The effect of volcanic eruptions on the chemistry of surface waters: The 1991 and 2000 eruptions of Mt. Hekla, Iceland

The Mt. Hekla eruptions in 1991 and 2000 have provided a unique opportunity to study the local environmental effects of high latitude volcanic eruptions in the middle of winter. Both eruptions started around sunset at sub-zero temperatures. In order to define better these effects we studied the chemistry of surface waters in the vicinity of the volcano. Additionally, we describe and predict the environmental consequences of these volcanic eruptions on the chemistry of surface waters on land and in the ocean. l dissolved elements in the polluted snow melt from the 2000 eruption (Cl, F, Al, Fe, Mn, As, Cd, Pb, Zn, and U) exceeded the limits for water intended for human consumption. The volcanic ash, the primary minerals and secondary mineral phases that commonly form in the weathering environment of Iceland were undersaturated in the polluted snow melts with the exception of fluorite, barite, goethite and amorphous FeOOH. These minerals can constrain the maximum initial concentration of the respective dissolved constituents in the meltwaters. It took few days after the first rainfall on the volcanic ash to flush out the readily soluble constituents in the vicinity of the headwaters of the Ytri–Rangá River. The polluted river waters were oversaturated with respect to several secondary minerals, resulting in precipitation of amorphous Al hydroxide and FeOOH and further, scavenging of some trace elements on the Al and Fe surfaces. Dissolved Al and F were the main contaminants in the Ytri–Rangá river water. Previous studies have shown that combined Al–F toxicity is directly related to the predominance of specific Al–F species in solution. Model calculations for mixing of a Hekla-type volcanic cloud with 4 surface water end-members shows Al–F species to be most important in rain and dilute river waters. This makes Al and F phyto-available but diminishes the concentration of the Al3+ species, the most toxic Al species. Mixing with high alkalinity river waters or seawater results in the domination of non toxic Al(OH)4− species at dilutions greater than 800, thus hindering Al toxicity and bioavailability. tudy indicates that H2SO4 contamination from volcanic eruptions is time and place dependent. Volcanic eruptions that take place during winter at high latitudes result in relatively high global sulfur contamination and relatively low local sulfur contamination due to the low oxidation rate of SO2 into H2SO4 because of the low solar radiation.