Discrete monitoring of chemical parameters in ground waters of Mt. Etna volcano: 2000-2006

Three sites of groundwater captation on Mt. Etna volcano (namely Currone, Ilice and Pavone) were sampled systematically for six years (from 2000 to 2006) for the determination of the major ionic composition of water. The monitored sites were chosen among those most representative of the geochemical characteristics of the groundwater systems of the volcano. The period studied was characterized by several strong eruptions of Mt. Etna, both at its summit craters and along its flanks. The overall composition of the waters sampled at Currone and Ilice falls in the group of bicarbonate-alkaline-earth compositions, whereas those from Pavone show a bicarbonate-alkaline composition. In all sites, however, some samples show input of chlorine-sulfate alkaline waters, likely due to interaction between fresh groundwater and either acidic waters from SO2-polluted rainwater, (more evident at Pavone) or geothermal brines (more evident at Currone). Significant temporal variations affected, in a more or less marked way, all of the parameters measured at the three sites. Basic statistical correlations among the parameters at each site allowed to discover a general coherent temporal behavior of all major ions dissolved in Mt. Etna's ground waters. Factor Analysis allowed showing up to three main groups of parameters with similar temporal behavior, depending on the site. A first group was explained by interaction between volcanic ground waters and geothermal fluids; a second group was related with leaching of the host volcanic rocks by CO2-rich volcanic water; a third group was explained as due to input of plume-SO2-derived sulfate through water recharge. Using normal probability plots for each parameter at the three sites it was possible to reveal different geochemical populations explained as background, anomalous values and, possibly, outliers. Plotting the temporal patterns of all the monitored parameters versus the concurrent eruptive episodes at Mt. Etna, we discovered significant correlations that, although with different intensity and rate depending on the parameter and on the site, highlighted several geochemical processes induced by interaction between cold groundwater and magmatic/hydrothermal fluids, mostly following changes in the ground permeability of the volcanic pile. These processes seemed to be enhanced during periods of shallow magma accumulation inside of the volcano that preceded summit or flank eruptions occurred at Mt. Etna during the monitored period.