2c-2: Sources and cycling of mercury in terrestrial ecosystems

Tibetan Plateau (TP) is generally known as the third pole of the world. Due to the sparse population and minimal industries, TP is considered as one of the cleanest regions of the world. Since the atmospheric environment monitoring in TP has been used for long-range transported pollutants assessment widely, the study of local atmospheric pollution sources in TP has raised a great deal of concern. Yak Dung is an important residential energy wildly used as the biomass fuel in TP for cooking and heating, which might contribute to the accumulation of pollutants in the air of the region. In order to evaluate Yak Dungs influences on mercury emission and air concentration, in this study, burning experiments were designed under local conditions. Yak dung samples were collected in a large area, ranging from southwest (Tingri) to northeast (Nagqu) TP and were analyzed under the local utilization condition. The vegetation atlas and population density are used to estimate the amount and distribution of Yak, which in turn can be used to estimate the quantity of local yak dung. The result shows that the concentration of total mercury in yak dung and total gaseous mercury in flue gas varies from place to place. Different pre-treatment technologies and dung source influence the water content, furnace temperature and the combustion process, and resulted in the differences in final mercury speciation and emission. The total amount and distribution of mercury emitted from yak dung to atmosphere in TP were estimated. More mercury is emitted from south central and east regions than other regions. Although TP is considered as ideal region representing atmospheric background, the influence of local emission from the burning of yak dung on atmospheric environment cannot be ignored.

Accumulation of mercury in crops threats the safety of terrestrial food chain. Understanding the mechanism of plant uptake of Hg is a crucial step for managing mercury accumulation in crop. In this study, a series of hydroponic experiment was conducted in a plant growth cabinet to investigate the Hg uptake kinetic, as well as endogenous Zn2+ supply on Hg uptake by Triticale which is a major staple food crop in Asian. Results showed that the environmental temperature greatly affects the kinetic of Hg uptake into the root in a short term (20 min) Hg2+ exposure experiment. At ice-cold (﹤2℃) condition, a linear relationship between hydroponic Hg (Maximum Hg: ) and root Hg was observed, showing a nonsaturable type of root uptake of Hg, probably corresponding to the bind of Hg2+ to cell wall. At 25 ℃, both linear and hyperbolic relationships were obtained where the form was shown at low endogenous Hg concentration (<5 M), the latter was shown at high Hg concentration, indicating both a nonsaturable (linear) and saturable (hyperbolic) type of root uptake of Hg, corresponding to both the bind of Hg2+ to cell wall and carrier-mediated Hg2+ across root-cell plasma membranes. The Michaelis-Menten equation was fit for the saturable data (r2=0.95), calculating the Michaelis constant (Km) of 7.7M and Maximum initial velocity (Vm) of 406 mgkg-1 DW. The effect of Zn2+ on root uptake of Hg2+ was variable. With increasing endogenous Zn2+ concentration, Hg concentration in roots decreased at low Hg level (<10 M) but increased at high Hg level (>20 M), suggesting that the increase of Zn2+ might competitively bind to cell wall against Hg2+ at low Hg level and might enhance the enzyme activities at plasma membranes by which increasing carrier-mediated Hg2+ uptake.

Mercury (Hg) is a ubiquitous, toxic and bioaccumulative environmental contaminant that undergoes reaction to change speciation and environmental fate. Some areas, like the Oakridge National Laboratory Y-12 National Security Complex (TN, USA), have experienced historical contamination of surrounding soils with Hg, and remediation of such sites has been complicated by poor mechanistic understanding of Hg reactions. Specifically, non-volatile Hg species in soil can undergo reduction to volatile Hg(0), which can then be lost to the atmosphere; however, the mechanism of this reaction is not known. This work used a computational study, coupled with laboratory experiments to identify a mechanism for the reduction of one environmentally relevant Hg species, mercury (II) chloride (HgCl2), in soil-like environments. Computational modelling using Gaussian software suggests that HgCl2 reduction might include a unimolecular dissociation driven by scissoring of the chlorine bonds to form Hg(0). Based on the energy of excitation suggested in these computational results, we hypothesized that ultraviolet-B (UVB; 280 320 nm) radiation would provide the energy necessary to cause the dissociation of HgCl2 to Hg(0), in the absence of a secondary electron donor. To test this, clean silica sand was spiked with an aqueous solution of HgCl2, and exposed to full spectrum radiation, while the flux of Hg(0) from this sand was quantified over time. Filters were applied to remove ultraviolet radiation, and preliminary results align with the molecular modeling data, suggesting that UVB radiation is particularly important in the reduction of HgCl2 in sand, and that this reduction will proceed with no secondary electron donor. Mechanistic understanding of these complex processes will assist in global mercury transport modelling, and may also contribute to the creation of effective remediation strategies for mercury contaminated soils.

A filter-based dynamic flux chamber method was used to measure gaseous reactive mercury (RM) and elemental (GEM) air-soil exchange. The soil materials used in this study include waste rock, heap leach ore, and tailings waste collected from industrial scale open pit gold mines in central Nevada, USA. Substrate concentrations ranged between 0.1 to 40 µg g-1 THg. Gaseous elemental mercury flux was quantified with a Tekran 2537 mercury analyzer, while cation exchange membrane (CEM) and Nylon filters were used to capture RM from the flux chamber sample lines. The CEM filters were used to quantify absolute RM concentrations, while Nylon filters were used to determine RM speciation by thermal desorption. Flux measurements were conducted for each material under wet and dry substrate conditions. The magnitude of RM flux was correlated with substrate Hg concentration, with lower Hg substrates showing negative or very small RM fluxes. Wet materials consistently showed higher positive RM fluxes compared to their dry counterparts. High substrate Hg tailings material showed RM fluxes up to 48 ng m-2 hr-1 under when wet. Initial thermal desorption analysis indicates a distinct difference in RM species deposited to lower Hg substrates and that emitted from tailings material.

Agricultural catchment is an important component around Three Gorges Reservoir. Frequent human activities in this area make soil subject to erosion, and produced surface runoff can carry large amounts of soil with mercury (Hg) into reservoir, resulting in increase of Hg output from agricultural catchment and Hg load in reservoir. However, limited data were published about the Hg transportation in this area, and the runoff output fluxes from such watershed and caused contribution to Hg load in aquatic system were still unclear. Therefore, a typical small agricultural catchment in Wangjiagou, Chongqing was chosen to be the study area, aiming to: 1) study the Hg distribution in soil and analyze the effect of human activities on Hg distribution; 2) understand the characteristics of Hg migration; 3) calculate the Hg outputs from this agricultural catchment. The results showed that the Hg concentrations in soil ranged 9.47-94.57 μg kg-1 with an average of 34.23 ± 16.23 μg kg-1. Significant spatial and vertical distributions of Hg were detected with higher THg level in forest land and in surface soil (0-5 cm). Soil erosion rate was in the range of 0-904.16 t hm-2 yr-1, and the annual soil erosion amount was 5632.68 t. Surface related with very slight and extremely severe erosion occupied about 50% and 23% of entire land, respectively. Hg migration fluxes in different landscape types were estimated to be 1.5952 (garden plot), 0.9040 (forest land), 0.1648 (dry land) and 0.0032 (paddy land) kg km-2 yr-1, and the total Hg surface erosion load was predicted to be 276 g yr-1. The erosion load of Hg from forest land located in steep zones was 238 g yr-1 accounting for 86% of total Hg load. While Hg migrated from upland (garden plot, forest land and dry land) could be intercepted by wetland (paddy soil) in flat bottom of this catchment, and the erosion load was just 0.7 g yr-1 which was comparable with the measured migration fluxes (0.8 g yr-1) from the only exit situated in paddy land, suggesting that the project of terracing of sloping land around the Three Gorges Reservoir could decrease the Hg output from agricultural catchment to water of reservoir.

Mercury (Hg), as a toxic and persistent pollutant, has a long-term impact on ecological system and human health. To briefly assess the effect of atmospheric deposition on semi-arid temperate grassland, contents of Hg and organic matter (TOC) in the topsoil and subsoil was analyzed for samples collected from 80 sites in central Inner Mongolia, China during 2012-2015. Results indicated that the Hg contents of topsoil varied from 1.08 to 46.0 µg/kg, while those of subsoil was in 0.74-29.7 µg/kg. The topsoil Hg content was positively correlated with TOC, with lower Hg to TOC ratio for topsoil with higher TOC. The Hg to TOC ratio of topsoil also showed significant positive correlation with atmospheric Hg deposition, modeled by the improved GEOS-Chem model on the bases of emission inventory including both anthropogenic and natural sources. Although 114 large coal-fired power plants (CFPPs), most of which were installed in recent years, were located surrounding the sampling sites in this region, they may not the main cause of increasing Hg content of topsoil in this region, because the Hg to TOC ratio of topsoil did not show significant correlation with the “power plant impact factor”, which is related to the installed capacity and the distance between CFPP and the sampling site within 150 km. It may be concluded that atmospheric deposition was the main source of Hg in topsoil instead of geological sources, and power plants is not the only source of soil Hg content. Great attention on other and the potential anthropogenic sources such as residential coal combustion should also be paid for Hg emission control in this region.

The main goal of this study is to constrain terrestrial Hg cycling in the arctic tundra which covers 4% of the global land surface. We present results on pool sizes and concentrations of Hg in tundra soils, vegetation, and runoff samples obtained from the northern Alaskan slope. Samples were mainly collected at Toolik Field station (68˚ 38’ N, 149˚ 36’ W) and along a 200km transect extending north from Toolik Field station to the Coastal Plains near Prudhoe Bay. Collected samples were analyzed for total Hg concentration, pH, bulk density, organic and total carbon, nitrogen, and major and trace elements. Runoff samples were analyzed for total Hg concentration and dissolved organic carbon. Additionally, we will present results of soil extraction experiments to assess the relevant factors related to mobility of Hg from tundra soils and transfer to aquatic systems.

Results from the Toolik Field Station show that tundra vegetation concentrations (average 112±15 µg kg-1) were 3 to 5 times higher than Hg levels generally measured at many temperate sites and are attributable to a high representation of lichen and mosses in bulk vegetation. Tundra soil Hg concentrations were 151±7 µg kg-1 in organic soils and 98±6 µg kg-1 in mineral soils, and much higher than the range of 20-50 µg kg-1 reported from upper soils in temperate areas. Permafrost soil Hg concentrations were lower than upper soils (average 40±0.2 µg kg-1) and methyl-Hg was generally 3% of total Hg. Vertical concentration patterns were relatively constant, in contrast to temperate sites showing strong declines with depth that follow the distribution of organic carbon. Mass calculations show that Hg mass in the upper 40-100 cm of the soil profile (200-500 g ha-1) was primarily stored in mineral soil layers (over 90%). Hg mass showed substantial spatial variability, particularly along an upland-wetland gradient where wetland Hg pools were much lower due to an absence of mineral soil layer. Average Hg concentrations in runoff were relatively low (4±0.4 ng L-1).

Principle component analyses including major and trace elements showed that soil Hg in surface organic layers was largely unassociated with geogenic soil elements indicating that surface soil Hg was not of lithogenic origin but derived from atmospheric sources. C-14 age-dating of deeper, mineral soil layers (14C age: 7,307 years) suggested that high concentrations of Hg present in these layers may be caused by a long legacy of atmospheric deposition and retention in soils.

Computational algorithms for simulating terrestrial and aquatic biogeochemical processes of Hg were developed for simulation in the Soil and Water Assessment Tool (SWAT), a watershed scale hydrologic model developed by U.S. Department of Agriculture. The SWAT-Hg model integrates wet/dry deposited Hg species into the watershed hydrological system and estimates the transport and fate of Hg in the landscape and through stream network. The output can be used to predict the Hg levels in the fish tissue in response to climate change and variability in atmospheric mercury deposition rates. The SWAT-Hg considers three Hg species, Hg(II), Hg(0) and MeHg, in various domains of a watershed such as soils, water bodies, and vegetation. Atmospheric Hg is deposited on leaf, surficial soils (litter), or snow pack, depending on the condition of the land cover. The accumulated Hg on leaf can be washed off to surficial soil via through fall or litter fall and the Hg on snow pack can leach into soil via snow melting. After Hg partitioning between dissolved and solid phase, dissolved Hg can percolate to deeper soils, such as organic, mineral soil, and to aquifer, and can be further transported to surface water by lateral flow and return flow. Solid phase Hg in surficial soil can be released to surface water by soil erosion. In each domain, mercury can be transformed to each other via various biogeochemical reactions, such as reduction/oxidation, methylation/demthylation, and etc. These complex mercury transport and transformation behaviors are simulated in HRUs (hydrologic response units). The integration of individual Hg species behaviors in HRUs provides the Hg fate and transport in a watershed. The newly developed SWAT-Hg was calibrated and validated in the Jangsung Dam (JSD) watershed which is a headwater subbasin of the Youngsan River basin located in the south-western end of the Korean peninsula. The watershed covers a 105 km2 area, mostly pristine forests, draining into Jangsung Dam. The dam has storage capacity of 8,480 tons. The SWAT-Hg has been calibrated and validated with the data obtained in the watershed and is being applied to simulate fish mercury levels in response to environmental perturbations. The updated SWAT-Hg can be applied to other watersheds and countries to study the Hg fate and transport, because of the proven robustness of SWAT applicability to various geophysical settings.