Investigation of the Source, Fate, and Transport of Mercury in Hunza River, Northern Areas, Pakistan

Mercury is one of the most hazardous and toxic elements with a high degree of mobility. Although it occurs naturally, anthropogenic activity can also release mercury to the environment. Globally, the use of mercury in gold mining has caused significant environmental pollution since Ancient Roman times. Small-scale gold mining activities along 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 on fluvial fate and transport of mercury in Hunza River. This research study aimed to investigate the source, fate and transport of river-borne mercury. Eleven (11) out of thirty-seven (37) river water samples showed higher dissolved mercury concentration than critical levels suggested by Environmental Protection Agency. These sites corresponded to the observed gold panning sites. Particulate mercury concentration was roughly three orders of magnitude higher than that of dissolved mercury per liter of water for all sampled rivers. Thus, suspended sediments represented the major pathway of mercury transport. A mass balance was performed to calculate annual mercury discharge to Hunza River basin. A conservative estimation resulted in 0.63 tonnes of annual mercury flux. This amount of mercury is difficult to result from anthropogenic discharge alone. An index based physical model was created to estimate mercury yield at sub-basin level. It was found that the dominant source of mercury is the mercury-rich soils upstream of Attabad Lake rather than the direct mercury release from mining activities. This result is supported by previous soil geochemistry data. 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 mining process seems important for the well-being of the miners, preventing further accumulation of mercury in Attabad Lake would be a much more effective mean of mercury control in the region.