Trace element mobility during interaction between rain and volcanic ash on Mt. Etna

Abstract

Volcanic emissions represent one of the most relevant natural sources of trace elements to the troposphere, both during and between eruptions. Due to their potential toxicity they may have important environmental impacts from the local to the global scale. Atmospheric precipitation was collected approximately every two weeks, from April 2006 to December 2007, using a network of five rain gauges, located at various altitudes on the upper flanks around the summit craters of Etna volcano. The collected samples were analysed for major (Ca, Mg, K, Na, F, SO4, Cl, NO3) and a large suite of trace elements (Ag, Al, As, Au, B, Ba, Be, Bi, Cd, Co, Cr, Cs, Cu, Fe, Hg, La, Li, Mn, Mo, Ni, Pb, Rb, Si, Sb, Sc, Se, Sr, Th, Ti, Tl, U, V, Zn) by using different techniques (IC and ICP-MS ). The raingauges occasionally collected also fresh volcanic ashes emitted by the volcano during paroxysmal events. The ashes were retained by the filter-system used to block the coarse material. More than twenty events of ashfall were collected, but only half of these samples were suitable for a complete chemical analysis, due to the small amount of sample. In order to obtain elemental chemical composition of ashes, powdered samples were analysed by a combination of methods, including XRF, total digestion ICP-MS, INAA, and IR. The chemistry of rainwater reveals that most of the investigated elements (both major and trace elements) have higher concentrations close to the emission vent of the volcano, confirming the prevailing volcanic contribution. Rainwater composition clearly reflects the volcanic plume input. Ash-normalised rainwater composition indicates a contrasting behaviour between volatile elements, which are highly-enriched in rainwater, and refractory elements, which have low rainwater/ash concentration ratios. The degree of interaction between collected ash and rainwater was variable, depending on several factors: (i) the length of the period in which tephra was present in the sampler (the ash fall may have occurred any day from the first to the last day of the rain collecting period); (ii) the amount of rainwater fallen on the collectors after the ash-fall event, and its acidity; (iii) the granulometry of the ash samples that was widely variable (from few centimetres to micrometric particles) with increasing interaction due to the decreasing dimensions of the tephra grains; (iv) the distance of collector with respect to the craters. All these factors govern the efficiency of the dissolution processes of the volcanic ash, and the amount of elemental leaching from their surface. The results suggest that sulphate and halide salt aerosols are adsorbed onto ash particles, and their rate of dissolution in rainwater depends on solubility. Moreover, rapid chemical weathering of the silicate glass by the corrosive volcanic acid (SO2, HCl and HF) can also explain the enrichment of several refractory elements (Na, K, Ca, Mg, Si, Al, Fe, Ti, Sc).