ACID RAIN DISSOLVED WITH VOLCANIC GASES AND ITS CHEMICAL REACTION WITH SOIL

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Chemical characteristics of rain water and areal extension of its acidification in the vicinity of fumarolic area at Kuju volcano were studied between 1994 and 1996. Rain water was collected using rain collectors of open air type set at eleven different sites as far as 7km from the fumarolic area, and sampled at intervals of two weeks. The pH values in the vary vicinity of the fumarolic area ranged from 2.5 to 4, and rose with an increase in distance. Within 1.5km from the fumarolic area, concentrations of Cl^- and SO_4^<2-> occupied more than 90% out of the total anion concentration, and H^+ concentration dominated the cation. Discharge rate of HCl gas emitted from the fumaroles were estimated using the total amount of deposited HCl calculated from areal distributions of Cl concentration and the duration of precipitation during each rain collecting periods. Calculated discharge rate of HCl quickly increased from 2 t/d to 6.3 t/d in July 1995 prior to the eruptive activity started on October 11,1995. This behavior implies that activity of magma at depth has increased two and half months before the eruption. Experiments on acid rain infiltration and chemical reaction with soil was carried out using soil column made with Akahoya soil having acid adsorption capacity. Artificial acid rain of pH3 was supplied to the column at a precipitation intensity of 11.5 mm/h for the total amount of being equivalent to 4000mm. Discharged water from the column showed pH values higher than 5.5 throughout the period of experiment, indicating infiltrated acid rain was neutralized while flowing down the column, Sulphate ion being a dominant anion component of acid rain was mostly adsorbed by soil and adsorbed amount of the ion decreased quickly with distance from the top of the column.