Heng Yao

Elevated atmospheric deposition and dynamics of mercury in a remote upland forest of southwestern China

Mt. Gongga area in southwest China was impacted by Hg emissions from industrial activities and coal combustion, and annual means of atmospheric TGM and PHg concentrations at a regional background station were 3.98 ng m(-3) and 30.7 pg m(-3), respectively. This work presents a mass balance study of Hg in an upland forest in this area. Atmospheric deposition was highly elevated in the study area, with the annual mean THg deposition flux of 92.5 microg m(-2) yr(-1). Total deposition was dominated by dry deposition (71.8%), and wet deposition accounted for the remaining 28.2%. Forest was a large pool of atmospheric Hg, and nearly 76% of the atmospheric input was stored in forest soil. Volatilization and stream outflow were identified as the two major pathways for THg losses from the forest, which yielded mean output fluxes of 14.0 and 8.6 microg m(-2) yr(-1), respectively.

Selenium in Soil Inhibits Mercury Uptake and Translocation in Rice (Oryza sativa L.)

A great number of studies have confirmed that mercury-selenium (Hg-Se) antagonism is a widespread phenomenon in microorganisms, fish, poultry, humans, and other mammals. However, by comparison, little attention has been paid to plants. To investigate the influence of Se on the uptake and translocation of methylHg/inorganic Hg (MeHg/IHg) in the rice-soil system, we determined the levels of Se, IHg, and MeHg in different parts of rice plants (including the root, stem, leaf, husk, and grain (brown rice)) and corresponding soils of root zones collected from a Hg mined area, where Hg and Se co-occur due to historic Hg mining and retorting activities. The results showed that, in general, the Se levels were inversely related to the levels of both IHg and MeHg in the grains. In addition, a consistent reduction in translocation of both IHg and MeHg in the aerial shoots (i.e., the stem, leaf, husk, and grain) with increasing Se levels in the soils was observed. Furthermore, the Se levels were positively correlated with the IHg levels in the soils and the roots. These results suggest that Se may play an important role in limiting the bioaccessibility, absorption, and translocation/bioaccumulation of both IHg and MeHg in the aerial rice plant, which may be related to the formation of an Hg-Se insoluble complex in the rhizospheres and/or roots.

Prediction of methyl mercury uptake by rice plants ( Oryza sativa L.) using the diffusive gradient in thin films technique.

Rice consumption is the primary pathway for methyl mercury (MeHg) exposure at inland mercury (Hg) mining areas of SW China. Mechanistic information on MeHg accumulation in rice is, however, limited. The process of MeHg exchange between paddy soil and rice plants predominantly occurs in pore water. The detection of bioavailable MeHg in pore water is therefore important to predict MeHg uptake by rice plants ( Oryza sativa L.). This study investigated MeHg dynamics and spatial MeHg trends in pore water during the rice growing season using the diffusive gradient in thin films (DGT) technique and tested the ability of DGT to predict MeHg uptake by rice. The MeHg uptake flux from soil to rice plants via roots was significantly correlated with the DGT-measured MeHg flux (R = 0.853, p < 0.01). Our study implies that DGT can predict the bioavailability of MeHg in rice paddy soil and that the DGT method can provide quantitative description of the rate of uptake of this bioavailable MeHg. The DGT technique is demonstrated as a useful indicator of the likely ecotoxicological risk that might be apparent where paddy rice is grown in MeHg contaminated soil.

Total mercury in wild fish in Guizhou reservoirs, China

The health hazard of mercury (Hg) compounds is internationally recognized, and the main pathways for methylmercury (MeHg) intake in humans are through consumption of food, especially fish. Given the large releases of Hg to the environment in China, combined with the fast development of hydropower, this issue deserves attention. Provided similar mobilization pathways of Hg in China as seen in reservoirs in North America and Europe one should expect increased Hg contamination in relation to future hydropower reservoir construction in this country. This study presents total Hg (THg) concentrations in wild fish from six Guizhou reservoirs, China. The THg concentrations in fish were generally low despite high background levels in the bedrock and depositions from local point sources. The over all mean +/- SD concentration of THg was (0.066 +/- 0.078) microg/g (n = 235). After adjusting for among-reservoir variation in THg, there were significant differences in THg among functional groups of the fish, assumed to reflect trophic levels. Predicted THg-concentration ratios, retrieved from a mixed linear model, between the functional groups were 9:4:4:1 for carnivorous, omnivorous, planktivorous and herbivorous fish. This result indicated that MeHg accumulation may prevail even under circumstances with short food chains as in this Chinese water system. No fish exceeded recommended maximum THg limit for human consumption set by World Health Organization and the Standardization Administration of China (0.5 microg/g fish wet weight (ww)). Only six fish (2.5%) exceeded the maximum THg limit set by US Environmental Protection Agency (0.3 microg/g fish ww).

Mercury and methylmercury concentrations in two newly constructed reservoirs in the Wujiang River, Guizhou, China

We studied the distribution of different mercury species in two newly created reservoirs (Hongjiadu [HJD] and Suofengying [SFY] Reservoir) within the Wujiang River, Guizhou Province, China. These reservoirs were sampled four times between January, 2007, and November, 2007. Water, soil, and fish tissue samples were collected and analyzed for organic (methylmercury) and total Hg. Dissolved organic carbon (DOC) content in water was measured as well. In both reservoirs, the DOC concentration was low (range 0.5-4.9 mg/L). Organic matter content in soil collected near the bank of both reservoirs was also low (range 0.4-6.9%). The total MeHg level did not increase significantly with depth in water column, nor did it exhibit a pronounced spatial pattern moving from upstream to the dam. The total MeHg level did not increase significantly with depth in water column, nor did it exhibit a pronounced spatial pattern moving from upstream to the dam. The total Hg content in fish tissue was on average 0.044 mg/kg wet weight, which is a very low content in an international comparison. It is suggested that the studied reservoirs were not active sites of net Hg methylating. Low levels of organic matter (OM) may constrict the evolution of the bulk Hg methylation process occurring in flooded soil. Therefore, we hypothesize that newly constructed reservoirs in the karstic region of Southwest China appear to be much less beset by the problems of MeHg pollution that have been reported for corresponding reservoirs in North America and Europe. However, further research is required to verify this finding.

The impacts of organic matter on the distribution and methylation of mercury in a hydroelectric reservoir in Wujiang River, Southwest China

To understand the impacts of organic matter on the geochemical processing of Hg in a hydroelectric reservoir, spatial and seasonal distributions of total mercury (THg) and methylmercury (MeHg) in both solid and liquid phases of sediment cores were investigated in the Wujiangdu Reservoir in Guizhou Province, China. Four sampling sites with different intensities of cage aquaculture activities were chosen and were evenly distributed from upstream to downstream of the Wujiangdu Reservoir. Elevated MeHg concentrations and the high percentage of MeHg in solid and liquid phases of the surface sediment demonstrated that the active Hg methylation process occurred downstream of the Wujiangdu Reservoir. In contrast, the absence of obvious peaks for MeHg in sediment from the upper reaches of the Wujiangdu Reservoir, consistent with low levels of organic matter, suggested that production of MeHg was limited. In sediment with high organic matter content, reduction occurred closer to the sediment surface, resulting in maximum MeHg concentration in these layers. The correlation between MeHg and organic carbon implied that organic matter content in the sediment originating from cage aquaculture activities can help to predict MeHg production rates in reservoirs, which could be used to assess possible MeHg contamination in a reservoir ecosystem.

Seasonal distributions of mercury species and their relationship to some physicochemical factors in Puding Reservoir, Guizhou, China

A comprehensive study was conducted in July 2006, January 2007 and March 2007 to determine the impacts of some major physicochemical parameters on the level of mercury (Hg) in Puding Reservoir, Guizhou, China. The concentrations of Hg species in the summer campaign were significantly higher (p<0.01, generally 2 to 3 times higher) than those in the winter and spring campaigns, and no statistical differences were found between the same parameters for the latter two campaigns (p>0.05). Ancillary parameters including suspended particulate matter (SPM), dissolved organic carbon (DOC), temperature (T), dissolved oxygen (DO), pH, nitrate (NO3-) and chloride (Cl-) were also measured. During the sampling campaign in July 2006, average values for SPM, DOC, T, and NO3- were all higher compared to the other two campaigns, which suggested a similar seasonal trend between these parameters and Hg species. Seasonal variability may be related to increased runoff. High runoff volume due to abundant precipitation in the summer carried Hg-laden particulates into the reservoir, whereas there was less precipitation in the winter and spring when Hg levels were lower. Increased agricultural activity in the summer season also increased Hg levels in Puding Reservoir.

Intercomparison and applicability of some dynamic and equilibrium approaches to determine methylated mercury species in pore water

To assess adequately the impact of methylmercury (MeHg) on sensitive wetland ecosystems, accurate measurements of MeHg in pore water are required. In the present study, the feasibility of three methods for porewater sampling was investigated with respect to MeHg in sediments and rice paddy fields. The performance of dialysis samplers (peepers), sediment core sectioning followed by porewater separation by centrifugation (core), and the thin film diffusive samplers (DGT) were evaluated. These methods were intercompared in field experiments at two sites in Guizhou province, SW China disparately impacted by mercury pollution. All the methods report that the concentrations of MeHg in the soils of the Gouxi (GX) rice paddy near Wanshan were much higher than that in the sediment of the Hongjiadu Reservoir (HR), which is located within the Wujiang River basin. The three methods also report different MeHg profiles at the same site. However, these methods exhibit different temporal and spatial resolution scales, due to the differing operations involved with the sampling methods, may also reflect different states of MeHg in pore water. This corresponds to MeHg derived from diffusive flux, equilibrium concentration, and bulk concentration in pore water detected by DGT, peeper, and sediment core, respectively. The advantages and limits of the three methods are also presented.

Compound specific stable isotope determination of methylmercury in contaminated soil

Rice is one of the main sources of methylmercury (MeHg) to humans, and soil is the main source of MeHg to rice grains. Determining the Hg isotope composition in environmental samples is a good way of characterizing sources of Hg pollution and investigating environmental processes. We developed a new compound-specific method for determining stable Hg isotopes in MeHg in contaminated soil and sediment. The method involved HNO3 leaching/solvent extraction, chemical ethylation, and separation by gas chromatography with a solenoid valve optimized to enrich MeHg. The method was optimized by using MeHg standard solution, certified reference materials and paddy soil samples. The δ202Hg precision for replicate MeHg isotope analyses was 0.14‰ (2 × standard deviation, n = 11), and no fractionation of Hg stable isotopes was found during the separation processes. The δ202Hg values for MeHg in paddy soils were -1.78‰ to -1.30‰, which were lower than the δ202Hg values for total Hg (-1.32‰ to -0.44‰). The results indicated that methylation (rather than demethylation) was the dominant process in the paddy soils. The method developed in this study can help us to better understand MeHg migration and transformation processes in paddy soil-rice ecosystem.

Wet Deposition Flux of Total Mercury and Methylmercury in Wujiang River Basin

Wet deposition is an important pathway for the removal of mercury (Hg) from the atmosphere and the loading of Hg to the terrestrial and aquatic ecosystem systems. In this chapter, we (1) measured total mercury (THg) and methylmercury (MeHg) in bulk precipitation at five rural sites in Wujiang River Basin; (2) studied the monthly and annual deposition fluxes of THg and MeHg at the sampling sites in Wujiang River Basin and the underlying factors, and (3) estimated the input of THg and MeHg to the studied reservoirs in Wujiang River Basin via wet deposition.