The source and fate of sediment and mercury in Hunza River basin, Northern Areas, Pakistan

Abstract

This article presents results of mercury in surface waters from Hunza River basin, Northern Areas, Pakistan. Small-scale gold mining activities along the Hunza and Gilgit rivers are long known to be discharging mercury in the amalgamation and roasting processes. Previous studies reported high mercury concentrations in soils close to mining operations as well as serious health problems for miners. However, none of the studies have focused on the level of contamination in aqueous environments. This is the first study on the investigation of source and fate of sediment and river-borne mercury in the Hunza River. The samples collected near gold panning sites showed higher mercury concentrations than critical levels established by the U.S. Environmental Protection Agency. The observed dissolved mercury concentrations ranged from 5.10 to 25.25 ng/l, whereas particulate-bound mercury ranged from 4.85 to 154.62 ng/l. Particulate-phase mercury corresponded to more than 75% of the total observed mercury concentrations for all of the sampled rivers. Thus, suspended sediments represented the major pathway of the riverine mercury transport. A mass balance calculation suggested an annual mercury flux of 48.6 g/km2 into the Hunza River basin. The samples collected from the most affected river, the Shimsal River, averaged to have 108 ng/l total mercury. This amount was close to the average soil mercury data of 151 ng/l as reported by the Pakistan Mineral Development Corporation in 2001. The dominant source of contamination was shown to be the leaching of large quantities of mercury from the mercury-rich sediment and flood plain soil into the rivers, rather than the direct release from mining activities. Significant decrease in both dissolved and particulate-bound mercury concentration downstream of Attabad Lake suggested that mercury is being accumulated or consumed in the lake. Although minimization or elimination of mercury loses from the mining process seems important for the well-being of the miners, preventing the remobilization of accumulated mercury is equally important in mercury control in this region. Copyright © 2014 John Wiley & Sons, Ltd.