Natural and anthropogenic impacts on mercury accumulation in Xiaohai Lagoon, South China over the last 1200 years

Anthropogenic use and release of mercury (Hg) have profoundly affected the global Hg cycle since preindustrial times. However, it is often difficult to quantify the relative contributions of natural and anthropogenic factors to environmental Hg accumulation. Here, we have presented a 1200-year record of Hg deposition in a sediment core from the Xiaohai Lagoon (South China), in combination with multiple environmental indicators (e.g., geochemical elements, grain size and total organic carbon, etc.). Using principal component analysis (PCA) and stepwise regression analysis (SRA), we aimed to explore the latent processes governing the accumulation of Hg over time and to quantitatively assess the natural and anthropogenic impacts on Hg deposition over the last millennium in Xiaohai Lagoon. Our results have demonstrated that between ∼ 870 and ∼ 1860 CE, natural factors were the main drivers controlling Hg concentrations in the lagoon. These were directly driven by higher soil erosion and increased inputs of fine-grained matter. However, from 1860 to 2013 CE, enhanced anthropogenic activities played a significant role in Hg accumulation in Xiaohai Lagoon. Anthropogenic Hg fluxes increased significantly from ∼ 1860 CE, peaked several times during the 1860s to the 1950s, accelerated from the late 1950s to the early 2000s, and then declined gradually owing to the stringent environmental protection strategies and efficient pollutant control technologies. Our results have suggested that the increased anthropogenic Hg inputs between the 1860s and mid-1970s were mostly attributed to wars, the "Westernization Movement", and global industrial activity, with a surge mainly after the 1980s dominated by industrial activities in China and numerous developing countries in Southeast Asia. This study has shown the natural and anthropogenic influences associated with mercury pollution through quantitative analysis and can deepen our understanding of the processes and mechanisms of mercury deposition in natural environments under the influence of human activities.

Predominant contributions through lichen and fine litter to litterfall mercury deposition in a subalpine forest

Litterfall, typically referring to needles/leaves, may stand for >50% of the total mercury (Hg) deposition in forest ecosystems. By detailed categorisation, we reveal for the first time that the contributions through lichens and fine litter, together 9.98 μg Hg m^-2 yr^-1, could be as high as that in needle litter (9.96 μg m^-2 yr^-1) to the annual total Hg deposition (44.6 μg m^-2 yr^-1) in a subalpine forest in Switzerland. Noticeably, needle litter had the highest contribution (53%) to total Hg in the autumn litterfall but lichens and fine litter together predominated in other seasons (47-59%). Such a seasonal pattern is caused by the high ability of lichens and fine litter to accumulate Hg and the high needle litterfall in autumn, which is related to a good rainfall in summer followed by a dry period in autumn. The constantly higher Hg levels in lichens and fine litter than in needle litter together with similar seasonal patterns of litterfall during 2009-2019 and rainfall during 1980-2019 suggest that our finding can be generally valid. Here, we highlight not only the considerable role of non-needle litterfall in Hg deposition but also the association with weather for seasonal Hg dynamics in different litterfall components.

Mercury accumulation and dynamics in montane forests along an elevation gradient in Southwest China

Understanding atmospheric mercury (Hg) accumulation in remote montane forests is critical to assess the Hg ecological risk to wildlife and human health. To quantify impacts of vegetation, climatic and topographic factors on Hg accumulation in montane forests, we assessed the Hg distribution and stoichiometric relations among Hg, carbon (C), and nitrogen (N) in four forest types along the elevation of Mt. Gongga. Our results show that Hg concentration in plant tissues follows the descending order of litter > leaf, bark > root > branch > bole wood, indicating the importance of atmospheric Hg uptake by foliage for Hg accumulation in plants. The foliar Hg/C (from 237.0 ± 171.4 to 56.8 ± 27.7 µg/kg) and Hg/N (from 7.5 ± 3.9 to 2.5 ± 1.2 mg/kg) both decrease along the elevation. These elevation gradients are caused by the heterogeneity of vegetation uptake of atmospheric Hg and the variation of atmospheric Hg° concentrations at different altitudes. Organic soil Hg accumulation is controlled by forest types, topographic and climatic factors, with the highest concentration in the mixed forest (244.9 ± 55.7 µg/kg) and the lowest value in the alpine forest (151.9 ± 44.5 µg/kg). Further analysis suggests that soil Hg is positively correlated to C (r² = 0.66) and N (r² = 0.57), and Hg/C and Hg/N both increase with the soil depth. These stoichiometric relations highlight the combined effects from environmental and climatic factors which mediating legacy Hg accumulation and selective Hg absorption during processes of organic soil mineralization.

Increase of litterfall mercury input and sequestration during decomposition with a montane elevation in Southwest China

Litterfall mercury (Hg) input has been regarded as the dominant Hg source in montane forest floor. To depict combining effects of vegetation, climate and topography on accumulation of Hg in montane forests, we comprehensively quantified litterfall Hg deposition and decomposition in a serial of subtropical forests along an elevation gradient on both leeward and windward slopes of Mt. Ailao, Southwest China. Results showed that the average litterfall Hg deposition increased from 12.0 ± 4.2 μg m^-2 yr^-1 in dry-hot valley shrub at 850-1000 m, 14.9 ± 6.8 μg m^-2 yr^-1 in mixed conifer-broadleaf forest at 1250-2400 m, to 23.1 ± 8.3 μg m^-2 yr^-1 in evergreen broadleaf forest at 2500-2650 m. Additionally, the windward slope forests had a significantly higher litterfall Hg depositions at the same altitude because the larger precipitation promoted the greater litterfall biomass production. The one-year litter Hg decomposition showed that the Hg mass of litter in dry-hot valley shrub decreased by 29%, while in mixed conifer-broadleaf and evergreen broadleaf forests increased by 22-48%. The dynamics of Hg in decomposing litter was controlled by the temperature mediated litter decomposition rate and the additional adsorption of environmental Hg during decomposition. Overall, our study highlights the litterfall mediated atmospheric mercury inputs and sequestration increase with the montane elevation, thus driving a Hg enhanced accumulation in the high montane forest.

Mercury consumption and human health: Linking pollution and social risk perception in the southeastern United States

The study of the relationships between freshwater organisms, pollution and public awareness has been little researched. The public's perception of risk from pollution is a fundamental component in determining consumer behavior and promoting healthy habits. For instance, understanding how consumers perceive the risks associated with pollution can help with adoption of safe behaviors to reduce the health hazard associated with pollutant exposure. This study focused on the southeastern United States, a region predicted to be exposed to high mercury stress by increasing mercury deposition and methylation. First, we placed our study region in the world map of regions more prone to suffer from increasing mercury stress in a climate change scenario. Second, mercury levels in fish tissues was quantified by direct mercury analyzer (DMA). Third, we explored human fish consumption habits and risk social perception, including willingness to adapt fish consumption based on two future hypothetical scenarios of mercury stress. From a global perspective, our analysis demonstrates that the southern US is one of five world areas of greatest conservation concern for mercury stress. In this region, the average mono-methyl mercury concentration in fish tissues exceeded the limits considered safe for human consumption. Even though many in the local population were aware of the health hazards associated with fish consumption, only women of reproductive age were willing to adopt safe consumption habits. Altogether, these results show how bringing together field data, social perceptions, and consumption habits can help in designing an adaptive strategy to confront mercury pollution. Although our results are for the United States, other world regions prone to suffer increasing mercury stress have been identified and should be the focus of future studies and prescriptions.

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.

Mercury accumulation in soil from atmospheric deposition in temperate steppe of Inner Mongolia, China

Mercury (Hg) is a toxic and persistent pollutant and has long-term impacts on ecological systems and human health. Coal-fired power plants (CFPPs) are the main source of anthropogenic Hg emission, and the emitted atmospheric Hg is deposited to the surrounding environments which causes soil pollution. To assess the effects of atmospheric Hg from CFPPs in China on the temperate steppe, Hg contents in the topsoil and subsoil were analyzed for samples collected from 80 sites in central Inner Mongolia during 2012-2015. The average content of Hg in topsoil and subsoil were 14.9 ± 10.4 μg kg^-1 and 8.9 ± 5.8 μg kg^-1, respectively. The principal components analysis (PCA) indicated that the soil organic matter content and atmospheric deposition were the main factors determining soil Hg content in Inner Mongolia. We used the power plant impact factor (PPIF) to evaluate the impacts of the surrounding CFPPs. The PPIF results showed the most positive correlation with Hg content in topsoil at more than 400 km distances, indicating that the contribution of the long-range transport of Hg emitted from CFPPs is regional in scale. Considering the potential of Hg accumulation in soil, long-term and regional measurements of soil Hg and stricter emission-limit standards for power plants should be implemented to control soil Hg pollution in China.

Source contribution analysis of mercury deposition using an enhanced CALPUFF-Hg in the central Pearl River Delta, China

Atmospheric mercury (Hg) poses human health and ecological risks once deposited and bio-accumulated through food chains. Source contribution analysis of Hg deposition is essential to formulating emission control strategies to alleviate the adverse impact of Hg release from anthropogenic sources. In this study, a Hg version of California Puff Dispersion Modeling (denoted as CALPUFF-Hg) system with added Hg environmental processes was implemented to simulate the Hg concentration and deposition in the central region of the Pearl River Delta (cPRD) at 1 km × 1 km resolution. The contributions of eight source sectors to Hg deposition were evaluated. Model results indicated that the emission from cement production was the largest contributor to Hg deposition, accounting for 13.0%, followed by coal-fired power plants (6.5%), non-ferrous metal smelting (5.4%), iron and steel production (3.5%), and municipal solid waste incineration (3.4%). The point sources that released a higher fraction of gaseous oxidized mercury, such as cement production and municipal solid waste incineration, were the most significant contributors to local deposition. In this intensive industrialized region, large point sources contributed 67-94% of total Hg deposition of 6 receptors which were the nearest grid-cells from top five Hg emitters of the domain and the largest municipal solid waste incinerator in Guangzhou. Based on the source apportionment results, cement production and the rapidly growing municipal solid waste incineration are identified as priority sectors for Hg emission control in the cPRD region.

Source Attribution for Mercury Deposition with an Updated Atmospheric Mercury Emission Inventory in the Pearl River Delta Region, China

We used CMAQ-Hg to simulate mercury pollution and identify main sources in the Pearl River Delta (PRD) with updated local emission inventory and latest regional and global emissions. The total anthropogenic mercury emissions in the PRD for 2014 were 11,939.6 kg. Power plants and industrial boilers were dominant sectors, responsible for 29.4 and 22.7%. We first compared model predictions and observations and the results showed a good performance. Then five scenarios with power plants (PP), municipal solid waste incineration (MSWI), industrial point sources (IP), natural sources (NAT), and boundary conditions (BCs) zeroed out separately were simulated and compared with the base case. BCs was responsible for over 30% of annual average mercury concentration and total deposition while NAT contributed around 15%. Among the anthropogenic sources, IP (22.9%) was dominant with a contribution over 20.0% and PP (18.9%) and MSWI (11.2%) ranked second and third. Results also showed that power plants were the most important emission sources in the central PRD, where the ultra-low emission for thermal power units need to be strengthened. In the northern and western PRD, cement and metal productions were priorities for mercury control. The fast growth of municipal solid waste incineration were also a key factor in the core areas. In addition, a coordinated regional mercury emission control was important for effectively controlling pollution. In the future, mercury emissions will decrease as control measures are strengthened, more attention should be paid to mercury deposition around the large point sources as high levels of pollution are observed.

Indoor and outdoor elemental mercury: a comparison of three different cases

Gaseous elemental mercury (GEM) concentrations were determined in three different indoor environments: an office in a building with no indoor sources of mercury (Bldg. I), an office affected by indoor mercury emissions from an adjacent laboratory (Bldg. II), and finally, an office where an outdoor mercury spill occurred accidentally (Bldg. III). The maximum recorded indoor GEM concentrations, with the largest variation in time, were observed in Bldg. II, with a continuous indoor mercury source (lower to upper quartile 15 to 62 ng m^-3). The lowest values were recorded in Bldg. I (lower to upper quartile 3 to 5 ng m^-3), where indoor GEM levels were affected mainly by the exhaust of vehicles in the parking lot of the building. The monitoring of GEM indoors (lower to upper quartile 15 to 42 ng m^-3), and outdoors (in several heights) of the Bldg. III, revealed that the cleaning up procedure that followed the spill was not adequate. Auxiliary measurements in the first two cases were the indoor microclimatic conditions, as well as the indoor CO₂ concentrations, and in the third case the outdoor meteorological data. The exhaust of vehicles, the chemical reagents, and an outdoor mercury spill were found to mainly affect the observed indoor GEM levels. People in Bldg. II and people walking through the area, where Hg⁰ was spilled, were found to be exposed to concentrations above some guide values.

Moss and lichen biomonitoring of atmospheric mercury: A review

Long-range transport and residence time of elemental Hg (Hg°) in air promote global dispersion and deposition in remote ecosystems. Many biotic and abiotic factors contribute to the photoreduction and phytovolatilization of Hg from terrestrial ecosystems, and the assessment of deposition and volatilization fluxes is very challenging. Mosses and lichens are widespread in nature and constitute the dominant vegetation in alpine and polar ecosystems. This review surveys the results of Hg biomonitoring with cryptogams in areas with different Hg sources and deposition processes. Lichen and moss ecophysiology, and factors affecting Hg uptake and bioaccumulation are discussed. Although some laboratory experiments indicate a linear accumulation of Hg in cryptogams exposed to Hg°, without any significant release, in nature the Hg accumulated in cryptogams is in a dynamic equilibrium with Hg in air and decreases when organisms are transplanted to clean environments. Mercury concentrations in mosses and lichens have often been used to estimate concentrations and deposition fluxes of atmospheric Hg; however, Hg° exchanges between cryptogams and air, and the time necessary for mosses and lichens to equilibrate elemental composition with changing atmospheric chemistry, preclude reliable estimates. Biological processes of Hg uptake and exchange with air cannot be reproduced by mechanical collectors, and comparisons between Hg concentrations in biomonitors and those in atmospheric deposition are scarcely reliable. However, the Hg biomonitoring with mosses and lichens is easy and cheap and allows to locate "hot spots" of natural or anthropogenic emissions and to assess spatio-temporal changes in Hg deposition patterns. Climate change is affecting the global Hg cycle through the melting of sea-ice in coastal Polar Regions, and modifying Hg sequestration in mountain ecosystems. Despite limitations, large-scale monitoring of Hg with mosses and lichens may be used as a tool to evaluate the impact of global processes in remote ecosystems.

Atmospheric chemistry of mercury in Antarctica and the role of cryptogams to assess deposition patterns in coastal ice-free areas

Mercury in the Antarctic troposphere has a distinct chemistry and challenging long-term measurements are needed for a better understanding of the atmospheric Hg reactions with oxidants and the exchanges of the various mercury forms among air-snow-sea and biota. Antarctic mosses and lichens are reliable biomonitors of airborne metals and in short time they can give useful information about Hg deposition patterns. Data summarized in this review show that although atmospheric Hg concentrations in the Southern Hemisphere are lower than those in the Northern Hemisphere, Antarctic cryptogams accumulate Hg at levels in the same range or higher than those observed for related cryptogam species in the Arctic, suggesting an enhanced deposition of bioavailable Hg in Antarctic coastal ice-free areas. In agreement with the newest findings in the literature, the Hg bioaccumulation in mosses and lichens from a nunatak particularly exposed to strong katabatic winds can be taken as evidence for a Hg contribution to coastal ecosystems by air masses from the Antarctic plateau. Human activities on the continent are mostly concentrated in coastal ice-free areas, and the deposition in these areas of Hg from the marine environment, the plateau and anthropogenic sources raises concern. The use of Antarctic cryptogams as biomonitors will be very useful to map Hg deposition patterns in costal ice-free areas and will contribute to a better understanding of Hg cycling in Antarctica and its environmental fate in terrestrial ecosystems.

Soil as an archive of coal-fired power plant mercury deposition

Mercury pollution is a global environmental problem that has serious implications for human health. One of the most important sources of anthropogenic mercury emissions are coal-burning power plants. Hg accumulations in soil are associated with their atmospheric deposition. Our study provides the first assessment of soil Hg on the entire Spanish surface obtained from one sampling protocol. Hg spatial distribution was analysed with topsoil samples taken from 4000 locations in a regular sampling grid. The other aim was to use geostatistical techniques to verify the extent of soil contamination by Hg and to evaluate presumed Hg enrichment near the seven Spanish power plants with installed capacity above 1000 MW. The Hg concentration in Spanish soil fell within the range of 1-7564 μg kg(-1) (mean 67.2) and 50% of the samples had a concentration below 37 μg kg(-1). Evidence for human activity was found near all the coal-fired power plants, which reflects that metals have accumulated in the basin over many years. Values over 1000 μg kg(-1) have been found in soils in the vicinity of the Aboño, Soto de Ribera and Castellon power plants. However, soil Hg enrichment was detectable only close to the emission source, within an approximate range of only 15 km from the power plants. We associated this effect with airborne emissions and subsequent depositions as the potential distance through fly ash deposition. Hg associated with particles of ash tends to be deposited near coal combustion sources.

Mercury profiles in sediment from the marginal high of Arabian Sea: an indicator of increasing anthropogenic Hg input

Total Hg distributions and its speciation were determined in two sediment cores collected from the western continental marginal high of India. Total Hg content in the sediment was found to gradually increase (by approximately two times) towards the surface in both the cores. It was found that Hg was preferentially bound to sulfide under anoxic condition. However, redox-mediated reactions in the upper part of the core influenced the total Hg content in the sediment cores. This study suggests that probable increase in authigenic and allogenic Hg deposition attributed to the increasing Hg concentration in the surface sediment in the study area.