Distribution and trends of mercury in deciduous tree cores

Riparian trees in mercury contaminated riverbanks: An important resource for sustainable remediation management

Mining operations generate sediment erosion rates above those of natural landscapes, causing persistent contamination of floodplains. Riparian vegetation in mine-impacted river catchments plays a key role in the storage/remobilization of metal contaminants. Mercury (Hg) pollution from mining is a global environmental challenge. This study provides an integrative assessment of Hg storage in riparian trees and soils along the Paglia River (Italy) which drains the abandoned Monte Amiata Hg mining district, the 3rd former Hg producer worldwide, to characterize their role as potential secondary Hg source to the atmosphere in case of wildfire or upon anthropic utilization as biomass. In riparian trees and nearby soils Hg ranged between 0.7 and 59.9 μg/kg and 2.2 and 52.8 mg/kg respectively. In trees Hg concentrations were below 100 μg/kg, a recommended Hg limit for the quality of solid biofuels. Commercially, Hg contents in trees have little impact on the value of the locally harvested biomass and pose no risk to human health, although higher values (195-738 μg/kg) were occasionally found. In case of wildfire, up to 1.4*10-3 kg Hg/ha could be released from trees and 27 kg Hg/ha from soil in the area, resulting in an environmentally significant Hg pollution source. Data constrained the contribution of riparian trees to the biogeochemical cycling of Hg highlighting their role in management and restoration plans of river catchments affected by not-remediable Hg contamination. In polluted river catchments worldwide riparian trees represent potential sustainable resources for the mitigation of dispersion of Hg in the ecosystem, considering i) their Hg storage capacity, ii) their potential to be used for local energy production (e.g. wood-chips) through the cultivation and harvesting of biomasses and, iii) their role in limiting soil erosion from riparian polluted riverbanks, probably representing the best pragmatic choice to minimize the transport of toxic elements to the sea.

Potential decoupling of CO2 and Hg uptake process by global vegetation in the 21st century

Mercury (Hg), a potent neurotoxin posing risks to human health, is cycled through vegetation uptake, which is susceptible to climate change impacts. However, the extent and pattern of these impacts are largely unknown, obstructing predictions of Hg's fate in terrestrial ecosystems. Here, we evaluate the effects of climate change on vegetation elemental Hg [Hg(0)] uptake using a state-of-the-art global terrestrial Hg model (CLM5-Hg) that incorporates plant physiology. In a business-as-usual scenario, the terrestrial Hg(0) sink is predicted to decrease by 1870 Mg yr-1 in 2100, that is ~60% lower than the present-day condition. We find a potential decoupling between the trends of CO2 assimilation and Hg(0) uptake process by vegetation in the 21st century, caused by the decreased stomatal conductance with increasing CO2. This implies a substantial influx of Hg into aquatic ecosystems, posing an elevated threat that warrants consideration during the evaluation of the effectiveness of the Minamata Convention.

Assessment of tree-ring mercury radial translocation and age effect in Masson pine: Implications for historical atmospheric mercury reconstruction

The tree ring has been regarded as an emerging archive to reconstruct historical atmospheric mercury (Hg) trends, but with the large knowledge gaps in the reliability. In this study, we comprehensively evaluated the Hg source, radial translocation and age effect of Masson pine (Pinus massoniana) tree ring at Mt. Jinyun in Chongqing, to assess the suitability of such tree ring as the archive of atmospheric Hg. Results showed that distinct variabilities among Masson pine tree-ring Hg concentration profiles. The Hg concentration significantly increased along with stem height (P < 0.05), indicating the Hg in tree rings mainly derived from foliage uptake atmospheric Hg. We found a distinct age effect that the tree ring of young trees had the higher Hg concentration. Besides, we used the advection-diffusion model to demonstrate how Hg concentration shifted by the advection or/and diffusion in tree rings. The modeling results showed that the advection induced radial translocation during the young growth period of tree was a plausible mechanism to result in the tree-ring Hg record largely different from the trend of anthropogenic Hg emissions in Chongqing. We finally suggest that in further Hg dendrochemistry, better discarding the tree-ring Hg profile of the young growth period to reduce impacts of the radial translocation and age effect.

Human Health Risk Assessment from Mercury-Contaminated Soil and Water in Abu Hamad Mining Market, Sudan

Artisanal and small-scale gold mining (ASGM) poses a significant global threat due to mercury emissions and resulting health hazards. This study focuses on assessing these risks in the Abu Hamad ASGM community in Sudan. Utilizing the Mercury Analyzer 3000 (NIC), analyses of twelve soil samples (including one tailings sample) and seven water samples revealed the highest concentrations near amalgam burning locations: 34.8 mg/kg in soil (S06) and 3.26 µg/L in water (W03). Concentrations decrease with distance, with soil near burning exceeding tailings (S05 = 19.0 mg/kg). Hazard quotients indicate mercury vapor inhalation as the primary exposure route from soil, with the Hazard Index reaching 5.34 for adults and 33.4 for children close to amalgam burning sites. Water samples generally pose little risk except for W03, where children face potential danger via ingestion (HI = 1.74). These findings emphasize the urgent need for adopting retorts and eco-friendly practices to reduce mercury emissions and protect ASGM communities.

Distribution characteristics of mercury concentration and estimation of mercury pools in different age groups of Larix gmelinii forests of Daxing'an Mountain

Forests are important sinks of atmospheric mercury. Quantifying mercury pools in forest ecosystem tissues are essential for understanding the global mercury cycle. To reveal the characteristics of Hg concentration and Hg pool distribution in natural forests at different ages, samples from the vegetation layer, organic horizons, coarse wood debris, and mineral soil layers were collected in young forest, middle forest, near-mature forest, and mature forest of Larix gmelinii forests at the Daxing'an Mountain. The results showed that there were differences in the absorption and accumulation of Hg by different tree species and tissues. In Larix gmelinii, the concentration of Hg followed the order of bark > branch > leaf > root > core, whereas in Betula platyphylla, the order was bark > leaf > branch > root > core. The mercury concentration in the organic horizons increased gradually with the decomposition process. There were no obvious regular patterns in the mercury concentrations of each tissue in different age groups Larix gmelinii forests. Furthermore, total biomass mercury pools (overstory, shrub layer, herb layer, moss layer, and coarse woody debris （CWD）) in the young, middle, near-mature, and mature forests of Larix gmelinii forests at Daxing'an Mountain were estimated to be 99.0 μg m-2，207 μg m-2，207 μg m-2 and 194 μg m-2, respectively. On ecosystem scale, total mercury pools were 16.9 mg m-2 (young), 27.5 mg m-2 (middle), 17.0 mg m-2 (near-mature), and 11.8 mg m-2(mature). The mineral soil mercury pool accounts for 94.0%-98.1% of the total ecosystem mercury pool, and its mercury pool proportion gradually decreased with the increase in forest age. These obtained results are quite valuable for further assessing the role of forest ecosystems in the atmospheric mercury cycle and estimating potential mercury emissions from biomass burning during forest wildfires.

Tree rings as historical archives of atmospheric mercury: A critical review

Historical concentrations of atmospheric mercury (Hg) are uncertain, as monitoring only began a few decades ago. Tree rings can serve as historical archives of Hg, providing centennial trends. The vast majority of tree-ring Hg studies have been published in the last decade, demonstrating the growing use of tree rings for Hg dendrochemistry. Thus, there is a need for a systematic review on current knowledge of tree rings as archives of atmospheric Hg. In this review, the predominant pathways of Hg uptake to tree rings are discussed, including the initial Hg uptake from the surrounding environment, fixation, and subsequent translocation. Foliar uptake of Hg was found to be the most important uptake route for Hg in tree rings, the root and bark route being negligible. Our summary of the suitability of different tree species indicates that radial translocation is the biggest limiting factor for Hg dendrochemistry, shifting and blurring historical Hg trends. Based on the review findings, Picea (spruce) and Larix (larch) are the most promising genera for Hg dendrochemistry. Additionally, the use of tree-ring Hg archives in combination with other co-located archives, namely lake sediments, peat, and ice, is suggested as it enhances the viability of observed tree-ring historical Hg trends. Finally, we propose future directions and recommendations for research using tree-ring Hg, including sampling protocols, experimental designs, and tree selection.

Pinus nigra bark from a mercury mining district studied with high resolution XANES spectroscopy

Tree bark near former mercury (Hg) mines and roasting plants is known to have exceptionally high (up to several mg kg^-1) Hg concentrations. This study explores the change of Hg speciation with depth (down to 25-30 mm from the outermost surface) in black pine (Pinus nigra) bark by means of high-resolution X-ray absorption near edge structure (HR-XANES) spectroscopy at the Hg L_III-edge. Principal component analysis and linear combination fitting applied to the HR-XANES spectra suggested that in the outermost layer (∼0-2 mm from the surface), roughly 50% of Hg is in the form of nanoparticulate metacinnabar (nano-β-HgS). A progressive increase in Hg-organic species (Hg bound to thiol groups) is found in deeper bark layers, while nano-β-HgS may decrease below the detection limit in the deepest layers. Notably, bark layers did not contain cinnabar (α-HgS), which was found in the nearby soils along with β-HgS (bulk), nor Hg⁰, which is the main Hg species in the atmosphere surrounding the sampled trees. These observations suggested that nano-β-HgS, at least in part, does not originate from mechanically trapped wind-blown particulates from the surrounding soil, but may be the product of biochemical reactions between gaseous elemental Hg and the bark tissue.

Reconstructing atmospheric Hg levels near the oldest chemical factory in central Europe using a tree ring archive

The Chemical Factory in Marktredwitz (CFM) is known as the oldest chemical factory in Germany (1778-1985), and from the beginning of the 20^th century focused primarily on the production of mercury (Hg) compounds. Due to extensive pollution, together with employee health issues, the CFM was shut in 1985 by a government order and remediation works proceeded from 1986 to 1993. In this study, tree ring archives of European Larch (Larix decidua Mill.) were used to reconstruct changes of air Hg levels near the CFM. Mercury concentrations in larch boles decreased from 80.6 μg kg^-1 at a distance of 0.34 km-3.4 μg kg^-1 at a distance of 16 km. The temporal trend of atmospheric Hg emissions from the CFM reconstructed from the tree ring archives showed two main peaks. The first was in the 1920s, with a maximum tree ring Hg concentration 249.1 ± 43.9 μg kg^-1 coinciding with when the factory had a worldwide monopoly on the production of Hg-based seed dressing fungicide. The second peak in the 1970s, with a maximum tree ring Hg concentration of 116.4 ± 6.3 μg kg^-1, was associated with a peak in the general usage and production of Hg chemicals and goods. We used the tree ring record to reconstruct past atmospheric Hg levels using a simple model of Hg distribution between the larch tree rings and atmosphere. The precision of the tree ring model was checked against the results of air Hg measurements during the CFM remediation 30 years ago. According to the tree ring archives, the highest air Hg concentrations in the 1920s in Marktredwitz were over 70 ng m^-3. Current air Hg levels of 1.18 ng m^-3, assessed in the city of Marktredwitz, indicate the lowest air Hg in the past 150 years, underscoring the effective remediation of the CFM premises 30 years ago.

The tree-ring mercury record of Klondike gold mining at Bear Creek, central Yukon

Use of elemental mercury (Hg⁰) to enhance placer gold recovery is an effective method dating back centuries, but is associated with significant atmospheric Hg⁰ losses. This method was widely used in the Canadian Klondike region during most of the 20th century when the mining industry experienced rapid growth. While the health risks associated with Hg⁰ pollution are now well understood, few studies have assessed the environmental legacy of Hg⁰ use in the Klondike. We used an annually resolved Picea glauca tree-ring Hg record (1864-2015) to reconstruct and evaluate changes in local atmospheric Hg⁰ concentrations associated with gold production at the Bear Creek mining camp. Major temporal trends in the record are consistent with the scale of Bear Creek operations and are distinct from background trends at an unimpacted control site. Tree-ring Hg concentration increased most rapidly from 1923 to 1930, a period when several major mining operations were consolidated at Bear Creek. The highest Hg concentrations, ∼2.5× greater than pre-mining era, occurred in the 1930s, coinciding with maximum gold production at this site. Post-World War II economic factors adversely affected the industry, causing declining tree-ring Hg concentrations from 1939 to 1966. Closure of the Bear Creek camp in 1966 coincided with the strongest tree-ring Hg decline, although a return to background levels did not occur until the 1990s, likely due to re-emission of legacy Hg⁰ from contaminated soils. Finally, a robust increase was observed over the last decade, similar to other tree-ring Hg records in N.W. Canada, which is linked to rising Hg⁰ emissions in Asia. The Bear Creek tree-ring Hg record provides a unique opportunity to study the impact of Klondike gold mining on the local environment at annual resolution and demonstrates great potential to use Picea tree rings to study past changes in atmospheric Hg⁰ from local and global emissions. MAIN FINDINGS: A 151-year long, annually resolved tree-ring Hg record was developed at a historic Klondike gold-mining site to investigate the influence of mining-related Hg⁰ emissions on the local atmosphere and environment. Compared to a control site, the tree-ring Hg record documents highly elevated atmospheric Hg⁰ concentrations during the period mining activities were ongoing at this site.

Comparison of co-located ice-core and tree-ring mercury records indicates potential radial translocation of mercury in whitebark pine

Tree-ring records are a potential archive for reconstructing long-term historical trends in atmospheric mercury (Hg) concentrations. Although Hg preserved in tree rings has been shown to be derived largely from the atmosphere, quantitative relationships linking atmospheric concentrations to those in tree rings are limited. In addition, few tree-ring-based Hg records have been evaluated against co-located proxies of atmospheric Hg deposition or direct atmospheric measurements. Here we develop long-term Hg records extending from 1800 to 2018 CE using cores collected from two stands of whitebark pine located near the Upper Fremont Glacier in the Wind River Range, Wyoming, where a long-term record of atmospheric Hg deposition previously was developed from an ice core. The tree ring record showed that Hg concentrations increased beginning in 1800 CE to a broad peak centered at ~1960 CE, before decreasing to present, generally paralleling the ice-core record of Hg deposition. The exact timing and magnitude of the Hg increases in the trees, however, is offset earlier relative to the ice-core record. These discrepancies potentially arise from biotic processes that impact Hg uptake and preservation in whitebark pine, and results from an advection-diffusion model indicate that the temporal differences are consistent with radial movement of Hg within the trees. The forms of atmospheric Hg and seasonality may also impact the Hg record preserved by each archive, but are less likely to affect long-term trends. Further work is needed to assess radial Hg translocation in more controlled studies with larger sample sizes.

Determination of Metals in Tree Rings by ICP-MS Using Ash from a Direct Mercury Analyzer

Elemental profiles in cores of tree trunks (bole wood) have been used for environmental monitoring and reconstruction of metal pollution history. Mercury (Hg) is a global pollutant that can be accurately measured in tree rings in a simple and pragmatic fashion using a direct mercury analyzer (DMA) that is based on thermal decomposition, amalgamation, and atomic absorption spectrophotometry. In this feasibility study, we demonstrate that the ash remaining after the DMA analyses can be used to quantify a wide range of other non-volatile elements (Ba, Be, Co, Cr, Cu, Fe, Ga, Mg, Mn, Ni, Pb, Sr, Th, and U) in that same sample of wood by inductively coupled plasma mass spectrometry (ICP-MS) after microwave-assisted acid digestion. Other elements (Ag, Cd, Cs, Rb, Tl, and V) exhibited poor recoveries, possibly due to losses during sample preparation. We assessed the accuracy with reference materials, spikes, and by comparison with EPA Method 3052 (Microwave Assisted Acid Digestion of Siliceous and Organically Based Matrices). For the first group of elements (deemed suitable for the method), recoveries ranged between 80% and 120% and the relative standard deviation was generally < 15%, indicating acceptable precision. We applied the method to five species of trees: eastern red cedar (Juniperus virginiana), loblolly pine (Pinus taeda), shortleaf pine (Pinus echinata), white oak (Quercus alba), and tulip poplar (Liriodendron tulipifera) from Holly Springs National Forest in north Mississippi, USA. Mercury concentrations (ng/g ± SE) were highest in the cedar (1.8 ± 0.3; n = 5), followed by loblolly pine (1.6 ± 0.3, n = 3), shortleaf pine (1.2 ± 0.2; n = 3), oak (1.1 ± 0.2; n = 5), and poplar (0.5 ± 0.1; n = 5). Concentrations of other elements were generally Fe > Mg > Ba ≈ Sr ≈ Mn > Cr ≈ Cu > Ni ≈ Rb > Co > Ga ≈ Ag, with the other elements generally below the method detection limit (MDL). Overall, we showed that the DMA can be used to not only determine total Hg in segments of tree core, but can serve as the ashing step in the preparation of wood for ICP-MS analysis, thus allowing the determination of non-volatile elements along with Hg in the very same sample.

Using Tree Rings to Track Atmospheric Mercury Pollution in Australia: The Legacy of Mining in Tasmania

Historical records of mercury (Hg) deposition in lake sediments have commonly been used to monitor historic atmospheric concentrations. In the Australian environment, however, freshwater lakes are limited, restricting the region for which depositional archives of Hg can be derived. In this study we show that dendrochemistry can provide a record of atmospheric concentrations at very high resolution. We measured Hg concentrations in growth rings of two tree species from a site in western in Tasmania-Huon Pine ( Lagarostrobus franklinii) and Celery Top Pine ( Phyllocladus aspleniifolius). This region has been heavily mined over the past 150 years. Although much previous work has linked atmospheric Hg to gold mining, the evidence in this study suggests that copper smelters in Queenstown and Zeehan, not gold mining activities, were the main sources of Hg emissions to the atmosphere in this location. Huon Pine had significantly higher background Hg concentrations ( x̅ = 5.62 ng/g) than Celery Top Pine ( x̅ = 2.95 ng/g). No significant increase in Hg concentration during the peak copper smelting phase (1896 to 1935) was observed in Celery Top Pine, while a significant 1.4 fold-increase was observed in Huon Pine. Our results show that of species examined across the globe, Huon Pine is one of the most efficient bioaccumulators of Hg, making it a good proxy for tracking historical Hg emissions in western Tasmania. This ability to measure Hg in the environment is essential if Australia ratifies the Minamata Convention.

Total Mercury in Plant Tissue from a Mining Landscape in Western Mexico

Environmental impacts of mining activities are well known, particularly on-site degradation, but long term effects are less known. Mercury content from vegetation samples from a mine dump and surrounding forests was quantified for understanding the fate of this element in the local the environment. The study area, Tlalpujahua, Michoacán, México, has a mining history going back more than 400 years. Including gold and silver extraction by means of mercury amalgamation for 352 years (1554-1906). Mercury was present in all sampled materials. The highest values correspond to wood samples from the mine dump (13.84 ± 3.88 ppm), while wood samples from adjacent forests had 4.3 ± 2.4 ppm, almost twice as much as coniferous needles, shrub leaves and corn seeds (2.2 ± 0.34 ppm). The highest concentration was found for J. deppeana wood (16.05 ± 2.3 ppm). The capacity of accumulating mercury by Juniperus trees when growing on the mine dumps suggests that this species has a potential to be used for biosequestration purposes.

Larch Tree Rings as a Tool for Reconstructing 20th Century Central European Atmospheric Mercury Trends

We propose the tree rings of European Larch ( Larix decidua) as a widely available and reliable geochemical archive of local and regional changes in atmospheric mercury (Hg). Mean Hg concentrations in larch tree rings from 8 background sites across the Czech Republic ranged from 2.2 to 4.8 μg kg^-1; the maximum concentrations occurred in the period 1951-1970. At 3 sites impacted by Hg-emission sources [gold amalgamation processing, caustic soda production, and lead (Pb) ore smelting] mean larch tree ring Hg concentrations were significantly elevated relative to background sites. Changes in larch tree ring Hg concentrations were temporally coherent with known activities at the sites that would alter Hg emissions; the nearly simultaneous response in tree rings indicated little or no translocation of Hg within the larch bole. Based on the present-day atmospheric Hg concentration of 1.63 ng m^-3 at the intensively monitored Czech Global Mercury Observation System site and the most recent mean tree ring Hg concentration of 2.8 μg kg^-1 in co-located larch trees, we developed a simple distribution model of Hg between the atmosphere and larch tree rings. We applied the model using observed changes of Hg in larch tree rings from the countrywide background sites to reconstruct past atmospheric Hg concentrations in central Europe. Modeled Hg concentrations were in agreement with annual means from the European Monitoring and Evaluation Programme observatories.

400-Year Record of Atmospheric Mercury from Tree-Rings in Northwestern Canada

Tree-rings are a promising high-resolution archive for gaseous atmospheric mercury (composed primarily of Hg⁰) reconstruction, but the influence of cambial age (ring number from pith) and tree-specific differences are uncertainties with potential implications for interpreting tree-ring Hg signals. We address these uncertainties and reconstruct the last 400 years of Hg⁰ change using a tree-ring Hg data set from 20 white spruce ( Picea glauca) trees from a pristine site in central Yukon. Cambial age has no significant influence on tree-ring Hg concentration, but tree-specific differences in mean concentration are prevalent and must be normalized to a common mean to accurately constrain long-term trends in the mean tree-ring Hg record. Our record shows stable, low Hg⁰ concentrations prior to ∼1750 CE, a persistent rise from ∼1750-1950 (increasing more rapidly post-1850), a pause from ∼1951-1975, and then a resumed increase to record-high levels at present. This general pattern is reflected in other proxy-based Hg reconstructions worldwide. This study provides a novel long-term Hg⁰ reconstruction in the Western subarctic from one of the most widely distributed boreal tree species in North America and, therefore this proxy may also hold potential for investigating broader spatial patterns in Hg⁰ cycling across the subarctic and northern boreal forest.

Wood and bark of Pinus halepensis as archives of heavy metal pollution in the Mediterranean Region

Natural levels of heavy metals (HM) have increased during the industrial era to the point of posing a serious threat to the environment. The use of tree species to record contamination is a well-known practice. The objective of the study was to compare HM levels under different pollution conditions: a) soil pollution due to mining waste; b) atmospheric pollution due to coal-fired power plant emissions. We report significant HM enrichment in Pinus halepensis tissues. Near a burning power plant, Pb content in a tree wood was 2.5-fold higher that in natural areas (no pollution; NP). In mining areas, Cd content was 25-fold higher than NP. The hypothesis that HM contents in tree rings should register pollution is debatable. HM uptake by pines from soil, detoxification mechanisms and resuspended local soil dust is involved in HM contents in wood and bark.

Concentrations and content of mercury in bark, wood, and leaves in hardwoods and conifers in four forested sites in the northeastern USA

Mercury (Hg) is deposited from the atmosphere to remote areas such as forests, but the amount of Hg in trees is not well known. To determine the importance of Hg in trees, we analyzed foliage, bark and bole wood of eight tree species at four sites in the northeastern USA (Huntington Forest, NY; Sleepers River, VT; Hubbard Brook, NH; Bear Brook, ME). Foliar concentrations of Hg averaged 16.3 ng g-1 among the hardwood species, which was significantly lower than values in conifers, which averaged 28.6 ng g-1 (p < 0.001). Similarly, bark concentrations of Hg were lower (p < 0.001) in hardwoods (7.7 ng g-1) than conifers (22.5 ng g-1). For wood, concentrations of Hg were higher in yellow birch (2.1-2.8 ng g-1) and white pine (2.3 ng g-1) than in the other species, which averaged 1.4 ng g-1 (p < 0.0001). Sites differed significantly in Hg concentrations of foliage and bark (p = 0.02), which are directly exposed to the atmosphere, but the concentration of Hg in wood depended more on species (p < 0.001) than site (p = 0.60). The Hg contents of tree tissues in hardwood stands, estimated from modeled biomass and measured concentrations at each site, were higher in bark (mean of 0.10 g ha-1) and wood (0.16 g ha-1) than in foliage (0.06 g ha-1). In conifer stands, because foliar concentrations were higher, the foliar pool tended to be more important. Quantifying Hg in tree tissues is essential to understanding the pools and fluxes of Hg in forest ecosystems.

Distribution of Mercury Concentrations in Tree Rings and Surface Soils Adjacent to a Phosphate Fertilizer Plant in Southern Korea

This study aimed to determine mercury concentrations in tree rings and surface soils at distances of 4, 26 and 40 km from a fertilizer plant located in Yeosu City, Korea. Mercury concentrations in all tree rings were low prior to the establishment of the plant in 1977 and became elevated thereafter. The highest average mercury concentration in the tree rings was 11.96 ng g^-1 at the Yeosu site located nearest to the plant, with the lowest average mercury concentration of 4.45 ng g^-1 at the Suncheon site furthest away from the plant. In addition, the highest mercury content in the surface soil was 108.51 ng cm^-3 at the Yeosu site, whereas the lowest mercury content in the surface soil was 31.47 ng cm^-3 at the Suncheon site. The mercury levels decreased gradually with increasing distance from the plant.

Mercury speciation in Pinus nigra barks from Monte Amiata (Italy): An X-ray absorption spectroscopy study

This study determined, by means of X-ray absorption near-edge structure (XANES) spectroscopy, the speciation of mercury (Hg) in black pine (Pinus nigra) barks from Monte Amiata, that were previously shown to contain exceptionally high (up to some mg kg^-1) Hg contents because of the proximity to the former Hg mines and roasting plants. Linear fit combination (LCF) analysis of the experimental spectra compared to a large set of reference compounds showed that all spectra can be fitted by only four species: β-HgS (metacinnabar), Hg-cysteine, Hg bound to tannic acid, and Hg⁰. The first two are more widespread, whereas the last two occur in one sample only; the contribution of organic species is higher in deeper layers of barks than in the outermost ones. We interpret these results to suggest that, during interaction of barks with airborne Hg, the metal is initially mechanically captured at the bark surface as particulate, or physically adsorbed as gaseous species, but eventually a stable chemical bond is established with organic ligands of the substrate. As a consequence, we suggest that deep bark Hg may be a good proxy for long term time-integrated exposure, while surface bark Hg is more important for recording short term events near Hg point sources.

The history of mercury pollution near the Spolana chlor-alkali plant (Neratovice, Czech Republic) as recorded by Scots pine tree rings and other bioindicators

We assessed >100years of mercury (Hg) pollution recorded in the tree rings of Scots Pine near a Czech chlor-alkali plant operating since 1941. Hg concentrations in tree rings increased with the launching of plant operations and decreased when Hg emissions decreased in 1975 due to an upgrade in production technology. Similar to traditional bioindicators of pollution such as pine needles, bark and forest floor humus, Hg concentrations in Scots Pine boles decreased with distance from the plant. Mean Hg in pine bole in the 1940s ranged from 32.5μg/kg Hg at a distance of 0.5km from the plant to 5.4μg/kg at a distance of >4.7km, where tree ring Hg was the same as at a reference site, and other bioindicators also suggest that the effect of the plant was no longer discernible. Tree ring Hg concentrations decreased by 8-29μg/kg since the 1940s at all study sites including the reference site. The lack of exact correspondence between changes at the plant and tree ring Hg indicated some smearing of the signal due to lateral translocation of Hg from sapwood to heartwood. Bole Hg concentrations reflected local and regional atmospheric Hg concentrations, and not Hg wet deposition.

Black pine (Pinus nigra) barks as biomonitors of airborne mercury pollution

Tree barks are relevant interfaces between plants and the external environment, and can effectively retain airborne particles and elements at their surface. In this paper we have studied the distribution of mercury (Hg) in soils and in black pine (Pinus nigra) barks from the Mt. Amiata Hg district in southern Tuscany (Italy), where past Hg mining and present-day geothermal power plants affect local atmospheric Hg concentration, posing serious environmental concerns. Barks collected in heavily Hg-polluted areas of the district display the highest Hg concentration ever reported in literature (8.6mg/kg). In comparison, barks of the same species collected in local reference areas and near geothermal power plants show much lower (range 19-803μg/kg) concentrations; even lower concentrations are observed at a "blank" site near the city of Florence (5-98μg/kg). Results show a general decrease of Hg concentration from bark surface inwards, in accordance with a deposition of airborne Hg, with minor contribution from systemic uptake from soils. Preliminary results indicate that bark Hg concentrations are comparable with values reported for lichens in the same areas, suggesting that tree barks may represent an additional useful tool for biomonitoring of airborne Hg.

Assessment of Global Mercury Deposition through Litterfall

There is a large uncertainty in the estimate of global dry deposition of atmospheric mercury (Hg). Hg deposition through litterfall represents an important input to terrestrial forest ecosystems via cumulative uptake of atmospheric Hg (most Hg(0)) to foliage. In this study, we estimate the quantity of global Hg deposition through litterfall using statistical modeling (Monte Carlo simulation) of published data sets of litterfall biomass production, tree density, and Hg concentration in litter samples. On the basis of the model results, the global annual Hg deposition through litterfall is estimated to be 1180 ± 710 Mg yr(-1), more than two times greater than the estimate by GEOS-Chem. Spatial distribution of Hg deposition through litterfall suggests that deposition flux decreases spatially from tropical to temperate and boreal regions. Approximately 70% of global Hg(0) dry deposition occurs in the tropical and subtropical regions. A major source of uncertainty in this study is the heterogeneous geospatial distribution of available data. More observational data in regions (Southeast Asia, Africa, and South America) where few data sets exist will greatly improve the accuracy of the current estimate. Given that the quantity of global Hg deposition via litterfall is typically 2-6 times higher than Hg(0) evasion from forest floor, global forest ecosystems represent a strong Hg(0) sink.

Dendrochemical assessment of mercury releases from a pond and dredged-sediment landfill impacted by a chlor-alkali plant

Although current Hg emissions from industrial activities may be accurately monitored, evidence of past releases to the atmosphere must rely on one or more environmental proxies. We used Hg concentrations in tree cores collected from poplars and willows to investigate the historical changes of Hg emissions from a dredged sediment landfill and compared them to a nearby control location. Our results demonstrated the potential value of using dendrochemistry to record historical Hg emissions from past industrial activities.

A review of passive sampling systems for ambient air mercury measurements

Atmospheric mercury (Hg) temporal and spatial patterns must be measured accurately in order to adequately understand the role of this pathway as it relates to Hg toxicity and exposure of humans and wildlife to Hg. It is also important to understand the distribution of the different chemical forms (elemental, oxidized, or particle bound) and specific compounds in air (e.g., HgCl2, HgBr2, HgO, Hg(NO3)2, and HgSO4). However, the current automated and passive sampling methods of measurement have limitations and artifacts impacting our ability to achieve this task. Both abiotic and biotic systems have been developed to measure the total gaseous Hg and oxidized Hg compounds (concentration and deposition). This study reviews and compares the performance of previously and currently applied passive sampling systems. Computable fluid dynamic modeling was conducted to gain additional understanding of a gaseous oxidized Hg (GOM) passive sampler. Case studies during which passive samplers were used are also presented to demonstrate the ability of passive samplers to capture atmospheric Hg variation. A network using passive samplers would be useful for monitoring global Hg trends due to the limits of the current automated method.

Pollutant plume delineation from tree core sampling using standardized ranks

There are currently contradicting results in the literature about the way chloroethene (CE) concentrations from tree core sampling correlate with those from groundwater measurements. This paper addresses this issue by focusing on groundwater and tree core datasets in CE contaminated site, Czech Republic. Preliminary analyses revealed strongly and positively skewed distributions for the tree core dataset, with an intra-tree variability accounting for more than 80% of the total variability, while the spatial analyses based on variograms indicated no obvious spatial pattern for CE concentration. Using rank transformation, it is shown how the results were improved by revealing the initially hidden spatial structure for both variables when they are handled separately. However, bivariate analyses based on cross-covariance functions still failed to indicate a clear spatial correlation between groundwater and tree core measurements. Nonetheless, tree core sampling and analysis proved to be a quick and inexpensive semi-quantitative method and a useful tool.

Cadmium stabilization with nursery stocks through transplantation: a new approach to phytoremediation

Disposal of heavy metal contaminated biomass after phytoremediation is still unfeasible. This paper presents a viable phyto-extraction approach in which metals in contaminated soils are stabilized by nursery stocks before transplantation for greening. In this respect, two pot-experiments are reported comparing seven nursery stocks species exposed to different Cd levels. The first experiment revealed that Cd was mainly stabilized in the roots of all species studied. Greater amounts of Cd were accumulated in the epidermis than cortex plus stele. Cupressus Blue Ice showed greatest tolerance to the 100 and 200 mg kg(-1) Cd stresses. The second experiment additionally evaluated the possible risk of Cd release after transplanting the Cd treated plants into uncontaminated soil. After 120 days of transplantation, the relatively trace amounts of Cd in the roots of Euonymus japonicus, Pittosporum tobira and C. Blue Ice had either been partially transferred into the shoots or released into the soil. The highest Cd concentration increase in bulk soil (0.428 mg kg(-1)), however, was much lower than the environmental quality standard for soils of China (1 mg kg(-1)). The potential effectiveness of this technique in the use of Cd-contaminated soil and further investigation needed in the field trials were also evaluated.

Potential sources of methylmercury in tree foliage

Litterfall is a major source of mercury (Hg) and toxic methylmercury (MeHg) to forest soils and influences exposures of wildlife in terrestrial and aquatic ecosystems. However, the origin of MeHg associated with tree foliage is largely unknown. We tested the hypothesis that leaf MeHg is influenced by root uptake and thereby related to MeHg levels in soils. Concentrations of MeHg and total Hg in deciduous and coniferous foliage were unrelated to those in soil at 30 urban and rural forested locations in southwest Ohio. In contrast, tree genera and trunk diameter were significant variables influencing Hg in leaves. The fraction of total Hg as MeHg averaged 0.4% and did not differ among tree genera. Given that uptake of atmospheric Hg(0) appears to be the dominant source of total Hg in foliage, we infer that MeHg is formed by in vivo transformation of Hg in proportion to the amount accumulated.