The vent-hosted hydrothermal system of Ruapehu volcano is normally covered by a c. 10 million m3 acidic crater lake where volcanic gases accumulate. Through analysis of eruption observations, granulometry, mineralogy and chemistry of volcanic ash from the 1995-1996 Ruapehu eruptions we report on the varying influences on environmental hazards associated with the deposits. All measured parameters are more dependent on the eruptive style than on distance from the vent. Early phreatic and phreatomagmatic eruption phases from crater lakes similar to that on Ruapehu are likely to contain the greatest concentrations of environmentally significant elements, especially sulphur and fluoride. These elements are contained within altered xenolithic material extracted from the hydrothermal system by steam explosions, as well as in residue hydrothermal fluids adsorbed on to particle surfaces. In particular, total F in the ash may be enriched by a factor of 6 relative to original magmatic contents, although immediately soluble F does not show such dramatic increases. Highly soluble NaF and CaSiF6 phases, demonstrated to be the carriers of ‘available’ F in purely magmatic eruptive systems, are probably not dominant in the products of phreatomagmatic eruptions through hydrothermal systems. Instead, slowly soluble compounds such as CaF2, AlF3 and Ca5(PO4)3F dominate. Fluoride in these phases is released over longer periods, where only one third is leached in a single 24-h water extraction. This implies that estimation of soluble F in such ashes based on a single leach leads to underestimation of the F impact, especially of a potential longer-term environmental hazard. In addition, a large proportion of the total F in the ash is apparently soluble in the digestive system of grazing animals. In the Ruapehu case this led to several thousand sheep deaths from fluorosis.
Excerpt from article:
3. Volcanic fluorosis
The first recorded cases of acute animal fluorosis were from volcanic ashfalls and gases in Iceland (Roholm, 1937). Tens of thousands of sheep, cattle and horses died mostly from acute fluorosis during the 1783 eruption of Lakagigar (Sigurdsson and Palsson, 1957; Thorarinsson, 1979). Following the 1970 Hekla eruption, F concentrations of ash-covered pastures exceeded 4000 µg g-1 and 7500 animal deaths ensued (Oskarsson, 1980).
Acutely lethal doses of F for most mammals are >100 µg g-1 of body weight (NRC, 1974), and grazing animals show immediate toxic impacts with >100 µg g-1 of dietary F (Underwood, 1981). Impacts of lower doses depend on the duration of exposure and chemical form of dietary F. In the form of CaF2, fluoride bioavailability is estimated to be around 50% of that in the highly soluble NaF (Clay and Suttie, 1985). Sheep can generally tolerate dietary intakes of up to 60 µg F g-1 (dry diet), but cattle generally only around 40 µg F g-1 (Thompson, 1978; NRC, 1980).
A chronic form of fluorosis in cattle (Underwood,1981) occurred during and following the 1988-1989 eruption of Lonquimay in southern Chile (Araya et al., 1990, 1993). Here, the pastures contained 240-315 µg F g-1 and the animals did not show signs of fluorosis until about 10 weeks after the eruption began. In the 2 years following the eruption, pasture F concentrations ranged between 7 and 34 µg g-1, which was enough to maintain fluorosis symptoms in cattle that lived during the eruption…
Full article available at Science Direct.