Kahayan river survey for mercury pollutant by gold mining in Kalimantan, Indonesia

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Gold mining activities have recently increased in the catchment of the Kahayan River in Central Kalimantan. Water contamination with mercury (Hg) due to the mining activities has been of concern to the residents along the Kahayan as well as those who live in Palangka Raya, which is the capital town of Central Kalimantan. In this study, the survey on Hg contamination in river water and sediment in the Kahayan and the major tributary, the Rungan River, were conducted to make clear the recent contamination level of river water and sediment.The surveys were carried out between Palangka Raya and Tubangmiri which is the town located about 100km up the stream in the Kahayan River and between the confluence point with the Kahayan and Mungukubaru located about 50km up the stream in the Rungan River, including the survey on the water in some tributary of the rivers.The total Hg concentrations in water in the main stream of the rivers were below 200 ng/L, and those were lower than 1000 ng/L, which is the standard values in Indonesia. Higher concentrations of Hg in some tributaries than those in the main stream were observed in both the Kahayan and the Rungan.The Hg contents in sediment were below 0.5 mg-Hg/kg-Dry Weight. The contents in sediment in this area were so far in a relatively low Hg contamination level comparing other gold mining areas. The Hg contents in fish were 0.02-0.5 mg・Hg/kg-Wet Weight, and Mystus was higher than other species.Continuous observation of the Hg concentration in water and sediment in the river is necessary to know the impact of gold mining activity on the contamination of Hg in the rivers as well as to predict the situation of Hg contamination in the watershed in the future.