Leaching characteristics of ash from the May 18, 1980, eruption of Mount St. Helens Volcano, Washington

Abstract

Leaching of freshly erupted air-fall ash, unaffected by rain, from the May 18, 1980,-eruption of Mount St. Helens volcano, Washington, shows that Ca , Na+, Mg^ , S0^~, and Cl" are the predominant chemical species released on first exposure of the ash to water. Extremely high correlation of Ca with SO* and Na with Cl in water leachates suggests the presence of CaS04 and NaCl salts on the ash. The amount of water soluble material on ash increases with distance from source and with the weight fraction of small (less than 63 micrometers) ash particles of high-surface area. This suggests that surface reactions such as adsorption are responsible for concentrating the soluble material. CaSO^, NaCl, and other salts are probably formed as microscopic crystals in the high-temperature core of the eruption column and are then adsorbed by silicate ash particles. The environment!y important elements Zn, Cu, Cd, F, Pb, and Ba are released by a water leach in concentrations which could pose short-term hazards to some forms of aquatic life. However, calculated concentrations are based on a water-to-ash ratio of 4:1 or less, which is probably an underestimation of the regionally operative ratio. A subsequent leach of ash by warm alkaline solution shows dramatic increases in the amount of dissolved SiC^, U, and V, which are probably caused by increased dissolution of the glassy component of ash. Glass dissolution by alkaline ground water is a mechanism for providing these three elements to sedimentary traps where they may coaccumulate as uraniferous silica or U-V minerals. Leaching characteristics of ash from Mount St. Helens are comparable to characteristics of ash of similar composition from volcanoes in Guatemala. Ashes from each locality show similar ions predominating for a given leachate and similar fractions of a particular element in the ash removed on contact with the leach solution.

An analysis of volcanic SO2 and ash emissions from Copahue volcano

“…Fluoride concentration in Copahue was similar to that of Yasur volcano, i.e., 178-281 mg/kg (Cronin and Sharp, 2002), under similar leachate analysis conditions. Comparisons were made with other volcanoes, e.g., St. Helens: mg/kg (Smith et al, 2017); Ruapehu: 350-850 mg/kg (Cronin et al, 2000); Soufriere de Guadalupe: 540-800 mg/kg (Le Guern et al, 1980).…”