Selenium around Etna Volcano, Italy

Humans are attracted by the fertile properties of volcanic soils and currently around 10% of the world population lives within active volcanic areas. Volcanoes emit significantly amounts of potentially toxic elements such as selenium (Se), even in the absence of obvious volcanic activity. Although Se is an essential element for humans, ingestion of an excess amount of Se can produce adverse effects. Mt. Etna, the biggest volcano in Europe, is persistently active for the last 200,000 years and one of the most intensely monitored volcanoes. We have studied selenium in rainwater and soils from Mt Etna volcano. Bulk depositions were collected from April 2006 to December 2007, using a network of five rain gauges, located at various altitudes around the summit craters. Highest Se concentrations (up to 13 µg/L) have been found close to the emission vent, confirming the prevailing volcanic contribution to rainwater composition close to the summit craters. The deposition rate for Se was estimated to be 1.7 µg m-2 day-1 nearby to the summit vents, to 0.5 µg m-2 day-1 at the local background site on the upwind western sector. Thirty soils collected at the flanks of the volcano show total concentrations between 0.1 and 2.2 ppm. Lab-controlled experiments with these soils and synthetic rainwater show that up to 5% of the Se is mobilized during soil-rain water interaction. The trace element deposition of the plume cannot explain the observed spatial trends. However, the stage of soil development plays an important role in the behaviour of Se within the soils. Our results have implications for the chemical composition of the Etnean aquifer. Additional, the mobility of soil Se influences the bioavailability and potential toxicity through agricultural activities, essential to the local economy.