Spatial distribution and temporal trends of mercury and arsenic in

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Spatial distribution and temporal trends of mercury and arsenic in remote timberline coniferous forests, eastern of the Tibet Plateau, China Ronggui Tang, Haiming Wang, Ji Luo, Shouqin Sun, Yiwen Gong, Jia She, Youchao Chen, Yang Dandan & Jun Zhou Environmental Science and Pollution Research ISSN 0944-1344 Volume 22 Number 15 Environ Sci Pollut Res (2015) 22:11658-11668 DOI 10.1007/s11356-015-4441-7

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Author's personal copy Environ Sci Pollut Res (2015) 22:11658–11668 DOI 10.1007/s11356-015-4441-7


Spatial distribution and temporal trends of mercury and arsenic in remote timberline coniferous forests, eastern of the Tibet Plateau, China Ronggui Tang 1,2 & Haiming Wang 3 & Ji Luo 1 & Shouqin Sun 1 & Yiwen Gong 1 & Jia She 1,2 & Youchao Chen 1,2 & Yang Dandan 1,2 & Jun Zhou 1

Received: 2 January 2015 / Accepted: 23 March 2015 / Published online: 9 April 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract An intensive investigation was conducted to study the spatial distribution and temporal variety trend of mercury and arsenic in plant tissue and soil profile in the eastern of the Tibet Plateau and to explore the possible sources of these two elements. At present, rare information is available on mercury (Hg) and arsenic (As) of timberline forests in the Tibet Plateau. Here, we present preliminary results on these two elements in leaves, twigs, root, litterfall, and soil. Geostatistical analyst of the ArcGIS 10.0 was used to determine the trait of spatial distribution of these two elements. Total arsenic (TAs) mean concentrations in the leaves, twigs, root, litterfall, and A- and C-layer soil ranged from 0.12 mg kg−1 (n=60), 0.35 mg kg−1 (n=60), 0.48 mg kg−1 (n=42), 1.52 mg kg−1 (n=84), 16.51 mg kg−1 (n=69), and 26.72 mg kg−1 (n=69), respectively. Total Hg (THg) mean concentrations in leaves, twigs, root, litterfall, and A- and Clayer soil were 0.0121 mg kg−1 (n=60), 0.0078 mg kg−1 (n= 60), 0.0171 mg kg−1 (n = 42), 0.0479 mg kg −1 (n = 84), Responsible editor: Stuart Simpson Electronic supplementary material The online version of this article (doi:10.1007/s11356-015-4441-7) contains supplementary material, which is available to authorized users. * Haiming Wang [email protected] 1

Key Laboratory of Mountain Surface Processes and Ecological Regulation, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences and Ministry of Water Conservancy, Chengdu, China


University of the Chinese Academy of Sciences, Beijing, China


Institute of Mountain Hazards and Environment, Chinese Academy of Sciences and Ministry of Water Conservancy, #9, Block 4, Renminnan Road, Chengdu 610041, China

0.0852 mg kg−1 (n=75), and 0.0251 mg kg−1 (n=75), respectively. In general, litterfall trended to accumulate high concentrations of Hg and As. Mercury in the timberline forest showed an increasing trend, whereas arsenic concentrations showed a decreasing trend in A-layer soil and an increasing trend in C-layer soil due to the easy mobile ability of As. Southwest and southeast monsoon could be the influencing factors, and Hg emission from India and China was the possible source of this study area through using a HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) model. It is believed that these observations may offer scientists and policymakers additional understanding of Hg and As concentrations in the remote timberline area, eastern of the Tibet Plateau. Keywords Geostatistical analyst . HYSPLIT model . GIS . IDW

Introduction Mercury (Hg) and arsenic (As) belong to the group of five toxic elements, which comprise mercury, arsenic, chromium, cadmium, and lead. Mercury which is released from a variety of sources including energy production, industrial applications, as well as production, use, and disposal of mercurycontaining products (Pacyna et al. 2008) is considered to be a global pollutant (Kabata-Pendias 2010; UNEP 2013a) due to its special physical and chemical characteristic (Feng et al. 2009). Extensive deforestation and agricultural land use also release mercury from soils, creating point sources of local, acute contamination (Barbosa et al. 2003). Once emitted into the atmosphere, Hg can be transported for a considerable distance by complex physical and chemical processes to many remote regions (Fu et al. 2010; Loewen et al. 2007). What is

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more, because mercury is capable of global distribution via the atmosphere, many remote ecosystems have been affected by this toxic element (Pacyna et al. 2008). The vision system and kidney of humans would be damaged when they are exposed to the environment with high Hg concentrations (da Costa et al. 2008; Feng et al. 2009). Unlike other heavy metals, Hg inputted in environment would transfer into greater toxic methyl mercury (MeHg) (Feng et al. 2009). The brain and nervous system are the target organs of toxicity of MeHg (Clarkson et al. 2003; Harada 1995). Arsenic which is ubiquitous in nature is a toxic and highly abundant metalloid that endangers human health through drinking water and the food chain (Bienert and Jahn 2010). The average content of As in the continental crust of the Earth is generally given as 1.8 mg kg−1, and the global soil average As concentration is 6.83 mg kg−1 (Kabata-Pendias 2010). Related media reported serious arsenic pollution in various locations like Shimen Town and Yangzong Lake from Hunan and Yunnan provinces, respectively, in China. For example, there were more than 400 persons died of cancer due to heavy As poisoning from the local abandoned As factory in Shimen Town (Huang 2014; 031f8d54-996c-4846-9418-e21d8a2495ec.shtml). Additional places like West Bengal, Bangladesh, and India also reported that long-term exposure to As also leads to cancer, affecting the lungs, bladder, and kidneys (Bienert and Jahn 2010). It was said that the man-induced mobilization of Hg and As into the biosphere (median values in thousand tons per year of terrestrial plus aquatic inputs minus atmospheric emissions) comes to about 120,000 ton for As and 11,000 ton for Hg (Nriagu and Pacyna 1988), which gives rise to potential threat of human health. Previous studies of five toxic elements of plant–soil ecosystem in the timberline of the Tibet Plateau mainly concentrated on Pb, Cd, and Cr (Luo et al. 2013a, b; Tang et al. 2015, 2014). Trace metals in topsoils, atmosphere, and snow of the Tibet Plateau also were reported (Cong et al. 2011, 2010; Huang et al. 2013a, b, 2012a, b; Li et al. 2009; Loewen et al. 2007; Sheng et al. 2012; Tripathee et al. 2014; Yanao and Junliang 1993; Zhang et al. 2002). Moreover, trace metals of soil in the eastern edge of the Tibet Plateau were reported (Wang et al. 2009; Wu et al. 2011). However, Hg and As of the environment exist in obvious divergence in different regions (Adriano 2001; Feng et al. 2009). For many regions, especially in sensitive and remote timberline forest area, the information of Hg and As is quite scarce. In addition, Hg and As in the Tibet Plateau may play an important role in the global biogeochemical cycle of itself as well as in the ecosystem and for human health. On the one hand, due to global change and regional economic development, a large amount of fossil fuel combustion and smelting produces different kinds of pollution. Meanwhile, under global climate warming and metabolic changing (Dillon et al. 2010), timberline forests are very


sensitive to input of pollutants. The fact is that the enhanced photosynthesis or respiration is likely to boost the absorption, transfer, and accumulation of trace metals (Hg and As). On the other hand, the eastern Tibetan Plateau is just considered as an ecological fragile region and it is adjacent to the two most populous and rapidly industrializing countries in the world (China and India). In addition, timberline forest areas are close to the head water of many large rivers, such as the Salween River, Lancang River, and Jinsha River, and long-term and extensive pollution can be a severe threat to its ecosystem stability and downstream (Luo et al. 2013a). Thus, the monitoring of trace metals in timberline forests could be an important step towards the response of global change and ecology safety research. Therefore, it is necessary to determine Hg and As concentrations in remote timberline forests. For this reason, the determination of Hg and As in timberline samples is extremely important for monitoring environmental pollution. The main objectives of the present investigation were (1) to explore the distribution tendency and possible sources of As and Hg in timberline coniferous forests using ArcGIS software and a HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) model, (2) to provide valuable first results which would contribute to the very small volume of biogeochemical data available for the Tibetan fragile ecology area, and (3) and to show the trend of Hg and As in a timberline area by total As (TAs) and total Hg (THg) concentrations in soil in different years.

Materials and method Study sites The study site was located in timberline forests in the eastern of the Tibet Plateau, north of Mount Hengduan (MH) (Table 1), which belongs to the hinterland of Asia (Fig. 1). MH is situated in the western of Sichuan and northern Yunnan provinces and in the eastern of the Tibet Autonomous Region, China. MH is a series of mountain ranges that stretch in the north–south direction, with nearly 900 km long, mean 4000– 5000 m above sea level, and commonly 1000 m or more elevation difference among mountain valleys. Under its unique geographical position and topography, this mountain region forms a diverse and regular vertical climate change that is Bspring at the foot, ice and snow on the peak, one mountain at four seasons, and different weather within 10 km.^ In the winter (October to the next April), the climate is mainly affected by the south branch flow of the westerlies. The Pacific and Indian Ocean water system produced southeasterly and southwesterly monsoon, respectively, in the summer. There are two seasons in the mountain ranges: the dry season and the wet season, with a deposition ranging 903–2595 mm. Most of precipitation is concentrated in June, July, and

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Fig. 1 Location of the sampling sites

August. The annual mean temperature ranges from 14 to 16 °C, and the mean temperature in the coldest month was 6/9 °C. Sampling The sampling was carried out in July 2012 from the remote region of the eastern of the Tibet Plateau, at a mean altitude of around 4000 m above sea level (a.s.l) from the district of timberline forests. There were no perceptible anthropogenic activities in the area, and human intrusion was very rare. From each sampling site, plant samples (leaves, twigs, and roots), litterfall, and soil materials were collected, respectively. Plant samples from separate trees and litter samples from the latest litterfall of needles (just including needles with rare decomposing) under the corresponding coniferous tree were homogeneously mixed, respectively. The latest litter was judged according to the level of decomposition of litterfall. All the samples were kept in plastic bags in a cold room. Soil profiles which were divided into three layers, including A layer, B layer, and C layer, were dug manually. A-horizon soil is humus in a completed decomposition state, B-horizon soil stands for illuvial horizon, and C-horizon soil trends to the unaltered soil parent materials. However, only A and C-layer soil was collected at each sampling site. In each layer, two to three replicates were collected (two replicates were collected for some samples). After being transferred to a laboratory, the samples were rinsed with distilled water for about 1 min to remove materials adhering to plant and litter surfaces. After washing, all the samples were oven-dried at 60 °C for 72 h, milled in an agate mortar, and passed through a 0.2-mm sieve. Then, the whole samples were stored in clean self-sealing plastic bags until the chemical analysis.

Element analysis For the analysis of the total amount of Hg concentrations in the leaf, twig, root tissue, and litterfall samples, 0.1–0.2-g dry sampling was digested in a water bath at 95 °C, with BrCl and mixed acid of HCl/H2SO4 (3:1 in volume) and HNO3/H2SO4 (8:2 in volume), respectively. Mercury concentrations in digested solution were then analyzed via reduction, purge, and trap, and CVAFS (US EPA, 2001). All the plant samples which were prepared for analysis have gone through the same procedure. Plant samples were wet digested with nitric acidhydrogen peroxide-hydrofluoric acid. The samples of Chorizon soil were digested with hydrochloric acid, nitric acid, and hydrofluoric acid, while besides these acid samples, the samples of A-horizon soil were digested with perchloric acid using a closed microwave (USA CEM-MARS6). Then, As concentrations in solutions were analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) (Profile DV, USA Teledyne Leeman Labs), with three replicates. The method was adopted from USA EPA Method 200.7 (revision 5.0, January 2001). Hg and As detection limits were 0.001 and 0.01 mg L−1, respectively. Quality assurance and quality control of analytical processes were addressed with reference materials, and the measurement errors were lower than 6 % for ICP-MS analysis. Precision was determined by relative standard deviations for duplicate samples, which were less than 5 % for element concentrations in all the samples. Recoveries of reference materials of Hg and As were in the range of 95–110 %. The measured mean TAs concentrations in reference vegetation and soil were 0.77 and 21.62 mg kg−1, which were comparable to the certified values of 1.25 mg kg − 1 (GBW07603) and 10.5 mg kg−1 (GBW07410), respectively.

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Geostatistical analyst and HYSPLIT model On the one hand, THg and TAs concentrations of plants and soil profiles in each sampling site were shown by the geostatistical analyst using a spatial interpolation method of inverse distance weighting (IDW) from ArcGIS 10. On the other hand, the 3-day backward trajectories were computed


using the HYSPLIT model in order to explore potential sources of aerosol and transported pathways of air masses ( archive). The first in each month was chosen as the time of forming map. Three respective sampling sites (P10, P24, and P29) were selected as a target place of forming map. So, 12 maps were showed using the HYSPLIT model.

Fig. 2 a–f Spatial distribution of THg and As concentrations in leaves, twigs, and litterfall. Faint yellow, orange yellow, and orange red colors represent the higher concentrations of THg and As, whereas light green and bottle green colors stand for lower concentrations of THg and As

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Results and discussion Spatial distribution of THg and TAs concentrations among leaves, twigs, litterfall, and A- and C-layer soil In this study, on the one hand, THg concentrations in leaves and twigs showed a higher value (>0.012 mg kg−1) in the northeast–southwest direction, while higher THg concentrations (>0.05 mg kg−1) in litterfall were in the southeast and northwest directions (Fig. 2). On the other hand, TAs concentrations in leaves and twigs presented higher value (>0.11 and >0.33 mg kg−1) in the east direction, whereas higher As concentrations (>1.8 mg kg−1) in litterfall were in the west direction. Figure 3 shows that TAs concentrations of both A- and Clayer soil showed higher value (>18 and >27.1 mg kg−1) in the west direction. The highest As concentration (41.2 mg kg−1) in A-layer soil was in P10, while TAs concentrations of more than 50 mg kg−1 in C-layer soil were in P19 (59.2 mg kg−1), P24 (53.7 mg kg−1), and P10 (51.4 mg kg−1). However, THg in A- and C-layer soil showed higher concentrations in the southwest direction (>0.087 mg kg−1) as well as in the southeast direction (>0.031 mg kg−1).

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THg concentrations in leaves, twigs, and A- and C-layer soil roughly showed spatial distribution (higher THg concentrations in northeast–southwest and southeast directions), while THg concentration in litterfall presented reverse distribution compared with leaves and twigs (higher THg concentrations in southeast and northwest directions). TAs concentrations in leaves, twigs, and A- and C-layer soil showed similar distribution characteristic (higher TAs concentrations in the southeast direction), whereas TAs concentrations in litterfall in the southeast direction were smaller (>0.942 mg kg−1). The relationship of THg and TAs concentrations among plant, litterfall, and soil In order to explore the relationship of THg and TAs between plant and soil, correlation analysis was performed. On the one hand, THg concentrations between leaves and twigs showed significant positive correlation (R 2 = 0.2482, P < 0.05) (Fig. 4a), which indicated that Hg can be transferred between leaves and twig. Similarly, THg concentrations between leaves and litterfall showed significant positive correlation relationship (R2 =0.3453, P twigs > leaves (Fig. 5a, b). Litterfall tended to accumulate high

Fig. 5 THg and TAs concentrations in diverse plant parts (a, b). Differences of THg and TAs concentrations in leaves and twigs (c, d) from the Electronic supplementary material (ESM). Correlation analysis between THg and TAs concentrations and TOC (e, f)

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concentrations of Hg and As. The THg concentrations in litterfall could increase to a higher level after litterfall decomposing process (Demers et al. 2007). In order to explore the possible cause, the correlation between the total organic carbon (TOC), THg, and TAs concentrations was determined (Fig. 5e, f). The significant correlation (R2 =0.4455, P0.05). This phenomenon presented that the factors of effect on THg concentrations in litterfall were not unique, and more investigations like Hg isotope analysis should be performed. Over all, litterfall showed a high accumulation of trace metals in this research and also in our previous results (Tang et al. 2014). Interestingly, TAs and THg concentrations in leaves and twigs showed opposite results (Fig. 5c, d). Maybe, needles easily adsorb and absorb the Hg materials coming from atmospheric sources due to its enormous surface area and wax rather than getting Hg from their root, resulting in higher Hg concentrations in leaves (Bishop et al. 1998; Demers et al. 2007; Ericksen et al.


2003; Lindberg et al. 1979; Rea et al. 2002). Then, partial Hg with litterfall was inputted to the floor of forests (terrestrial ecosystems) due to the significant correlation between leaves and litterfall (Fig. 4b) (Bushey et al. 2008; Ericksen et al. 2003) in terms of biogeochemical cycling of Hg. According to higher As concentrations in twigs, twigs of coniferous tree can be used as a rough indicator where potential As pollution may be expected. In the next step of work, because it is very important to quantify the input of mercury into different ecosystems (Bieser et al. 2014), Hg and As storage of different plant parts and soil profiles should be calculated; that is, trace metals are connected with biomass of ecosystem so that Hg and As of biogeochemistry cycling and eco-risk assessment in this area will be clearly showed and performed. TAs and THg concentrations between 1990 and 2013 in the soil In order to understand the present level of As concentrations in soil, As concentrations in the background of Tibet soil in 1990 were compared. Figure 6c, d shows that As concentrations in

Fig. 6 Comparison of As and Hg concentrations of different years in A and C layers (a–d). Comparison of TAs and Hg concentrations in A- and C-layer soil (e, f) from the ESM

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A- and C-layer soil were 19.7 and 19.3 mg kg−1 (Chen et al. 1991). As concentrations which were in the soil of the forests of the southeast of Tibet were close to mean As concentrations in the whole Tibetan soil (Yanao and Junliang 1993). So, the data from both 1990 and 2013 were comparable. The present As concentration (16.52 mg kg−1, n=69) in A-layer soil was lower than the value in 1990, whereas As concentration (26.72 mg kg−1) in C-layer soil was higher than the account in 1990. The possible cause was that As was leached from Ato C-layer soil due to the easy mobile ability of As (KabataPendias 2010), resulting in higher As concentrations in Clayer soil. Regarding THg concentrations in the soil, C-layer soil showed the same value between 1990 and 2013, while the THg concentration in A-layer soil (0.085 mg kg−1) in 2013 was significantly higher (P < 0.05) than the figure (0.025 mg kg−1) in 1990 (Fig. 6a, b). In addition, the residence time of mercury in the coniferous stand was more than 200 years, which can reflect historic Hg deposition and retention (Ericksen et al. 2003; Grigal et al. 2000; Lindqvist et al. 1991; Schwesig and Matzner 2000). Therefore, a sample conclusion can be drawn that Hg in the timberline forests is still increasing in this study. This result is in accordance with the fact that the amount of mercury available in the environment is steadily increasing due to anthropogenic emissions from fossil

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fuel burning (Bieser et al. 2014). Recent studies have shown that the atmospheric mercury burden has tripled since preindustrial times (Amos et al. 2012), which supports our study. Possible source of Hg and As in timberline forests It is easy to see that, at each site, the present TAs concentration in C-layer soil was higher than the figure in A-layer soil, whereas THg concentration in A-layer soil was higher than the value in C-layer soil (Fig. 6e, f). Probably, exterior Hg sources are being put into the timberline forests and accumulate in the topsoil, whereas As concentrations in A-layer soil could be related to the weathering of soil parent material. The HYSPLIT model from Fig. 7 was used to explore the possible Hg sources of this study area. The air mass of this area was mainly influenced by southwestern monsoon, southeastern monsoon, and westerlies. Sources of air mass mainly came from the northeast of India, southwest of China. However, these two areas also were considered as a huge source of Hg emission due to fast development (Wilson et al. 2006). It is estimated that atmospheric mercury emissions in China account for 25–40 % of global mercury emissions (UNEP 2013b). Maybe, Hg pollutants were long-distantly transported over a long range and deposited into the timberline forests due

Fig. 7 Seventy-two-hour back trajectories of the sampling period of this area, from January to December in 2012. Three target sites (P10, P24, and P29) were selected as the representative area

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to the impact of the southwest and southeast monsoon. The result could provide important reference of early warning for pollution damage to timberline forests. In general, Hg in timberline forests showed an increasing tendency in this study. Southwest and southeast monsoon could be the influencing factor, and Hg emission in India and China was a possible source in this study area.

Conclusion This study contains an analysis of spatial distribution of THg and TAs concentrations in plant and soil. Soil THg and TAs concentrations in diverse years were also compared. In general, Hg in timberline forests showed an increasing trend in this study. Southwest and southeast monsoon could be the influencing factor, and Hg emission of India and China was a possible source in this study area. Litterfall trended to accumulate high concentrations of Hg and As. The proposed methodology gives a clear way to estimate As and Hg concentration, which makes a good contribution to the environment management of timberline forests and help policymakers to take successful mitigation measures. To better understand the actual risk to human and mammal (yak) of Tibet with multiple sources of Hg and As, more intensive samplings, isotope analysis, different element forms, and necessary belt transect between the northeast of India and this study area are needed to assess the spatial distribution of risk. Therefore, new research directions may need to be proposed for future research upon discussion of these key issues: (1) collecting a greater amount of investigative field data in mild pollution areas, especially sensitive timberline forests, and (2) on the basis of this initial assessment, establishing and implementing coordinated monitoring programs for the continuing assessment of environmental quality of timberline forests in the eastern of the Tibet Plateau.

Acknowledgments This work was funded by the National Natural Science Foundation of China (grants nos. 41471416, 41473078, and 40871042). The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model.

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