Himalayan rivers may export more mercury (Hg), one of the environment’s most toxic pollutants, amid climate change, says a recent study published in Science of the Total Environment, which was led by Prof. ZHANG Qianggong from the Institute of Tibetan Plateau Research, Chinese Academy of Sciences (ITPCAS). “The severity and complexity of Hg pollution far exceeds conventional pollutants and the riverine export of Hg is an important link in both the global and regional Hg cycles,” said ZHANG. “The study has improved our understanding of riverine Hg export in Himalayan rivers.”
The conclusion is drawn from samples collected from the Koshi River Basin (KRB) in different seasons. The Koshi River is one of the main tributaries of the Ganges. It originates in Tibet, China and flows through Nepal and India. Weather in the basin is divided into four seasons by the Indian summer monsoon, i.e., pre-monsoon (March-May); monsoon (June-September); post-monsoon (October-November); and winter (December-February). ZHANG and his co-authors found that the Koshi River exports the most Hg during the monsoon season and the least during the pre-monsoon. “Riverine Hg across all seasons mostly exists as particulate Hg. A combination regime of river flow and suspended particulate matter was the primary mechanism governing the Hg variation in the water of the KRB and is very sensitive to climate change in the trans-Himalayan region,” said ZHANG.
A bird’s eye view of the Koshi River（Photo by ZHANG Qianggong）
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rapid climate change at the Third Pole, Hg export from the KRB is expected to increase in the future, which is also the likely scenario for other Himalayan rivers. “Studies have suggested that precipitation pattern and river flow will vary across different seasons, with a particular increase during the post-monsoon season, which will enhance the export of Hg via the trans-Himalayan rivers to lowlands,” said ZHANG. “More observation efforts are required on Hg export and its environmental impact in the region.”
Editor’s note: The study was supported by Pan-TPE, a TPE-related science project.
