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

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.

Assessment of mercury bioavailability in garden soils around a former nonferrous metal mining area using DGT, accumulation in vegetables, and implications for health risk

Mercury (Hg) is a toxic, non-essential element for living organisms, frequently present in high concentrations in soils from industrial areas. The total, dissolved, and labile Hg concentrations in garden soils and their accumulation in edible vegetables (onion, garlic, lettuce, and parsley) grown on contaminated soils in localities situated a former mining area were evaluated. The labile Hg fraction was estimated by diffusive gradient in thin films (DGT). The soil-to-vegetable transfer factors, as well as the health risk by exposure to Hg, were calculated based on the labile Hg concentration in soil. The total Hg concentration in soil varied widely (0.11-3.77 mg kg-1), Hg in soil solution ranged between 2.14 and 20.2 μg L-1 and labile Hg between 1.13 and 18.6 μg L-1. About 36-96% (84% on average) of the Hg concentration in soil solution was found in labile form. Multivariate analysis revealed significant correlations between the labile Hg concentration in soil and Hg accumulated in vegetables. The hazard indices showed that, although the study area is affected by legacy pollution, exposure to soil and consumption of locally grown vegetables do not pose health risks.

Needle age and precipitation as drivers of Hg accumulation and deposition in coniferous forests from a southwestern European Atlantic region

Vegetation and climate are critical in the biogeochemical cycle of Hg in forest ecosystems. The study assesses the influence of needle age and precipitation on the accumulation of Hg in needle biomass and its deposition by litterfall in thirty-one pine plantations spread throughout two biogeographical regions in SW Europe. Well-developed branches of Pinus pinaster were sampled and pine needles were classified according to 4 age classes (y₀, y₁, y₂, y₃). The concentration of total Hg (THg) was analyzed in the samples and Hg content in needle biomass and its deposition by litterfall were estimated. The concentration of total Hg (THg) increased with needle age ranging from 9.1 to 32.7 μg Hg kg^-1 in the youngest and oldest needles, respectively. The rate of Hg uptake (Hg_R) three years after needle sprouting was 10.2 ± 2.3 μg Hg kg^-1 yr^-1, but it decreased with needle age probably due to a diminution in photosynthetic activity as needles get older. The average total Hg stored in needle biomass (Hg_Wt) ranged from 5.6 to 87.8 mg Hg ha^-1, with intermediate needle age classes (y₁ and y₂) accounting for 70% of the total Hg stored in the whole needle biomass. The average deposition flux of Hg through needle litterfall (Hg_Lt) was 1.5 μg Hg m^-2 yr^-1, with the y₂ and y₃ needles contributing most to the total Hg flux. The spatial variation of THg, Hg_Wt and Hg_Lt decreased from coastal pine stands, characterized by an oceanic climate, to inland pine stands, a feature closely related to the dominant precipitation regime in the study area. Climatic conditions and needle age are the main factors affecting Hg accumulation in tree foliage, and should be considered for an accurate assessment of forest Hg pools at a regional scale and their potential consequences in the functioning of terrestrial ecosystems.

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.

Canopy-Level Flux and Vertical Gradients of Hg0 Stable Isotopes in Remote Evergreen Broadleaf Forest Show Year-Around Net Hg0 Deposition

Vegetation uptake represents the dominant route of Hg input to terrestrial ecosystems. However, this plant-directed sink is poorly constrained due to the challenges in measuring the net Hg⁰ exchange on the ecosystem scale over a long period. Particularly important is the contribution in the subtropics/tropics, where the bulk (∼70%) of the Hg⁰ deposition is considered to occur. Using the relaxed eddy accumulation technique, this study presents for the first time a whole ecosystem Hg⁰ flux record over an evergreen hardwood forest. This tower-based micrometeorological method gauged a cumulated net Hg⁰ flux of -41.1 μg m^-2 over 16 months, suggesting that the subtropical montane forest acts as a large and continuous sink of atmospheric Hg⁰. The monthly net fluxes were consistently negative (-7.3 to -1.0 μg m^-2 month^-1) throughout the year, with the smallest absolute values occurring during the mild and dry subseason in spring, which was also the annual lowest in vegetation activity. Colocated measurements of multilevel gradients of Hg⁰ concentration and its stable isotopic composition support the finding of year-round Hg⁰ deposition. The stable Hg isotope measurements also show that in-canopy bi-directional Hg⁰ exchange is prevalent.

Atmospheric Hg Levels in Tree Barks Due to Artisanal Small-Scale Gold Mining Activity in Bunut Seberang Village in Indonesia

Mercury (Hg) is a useful heavy metal; however, it is toxic to both humans and the environment. Tree bark is an excellent bioindicator, which has been proven to be effective in studying the level of atmospheric Hg contamination. This study aimed to determine the distribution of evaporated Hg using the total weight of Hg (THg) in tree barks in Indonesia at the artisanal and small-scale gold mining (ASGM) area of Bunut Seberang Village and Lampung University, respectively. Samples were taken using purposive sampling, based on the criteria of forestry trees at a height level of 1.3 m above ground as wide as 100 cm2. The samples were analyzed by atomic absorption spectrometry and Scanning electron microscopy to determine the levels of THg and to investigate the bark structures. Results showed that the highest THg values were found in a Magnolia champaca sample (56.5 µg), followed by Swietenia mahagoni (45.8 µg) and Swietenia mahagoni (33.5 µg). All species studied showed THg levels in the tree barks at an elevation from 30 to 320 m above sea level. The Hg amounts found in the sampled barks indicated the dispersion of Hg throughout the ASGM area, which signified hazardous atmospheric conditions in the area.

Mapping Atmospheric Mercury in Lampung Province, Indonesia Using Bark of Multipurpose Tree Species

The use of mercury in gold refining causes air pollution and results in the contamination of multipurpose tree species (MPTS). Tree bark has properties that cause it to store mercury for quite a long time. The purpose of this study was to determine mercury contamination of MPTS and map the mercury contamination distribution in the atmosphere using tree barks as bioindicators. Sampling was performed using purposive sampling. The mercury concentration was obtained by atomic absorption spectroscopy, and the highest THg contents were analyzed using a scanning electron microscope. The analysis was carried out by gauging total mercury (THg), distance, elevation to THg, and interpolation of THg at the research site. The results showed that there were 10 types of MPTS trees whose bark could accumulate mercury. The bark of the Tamarindus indica tree stored the greatest amount of THg (74.4 µg dry weight (DW)), followed by Persea americana (58.7 µg DW), and Annona muricata (44.2 µg DW), respectively. This result was influenced by the roughness of the bark and the location of the plants. No correlation was found between distance and elevation to THg on tree bark. The mercury interpolation in the atmosphere showed that mercury moves from the purification point to the southeast of the purification location.

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.

Monitoring of Airborne Mercury: Comparison of Different Techniques in the Monte Amiata District, Southern Tuscany, Italy

In the present study, mercury (Hg) concentrations were investigated in lichens (Flavoparmelia caperata (L.) Hale, Parmelia saxatilis (L.) Ach., and Xanthoria parietina (L.) Th.Fr.) collected in the surrounding of the dismissed Abbadia San Salvatore Hg mine (Monte Amiata district, Italy). Results were integrated with Hg concentrations in tree barks and literature data of gaseous Hg levels determined by passive air samplers (PASs) in the same area. The ultimate goal was to compare results obtained by the three monitoring techniques to evaluate potential mismatches. Lichens displayed 180–3600 ng/g Hg, and Hg concentrations decreased exponentially with distance from the mine. Mercury concentration was lower than in Pinus nigra barks at the same site. There was a moderate correlation between Hg in lichen and Hg in bark, suggesting similar mechanisms of Hg uptake and residence times. However, correlation with published gaseous Hg concentrations (PASs) was moderate at best (Kendall Tau = 0.4–0.5, p > 0.05). The differences occurred because a) PASs collected gaseous Hg, whereas lichens and barks also picked up particulate Hg, and b) lichens and bark had a dynamic exchange with the atmosphere. Lichen, bark, and PAS outline different and complementary aspects of airborne Hg content and efficient monitoring programs in contaminated areas would benefit from the integration of data from different techniques.

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.

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.