 μg/g) sediments had a distinct Hg isotopic composition (δ²⁰²Hg of 0.02 ± 0.15‰ and Δ¹⁹⁹Hg of −0.07 ± 0.03‰; mean ± 1SD, <i>n</i> = 12) compared to sediments from relatively uncontaminated streams in the region (δ²⁰²Hg = −1.40 ± 0.06‰ and Δ¹⁹⁹Hg of −0.26 ± 0.03‰; mean ± 1SD, <i>n</i> = 6). Additionally, several streams that are nearby but do not drain Y12 had sediments with intermediate THg (0.06 to 0.21 μg/g) and anomalous δ²⁰²Hg (as low as −5.07‰). We suggest that the low δ²⁰²Hg values in these sediments provide evidence for the contribution of an additional Hg source to sediments, possibly derived from atmospheric deposition. In sediments directly downstream of Y12 this third Hg source is not discernible, and the Hg isotopic composition can be largely explained by the mixing of low THg sediments with high THg sediments contaminated by Y12 discharges

Restoring Soil Calcium Reverses Forest Decline

Forest decline in the northeastern United States has been linked to the effects of acid deposition on soil nutrients. To test this link, we added a calcium silicate mineral to a paired watershed at the Hubbard Brook Experimental Forest, New Hampshire, in an amount designed to gradually replace the estimated amount of calcium lost as a result of human activity in the 20th Century (primarily because of acid deposition). The experimental restoration resulted in a recovery of tree biomass increment. The improved calcium nutrition also promoted higher aboveground net primary production and increased the photosynthetic surface area in the treated watershed relative to that in the reference watershed. These results demonstrated that soil acidification accelerated by acid deposition has contributed to the decline of forest growth and health on naturally acidic soil in the northeastern United States and that decline can be reversed by the addition of calcium

New Insights on Ecosystem Mercury Cycling Revealed by Stable Isotopes of Mercury in Water Flowing from a Headwater Peatland Catchment

Stable isotope compositions of mercury (Hg) were measured in the outlet stream and in soil cores at different landscape positions in a 9.7-ha boreal upland-peatland catchment. An acidic permanganate/persulfate digestion procedure was validated for water samples with high dissolved organic matter (DOM) concentrations through Hg spike addition analysis. We report a relatively large variation in mass-dependent fractionation (δ²⁰²Hg; from −2.12 to −1.32‰) and a smaller, but significant, variation of mass-independent fractionation (Δ¹⁹⁹Hg; from −0.35 to −0.12‰) during two years of sampling with streamflow varying from 0.003 to 7.8 L s^–1. Large variations in δ²⁰²Hg occurred only during low streamflow (<0.6 L s^–1), which suggest that under high streamflow conditions a peatland lagg zone between the bog (3.0 ha) and uplands (6.7 ha) becomes the dominant source of Hg in downstream waters. Further, a binary mixing model showed that except for the spring snowmelt period, Hg in streamwater from the catchment was mainly derived from dry deposition of gaseous elemental Hg (73–95%). This study demonstrates the usefulness of Hg isotopes for tracing sources of Hg deposition, which can lead to a better understanding of the biogeochemical cycling and hydrological transport of Hg in headwater catchments

Carbon, Nitrogen, and Mercury Isotope Evidence for the Biogeochemical History of Mercury in Hawaiian Marine Bottomfish

The complex biogeochemical cycle of Hg makes identifying primary sources of fish tissue Hg problematic. To identify sources and provide insight into this cycle, we combined carbon (δ¹³C), nitrogen amino acid (δ¹⁵N_Phe), and Hg isotope (Δ¹⁹⁹Hg, Δ²⁰¹Hg, δ²⁰²Hg) data for six species of Hawaiian marine bottomfish. Results from these isotopic systems identified individuals within species that likely fed from separate food webs. Terrestrial freshwater inputs to coastal sediments were identified as the primary source of tissue Hg in the jack species, <i>Caranx ignobilis</i>, which inhabit shallow marine ecosystems. Thus, coastal <i>C. ignobilis</i> were a biological vector transporting Hg from freshwater environments into marine ecosystems. Depth profiles of Hg isotopic compositions for bottomfish (excludung <i>C. ignobilis</i>) were similar, but not identical, to profiles for open-ocean pelagic fishes, suggesting that in both settings inorganic Hg, which was ultimately transformed to monomethylmercury (MeHg) and bioaccumulated, was dominantly from a single source. However, differences between pelagic fish and bottomfish profiles were attributable to mass-dependent fractionation in the benthos prior to incorporation into the food web. Results also confirmed that bottomfish relied, at least in part, on a benthic food web and identified the incorporation of deeper water oceanic MeHg sources into deeper water sediments prior to food web uptake and transfer

Tracking the Fate of Mercury in the Fish and Bottom Sediments of Minamata Bay, Japan, Using Stable Mercury Isotopes

Between 1932 and 1968, industrial wastewater containing methylmercury (MeHg) and other mercury (Hg) compounds was discharged directly into Minamata Bay, Japan, seriously contaminating the fishery. Thousands of people who consumed tainted fish and shellfish developed a neurological disorder now known as Minamata disease. Concentrations of total mercury (THg) in recent fish and sediment samples from Minamata Bay remain higher than those in other Japanese coastal waters, and elevated concentrations of THg in sediments in the greater Yatsushiro Sea suggest that Hg has moved beyond the bay. We measured stable Hg isotope ratios in sediment cores from Minamata Bay and the southern Yatsushiro Sea and in archived fish from Minamata Bay dating from 1978 to 2013. Values of δ²⁰²Hg and Δ¹⁹⁹Hg in Yatsushiro Sea surface sediments were indistinguishable from those in highly contaminated Minamata Bay sediments but distinct from and nonoverlapping with values in background (noncontaminated) sediments. We conclude that stable Hg isotope data can be used to track Minamata Bay Hg as it moves into the greater Yatsushiro Sea. In addition, our data suggest that MeHg is produced in bottom sediments and enters the food web without substantial prior photodegradation, possibly in sediment porewaters or near the sediment-water interface

Absence of Fractionation of Mercury Isotopes during Trophic Transfer of Methylmercury to Freshwater Fish in Captivity

We performed two controlled experiments to determine the amount of mass-dependent and mass-independent fractionation (MDF and MIF) of methylmercury (MeHg) during trophic transfer into fish. In experiment 1, juvenile yellow perch (<i>Perca flavescens</i>) were raised in captivity on commercial food pellets and then their diet was either maintained on unamended food pellets (0.1 μg/g MeHg) or was switched to food pellets with 1.0 μg/g or 4.0 μg/g of added MeHg, for a period of 2 months. The difference in δ²⁰²Hg (MDF) and Δ¹⁹⁹Hg (MIF) between fish tissues and food pellets with added MeHg was within the analytical uncertainty (δ²⁰²Hg, 0.07 ‰; Δ¹⁹⁹Hg, 0.06 ‰), indicating no isotope fractionation. In experiment 2, lake trout (<i>Salvelinus namaycush</i>) were raised in captivity on food pellets and then shifted to a diet of bloater (<i>Coregonus hoyi</i>) for 6 months. The δ²⁰²Hg and Δ¹⁹⁹Hg of the lake trout equaled the isotopic composition of the bloater after 6 months, reflecting reequilibration of the Hg isotopic composition of the fish to new food sources and a lack of isotope fractionation during trophic transfer. We suggest that the stable Hg isotope ratios in fish can be used to trace environmental sources of Hg in aquatic ecosystems