ENVIRONMENTAL ASSESSMENT OF 1991 HUDSON VOLCANO ERUPTION ASHFALL EFFECTS ON SOUTHERN PATAGONIA REGION, ARGENTINA

The August 1991 eruption of the Hudson volcano in the southern Andean volcanic zone affected an area of 1000 km in radius to the east in the Argentine Patagonic meseta. The thickness of ash ranged from 20 cm in the Andean area to less than 1 mm in the Atlantic coast zone. Wind st orms reactivated the ash deposits, together with terrigenous material, more than one year after the eruption. In order to assess the potenti al risk of the ash, analyses of concentration, size, mineralogical com position, toxic elements, and irritating effects of gases were perform ed. In all samples, the percentage of particles of the 2- to 5-mu m ra nge is below the toxic threshold level. Trace elements are below the t oxic threshold concentration for humans and animals. The major impact of the ashfall was on sheep herds; about one third of them were lost i n the areas close to the volcano. Soil incorporated the ash layers, an d a fast recovery of orchards was reported two years later. Rivers wer e loaded with sediment in the immediate aftermath, but one year later returned to previous conditions. Along the shores of the Buenos Aires Lake, a fresh tephra layer can be distinguished.