Excerpt from article:
Fluoride concentrations in Etna groundwaters range from 20 to 995 mg/l (average=434 mg/l; s=217). None of the analysed waters exceeds the national MAC for drinking water.
Volcanic activity is an important source of F to the atmosphere, mainly in the form of hydrofluoric acid. Massive F emissions are often related to explosive eruptions leading to acute F intoxication of animals that graze contaminated grass (Fridriksson, 1983). A significant release of F is also associated with persistent non-eruptive degassing. Etna, which emits through its summit crater plume about 40–200 Gg/a of HF, is the largest known point source of this gas (Pennisi and Le Cloarec, 1998; Francis et al., 1998). Due to these huge emissions, meteoric recharge is thought to be the main F source of the groundwater system. Estimated annual inputs through precipitation for the entire Etna area range from 0.54 Gg (Oct. 2000–Oct. 2001—D’Alessandro et al., 2002) to 0.94 Gg (Oct. 1997–Oct. 1998— Aiuppa et al. 2001a). Volume-weighted F content in rainwater for the period Oct. 1997–Oct. 2001 is 1950 mg/l, a value that is significantly higher than those observed in the Etna groundwater. Fluorite precipitation is probably not a F limiting process since all the analysed groundwaters are undersaturated with respect to this mineral. Zevenberger et al. (1996) have shown the great F adsorbing capacity of andosoils which are typical for active volcanic areas. Also in the Etna area most of the volcanogenic F carried by rainwater is captured by soils, whose adsorbing capacity has recently been demonstrated (D’Alessandro et al., 2002). Adsorption by soils is probably a very effective F limiting factor: in fact, while meteoric contribution is highest at high altitudes, close to the main craters, the highest values in groundwaters are observed on the lower flanks (Fig. 5). Most of the dissolved F probably comes from water–rock interaction processes since maxima are observed in the W and SW sectors where these processes are relatively more effective. On the other hand the high values observed in the SE sector (Fig. 5d), which is the most densely inhabited area, are probably derived from pollution by urban wastewaters. Enhanced F concentrations in this area are, in fact, associated with high B and PO4 contents.
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