Insights into mercury deposition and spatiotemporal variation in the glacier and melt water from the central Tibetan Plateau

Long-term monitoring of global pollutant such as Mercury (Hg) in the cryosphere is very essential for understanding its bio-geochemical cycling and impacts in the pristine environment with limited emission sources. Therefore, from May 2015 to Oct 2015, surface snow and snow-pits from Xiao Dongkemadi Glacier and glacier melt water were sampled along an elevation transect from 5410 to 5678m a.s.l. in the central Tibetan Plateau (TP). The concentration of Hg in surface snow was observed to be higher than that from other parts of the TP. Unlike the southern parts of the TP, no clear altitudinal variation was observed in the central TP. The peak Total Hg (Hg_T) concentration over the vertical profile on the snow pits corresponded with a distinct yellowish-brown dust layer supporting the fact that most of the Hg was associated with particulate matter. It was observed that only 34% of Hg in snow was lost when the surface snow was exposed to sunlight indicating that the surface snow is less influenced by the post-depositional process. Significant diurnal variation of Hg_T concentration was observed in the river water, with highest concentration observed at 7pm when the discharge was highest and lowest concentration during 7-8am when the discharge was lowest. Such results suggest that the rate of discharge was influential in the concentration of Hg_T in the glacier fed rivers of the TP. The estimated export of Hg_T from Dongkemadi river basin is 747.43gyr^-1, which is quite high compared to other glaciers in the TP. Therefore, the export of global contaminant Hg might play enhanced role in the Alpine regions as these glaciers are retreating at an alarming rate under global warming which may have adverse impact on the ecosystem and the human health of the region.

Distribution and transportation of mercury from glacier to lake in the Qiangyong Glacier Basin, southern Tibetan Plateau, China

The Tibetan Plateau is home to the largest aggregate of glaciers outside the Polar Regions and is a source of fresh water to 1.4 billion people. Yet little is known about the transportation and cycling of Hg in high-elevation glacier basins on Tibetan Plateau. In this study, surface snow, glacier melting stream water and lake water samples were collected from the Qiangyong Glacier Basin. The spatiotemporal distribution and transportation of Hg from glacier to lake were investigated. Significant diurnal variations of dissolved Hg (DHg) concentrations were observed in the river water, with low concentrations in the morning (8:00am-14:00pm) and high concentrations in the afternoon (16:00pm-20:00pm). The DHg concentrations were exponentially correlated with runoff, which indicated that runoff was the dominant factor affecting DHg concentrations in the river water. Moreover, significant decreases of Hg were observed during transportation from glacier to lake. DHg adsorption onto particulates followed by the sedimentation of particulate-bound Hg (PHg) could be possible as an important Hg removal mechanism during the transportation process. Significant decreases in Hg concentrations were observed downstream of Xiao Qiangyong Lake, which indicated that the high-elevation lake system could significantly affect the distribution and transportation of Hg in the Qiangyong Glacier Basin.

Synthesis of current data for Hg in areas of geologic resource extraction contamination and aquatic systems in China

China has become the largest contributor of anthropogenic atmospheric mercury (Hg) in the world owing to its fast growing economy and the largest of populations. Over the last two decades, Hg has become of increasing environmental concern in China and much has been published on its distribution, transportation, methylation, and bioaccumulation in aquatic systems and areas of geologic resource extraction contaminated sites, such as coal-fired power plants, non-ferrous smelters, Hg mining and retorting sites, Au amalgam, landfills, chemical plants, etc.. Environmental compartments, like soil, water, air, and crop from areas of geologic resource extraction contamination, especially from Hg mining regions, exhibit elevated values of total-Hg and MMHg. Risk assessments indicate that the consumption of rice, which has a high bioaccumulation of MMHg, has become the dominant pathway of MMHg exposure of inhabitants living in Hg mining areas. Low concentrations less than 5ngl(-1) in total-Hg can be observed in rivers from remote areas, however, high concentrations that reached 1600ngl(-1) in total-Hg can be found in rivers from industrial and urban areas. The studies of hydropower reservoirs of southwest China indicated the old reservoirs act as net sinks for total-Hg and net sources of MMHg, while newly established ones act as net sinks for both total-Hg and MMHg, which is in sharp contrast to the evolution of biomethylation in reservoirs established in the boreal belt of North America and Eurasia. Fish from those reservoirs have relatively low levels of total-Hg, which do not exceed the maximum total-Hg limit of 0.5mgkg(-1) recommended by WHO. Currently, however, there is still a large data gap regarding Hg even in the areas mentioned above in China, which results in poor understanding of its environmental biogeochemistry. Moreover, for a better understanding of human and environmental health effects caused by the fast growing economy, long-term Hg monitoring campaigns are urgently needed.