Recent Changes in Atmospheric Mercury Deposition Recorded in the Sediments of Remote Equatorial Lakes in the Rwenzori Mountains, Uganda

We analyzed sediment cores collected from three equatorial zone lakes in the Rwenzori Mountains of Uganda for Hg and dated them using Pb-210. The results show that the lakes have been contaminated by anthropogenic Hg from atmospheric deposition and that the onset of Hg pollution in the region began at least by the late 19th century. Mercury accumulation in all sediment cores increased by about 3-fold since the mid-19th century, a similar increase to that shown in other remote regions worldwide. These results from tropical high-elevation sites are the first for this region and contribute to our understanding of global Hg pollution trends. The atmospheric boundary layer is at a higher altitude in equatorial areas than at midlatitudes, and therefore, Hg deposition in these regions may not be enhanced by diurnal penetration of tropospheric air and associated reactive gaseous mercury as has been reported for mountain lakes at higher latitudes. Furthermore, the relatively low abundance of atmospheric oxidants may limit the amount of gaseous elemental mercury oxidized to the reactive gaseous form in equatorial Africa. These Rwenzori Hg records therefore have important implications for the understanding of Hg dynamics at high elevations in equatorial regions.