PHYSICOCHEMICAL IMPACTS OF VOLCANIC ASH ON SURFACE WATERS: CASE STUDY OF MAY 2008 ERUPTION OF VOLCAN CHAITéN, CHILE

Surface coatings on freshly-erupted volcanic ash are highly acidic, due to the influence in volcanic plumes of aerosols composed of strong mineral acids (primarily H₂SO₄, HCl and HF). Fresh ash also contains a wide range of readily soluble components arising from the adsorption of volatile elements onto ash in volcanic plumes, including metallic elements potentially hazardous to human health such as mercury, arsenic and lead. Thus, when fresh ash comes into contact with surface water bodies, or interacts with rain and meltwaters which feed them, it has the potential to cause changes that may compromise the safety of drinking-water supplies and threaten aquatic habitats. At present, there is no real substitute for monitoring surface waters and water supplies to determine any chemical changes.

After more than 9000 years of inactivity, Volcan Chaitén began erupting on 2 May 2008, with emissions of ash to an altitude of 20 km. On 6 May the eruption intensity increased significantly with the eruption column reaching 30 km. By 12 May the total volume of erupted material was estimated to be >2 km³. The prevailing westerly winds deposited much of this material over Argentina. This eruption is of particular geochemical interest as it was the first major explosive rhyolitic eruption in almost a century.

We visited the region of Patagonia affected by ashfall from the eruption of Volcan Chaitén during the period 23 January to 12 February 2009. Water samples were taken and field measurements recorded from lakes, rivers and streams. Analyses of these samples suggest that even after eight months, the influence of the ash on surface water composition is apparent, particularly in proximal surface waters. Turbidity levels are still very elevated; enrichment was also noted to varying extents for the elements Cu, Fe, Mn, Al, F, As and Pb. However, pH was circumneutral in almost all samples. Comparison of data to water quality protection guidelines indicated that for suitability for drinking water, the parameters most likely to be exceeded are turbidity, iron, manganese and aluminium. In relation to ecosystem protection guidelines, the problematic parameters are turbidity, copper, zinc and aluminium.