ENVIRONMENTAL IMPACT OF MT. ETNA’S VOLCANIC EMISSIONS: TRACE METAL BULK-DEPOSITION

Abstract

Volcanoes contribute to atmospheric pollution by increasing the amount of reactive and greenhouse gases and aerosols, making volcanic emissions one of the major natural sources of several trace elements to the atmosphere. In particular, Mt. Etna is considered to be, on the long-term average, the major atmospheric point source of many environmental harmful compounds. Their emission occurs in the form of gases, aerosols or particulate, both through continuous passive degassing from open-conduit activity and through sporadic paroxysmal eruptive activity. To estimate the environmental impact of magma-derived trace metals and their depositions processes, five bulk collectors have been deployed at various altitudes on the upper flanks around the summit craters of the volcano. Samples were collected every second week for a period of one year and analyzed for the main chemical-physical parameters (electric conductivity and pH) and for major and trace elements concentrations. The first data obtained clearly show that the volcanic contribution is always prevailing in the sampling site closest to the summit craters (∼1.5 km). In the distal sites (5.5-10 km from the summit) downwind of the summit craters, the volcanic contribution is also detectable but often overwhelmed by anthropogenic or other natural (seawater spray, geogenic dust) contributions. Volcanic contribution may derive from both dry and wet deposition of gases and aerosols from the volcanic plume, but sometimes also from leaching of freshly emitted volcanic ashes. In fact, in our background site (7.5 km in the upwind direction,) volcanic contribution has been detected only following an ash deposition event. Fluorine, S and Cl, are the major elements that prevailingly characterize the volcanic contribution in bulk deposition on Mt. Etna, but high concentrations of many trace elements are also detected in the studied samples. In particular, Si, Al, Fe, Ti, Cu, As, Rb, Pb, Tl, Cd, Cr, U and Ag display, in the site most exposed to the volcanic emissions, median concentration values about two orders of magnitude higher than those measured in our background site. Furthermore some of the analysed elements display very high enrichment values with respect to the average crust and, in the closest site to the summit craters, also deposition values higher than those measured in polluted urban or industrial sites.