G2 (II) Mercury stable isotope biogeochemistry

Mercury (Hg) is a toxic trace metal that enters many ecosystems via atmospheric deposition and poses serious threats to human health through the consumption of high trophic level fish. Although fossil fuel combustion (primarily coal) accounts for approximately two-thirds of anthropogenic Hg emissions to the atmosphere and enhanced deposition of Hg can occur close to point sources such as coal-fired utility boilers (CFUBs), there are currently no nationwide regulations in the U.S.A. on emissions from CFUBs. Previous studies have used trace element concentrations to indirectly identify Hg sources, but complex atmospheric Hg chemistry and mixing of emissions from local and long-range sources make it challenging to directly trace Hg pollution.

We investigated the use of Hg stable isotope ratios to directly identify Hg emissions from a large, isolated CFUB in Crystal River, Florida, U.S.A. During July 2009, daily-event precipitation samples were collected at four sites surrounding the CFUB at distances of 5.0 to 10.6 km and at nine other sites across FL. Crystal River precipitation samples were characterized by large negative mass-dependent fractionation (MDF) with d²⁰²Hg values as low as -4.37 ± 0.13‰, 2 s.d. (mean d²⁰²Hg = -2.55‰, 1 s.d = 1.05‰, n = 31). This magnitude of negative MDF of Hg greatly exceeds that which has been previously reported in natural atmospheric samples. In contrast, precipitation samples collected at the other sites did not exhibit large negative MDF and were characterized by a narrow range of d²⁰²Hg values close to 0‰ (mean d²⁰²Hg = 0.07‰, 1 s.d. = 0.16‰, n = 16).

These results indicate that Hg emitted by the CFUB and deposited nearby is isotopically distinct from Hg deposited at sites that are not heavily impacted by local coal combustion. Several factors including source coal isotopic composition and Hg isotope fractionation in the power plant system could influence the observed Hg isotope ratios. At locations influenced primarily by local coal combustion, it is likely that the anomalous Hg isotope signature of this pollution will be useful in tracing and quantifying its impact on local aquatic and terrestrial ecosystems.

Hg stable isotopes have recently gained application for the identification of Hg sources, and for understanding the behavior of Hg in aquatic systems. In order to trace Hg contaminant sources into fish, an understanding of Hg isotope fractionation during trophic transfer is required. In this study we performed a controlled experimental determination of both mass-dependent (ð²⁰²Hg) and mass-independent (∆¹⁹⁹Hg) fractionation during trophic transfer by raising yellow perch (Perca flavescens) in captivity and feeding them food pellets with varying amounts of added synthetic methylmercury (MeHg). Hg isotope values were measured by on-line cold vapor generation MC-ICP-MS using Tl as an internal standard and sample-standard bracketing, with an external uncertainty of better than ±0.1‰ (2sd) for ð²⁰²Hg and ∆¹⁹⁹Hg. Juvenile yellow perch (3 month old) were raised in three separate tanks and fed commercial food pellets with a) no added MeHg (background MeHg concentration of 0.0691ppm), b) 1ppm added MeHg and c) 5ppm added MeHg. The muscle tissues of yellow perch fed with these treatments had average Hg concentrations (dry weight) of 0.322ppm, 2.57ppm and 13.8ppm and bioaccumulation factors of 4.7, 3.0 and 2.9, respectively. The food pellets were characterized by ð²⁰²Hg values of 0.20‰, -0.63‰, and -0.70‰ and ∆¹⁹⁹Hg values of 1.40‰, 0.15‰, and 0.04‰ for 0ppm, 1ppm, and 5ppm pellets, respectively. The food pellet values are explained by simple mixing of MeHg with high ð²⁰²Hg and ∆¹⁹⁹Hg from fish by-products in the untreated pellets with added synthetic MeHg, which has lower ð²⁰²Hg and ∆¹⁹⁹Hg. The ð²⁰²Hg and ∆¹⁹⁹Hg values of the muscle tissues closely reflected their food sources and for each treatment were identical within analytical uncertainty. The fish muscle tissue values were characterized by ð²⁰²Hg values of 0.36‰, -0.69‰, and -0.76‰, and ∆¹⁹⁹Hg values of 1.55‰, 0.18‰, and 0.05‰, for 0ppm, 1ppm, and 5ppm pellets, respectively. Thus the isotopic composition of the food was directly passed to the fish muscle tissue without significant fractionation during trophic transfer. We infer that photochemical reduction and degradation of MeHg prior to incorporation into the food web is largely responsible for MIF in young fish. Moreover, this study supports the use of young fish as biosentinels for measurement of the Hg isotopic composition of their food sources and potentially for isotopic identification of Hg sources in the environment.

In recent years, mercury (Hg) has become a major concern in the arctic region. Mercury deposition is particularly enhanced in this region distant from direct anthropogenic emission sources. Following snow-melt, Hg undergoes various and complex reactions such as methylation leading to the formation of methylmercury (MeHg). This organic and toxic form of Hg displays strong bioaccumulative properties making its concentration to increase along the food chain. Marine mammal’s exposure to MeHg is mainly linked to their feeding behavior and habitat utilization. Regional variations in marine mammal mercury levels have raised concern on the ecological, geochemical and/or possible variation in Hg sources behind these trends. Recent work on arctic seabird eggs (Point et al. 2011) has shown a possible link between sea-ice and Hg isotopic signatures. In this work we measured the stable isotopic composition of mercury in marine mammal liver samples collected in the same area. Special attention was paid to investigate ringed seals, a proven key sentinel species of the Arctic sea-ice ecosystem.

We analyzed 53 ringed seal liver samples collected as part of the Alaska Marine Mammal Tissue Archival Project. Each individual liver sample was cryogenically prepared and archived at the National Institute of Standards and Technology (NIST) Environmental Specimen Bank (Charleston, SC, USA). Sample collection covered a time trend of 14 years (1988-2002) at two locations; Nome in the Bering Sea, and Barrow in the Chukchi Sea. Average total mercury concentration in ringed seal liver samples was 2.9 ±3.5 mg /g (SD, n=53), an order of magnitude lower than mercury levels observed in livers from other marine mammals collected in the same area (Belugas, HgT=22.9 ±32.0 mg/g (SD, n=54) and polar bears, 13.3±12.8 (SD, n=15) mg/g).

Hg stable isotope compositions were measured on a Neptune MC-ICP-MS with cold vapor generation using a previously referenced study (Point et al. 2011). Particular attention was paid to investigate (i) the influence of physiological factors on Hg stable isotope fractionation, in particular age and size that have a proven effect on Hg levels in ringed seal livers, (ii) the spatial and temporal variations in Hg stable isotopes fractionation, (iii) variations in Hg stable isotopic composition at different trophic levels of the Alaskan Arctic food chain.

In order to further evaluate of environmental impacts of atmospheric Mercury (Hg) emission from coal combustion in China. Hg stable isotope analysis from 60 coal deposits through China were carried out using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) in this study (Yin et al., 2010). Study shows that Hg which occurs in Chinese coal deposits appears to have a particular Hg stable isotope “fingerprint” effects from other parts of the world (Biswas et al., 2008). Hg isotope fingerprint also shows that coal deposits located at the edge of cratons differ slightly in isotopic composition and are enriched by up to 1.0 ‰ in ²⁰²Hg relative to ¹⁹⁸Hg when compared to coals collected from inner cratons, which may provide some important geological information.

Mercury pollution from industrial sources is a widespread environmental problem due to the extensive use of Hg in various industrial processes. Soils and sediments represent important sinks for anthropogenic Hg, but source allocation and predictions on the long-term fate of Hg in contaminated ecosystems often remain difficult. Recent analytical advances resulted in the discovery of variable Hg isotope ratios caused by mass-dependent (MDF) and mass-independent fractionation (MIF). Thus, stable Hg isotope signatures provide a promising new tool to trace sources and transformation processes in biogeochemical Hg cycling in nature.

Here, we present Hg isotope data from sediments which were collected in the vicinity of local industrial sources in Sweden. Most of the samples were affected by elemental Hg pollution, mainly from the chlor-alkali process, whereas other samples were contaminated with phenyl-Hg used as a preservative in the pulp and paper industry. The sampling sites further exhibited a range of environmental conditions from freshwater lakes to brackish water estuaries with different climatic conditions and biological activities. The samples have been previously characterized (Skyllberg et al., 2007, Ambio 36: 437-442) and assessed for their Hg methylation potential (Drott et al., 2008, ES&T 42: 153-158).

The sediment samples were digested with aqua regia and Hg isotope ratios were analyzed by CV-MC-ICPMS (Nu Plasma) using Tl mass bias correction and standard bracketing (Wiederhold et al., 2010, ES&T 44: 4191-4197). The results exhibited significant variations in Hg isotope signatures ranging from -2.1‰ to -0.2‰ in d²⁰²Hg (MDF) relative to NIST-3133. In contrast, only small MIF effects with variations of ~±0.1‰ in ∆¹⁹⁹Hg were detected. The sampling sites could be distinguished based on their ranges in d²⁰²Hg suggesting that different Hg isotope source signatures were emitted. The lightest MDF signatures with a d²⁰²Hg range of -1.5‰ to -2.1‰ were measured in the sediments of a brackish water estuary which also exhibited the highest Hg concentrations (>200 mg kg^-1) and in which traces (<5%) of remaining elemental Hg were detected. In addition, the variability of d²⁰²Hg values from different depths of a sampling site of up to 0.8‰ points toward mass-dependent fractionation which occurred after the contamination and which could be potentially used to trace Hg transformation processes at the sites.

One of the anthropogenic sources of Hg in environments impacted by gold-mining activities is liquid Hg(0), widely used in South America to amalgam gold. Excess mercury is first separated from the amalgam, generally by hand pressing, and then released into the river, increasing total Hg concentrations in sediments in Bolivian gold-mining areas. In this study, Hg isotope signatures have been analyzed in sediments sampled in mine tailings, along an impacted river and compared with a pristine area both in the Bolivian Amazon (Beni River basin) and in French Guiana (Oyapock River basin).

In the Beni R. sediments from mine tailings, d202Hg=–0.33±0.14‰ and D201Hg=–0.02±0.05‰ (2SD, n=4) are very close to d202Hg=–0.37±0.09‰ and D201Hg=–0.07±0.03‰ (2SD, n=3) of liquid Hg(0) used by gold-miners. On the other hand, isotopic signatures of Beni R. sediments downstream gold-mining areas, d202Hg=–1.02±0.36‰ and D201Hg=–0.06±0.07‰ (2SD, n=7), are not significantly different from Mamore River sediments that are not impacted by gold-mining activities, d202Hg=–1.03±0.22‰ and D201Hg=–0.09±0.10‰ (2SD, n=11) with P=0.84 for d202Hg and P=0.19 for D201Hg (student t-test, n=18). These results confirm preliminary study made on the Beni R. basin by Maurice-Bourgoin et al. (2003) who estimated that 26 t Hg.year–1 transit this river by natural erosion and dilute the anthropogenic Hg inputs at the basin scale (67500 km²).

In the Oyapock R. basin (28620 km²), Hg isotopic signatures of sediments from gold-mining tributaries d202Hg=–0.46±0.09‰ and D201Hg=–0.41±0.05‰ (2SD, n=3) are significantly different to the ones of the upstream pristine river d202Hg=–1.85±0.30‰ and D201Hg=–0.56±0.05‰ (2SD, n=8) with P<0.001 (student t-test, n=11) for both d202Hg and D201Hg. The D201Hg difference can be explained by a liquid Hg(0) contamination due to gold-mining: a mix of natural Hg with D201Hg=–0.56‰ and anthropogenic Hg with D201Hg=–0.02‰ is obtained. Based on a D201Hg binary mixing model, gold-mining inputs represent 29% of the total Hg in contaminated sediments of the Oyapock R. basin. Obviously more data are needed to refine this first estimate.

Artisanal small-scale gold mining (ASGM) is a central economic activity throughout the developing world involving over 13 million people; however, it leads to extensive pollution of the environment through the use of Hg to extract gold. This is a major health concern especially in regions in which polluted aquatic systems serve as a source of fish, an important dietary component. Studies conducted in the Amazon show elevated levels of Hg in fish and sediment downstream of ASGM sites; however, there is no definitive scientific evidence verifying Hg in specific aquatic ecosystems is from Hg use in ASGM rather than from other Hg sources including erosion. To test whether stable Hg isotopic analysis can trace sources of Hg through aquatic ecosystems, this research focused on Hg pollution of waterways in the context of ASGM in Amapá, Brazil. We measured the Hg isotopic composition of Hg in sediment cores from two lakes, one of which has elevated Hg and was thought to be impacted by the use of elemental Hg in ASGM. We also analyzed Hg in grab samples at the AGSM site, upstream and downstream the AGSM site along the river connecting the polluted lake to the ASGM site, and in forest soils. Hg from all these samples was purified via combustion and trapped in permanganate solution. Both mass-independent and mass-dependent Hg isotopic signatures were analyzed using cold vapor multi-collector inductively coupled plasma mass spectrometry (CV-MC-ICP-MS). Isotopic analysis demonstrates that ASGM is associated with elevated Hg concentrations near the ASGM site. However the unique isotopic signature detected in samples immediately downstream of the ASGM site was not observed farther downstream or in the lake sediment, which have two to three times more Hg than the non-polluted lake. This suggests that this elevated Hg is not from elemental Hg used during the amalgamation process. Instead the isotope data suggests that the Hg in the polluted lake is from erosion and runoff, likely due to land-use change associated with ASGM, agriculture, and cattle production. This study indicates that, through the analysis of stable Hg isotopes, it is possible to trace Hg from ASGM and to assess the extent to which it is impacting local ecosystems and food webs. Hg biomarkers in hair from individuals living around both lakes will also be discussed. This information may be useful to guide future public health interventions to reduce Hg exposure in these highly vulnerable populations.

Recent studies showed that rice intake may pose a big health risk of mercury (Hg) exposure to local habitants, especially in Wanshan (WMM) Hg mine (in Guizhou Province, SW China), where active transformation of inorganic Hg to Me-Hg takes place. The mechanism of Hg transformation in rice should be therefore investigated so that remediation can be ap­propriately targeted. In order to study of sources and transportations of Hg in the rice plant (Oryza sativaL.), stable Hg isotope variations in different rice tissues (root, stem, foliage and seed) which collected from WMM were investigated by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) technique. In comparison, Hg isotope ratios of paddy soil, Hg-ores and mine-waste calcines of WMM were also analyzed. From our study, rice tissues differ slightly in isotopic composition and are depleted by up to 3.0‰ in ²⁰²Hg relative to ¹⁹⁸Hg when compared to the growth mediums. Mass independent fractionation (MIF) signature of Hg (MIF) in rice samples affected the odd Hg isotopes to produce a 0.36‰ range in both ∆¹⁹⁹Hg values. It’s suggest that the isotopic anomalies measured in rice plants represented the composition of atmospheric Hg. We therefore concluded that the mix of atmospheric Hg with soil Hg is the cause for the measured isotopic variations. This observation leads to the hypothesis that the magnitude of MIF measured can certainly reflect the proportion of atmospheric Hg in different rice tissues. Finaly, according to an end-member mixing model, the atmospheric Hg contributes for more than 50% of rice stem, the rice root exhibit a predominance of paddy soil Hg with a percentage of Hg from the ambient air of approximately ~10%. The rice grain is thought to be largely controlled by the paddy soil, and the atmospheric mercury only contributes for less than 40%.