Effect of Eruption and Typhoon to the Chemical Composition of Rainwater Collected in Saga City

Upto 1989, the pH of the rainwater at Saga city was kept around 5.0. However, in 1990, Mt. Fugen on Unzen erupted and in 1991, some extraordinary strong typhoons visited Saga prefecture. In this work, every event rainwater was collected for 24 months from June, 1990 to May, 1992. Electric conductivity, pH, concentrations of Na+, K+, Ca2+, Mg2+, SO42-, NO3-, NHt and Zn2+ were determined. The results showed that the rainwater at Saga city has a tendency of getting a higher acidity. The lowest pH value was 3.8. The reason is considered to be the effect of volcanic eruption and a small amount of rainfall. The concentration of NO3-, SO42- and Cl- in the rainwaters of these two years (1990-1992) were found to be higher than those of 1989. The rainwaters collected under typhoons contained high concentrations of SO42- and Cl- which were considered to be originated from sea salt. The difference between [nss-SO42- +NO3-] and [Ca2 +NH4+] in rainwaters on some typhoons is lower than that in us ual rainwater. Onthe other hand, that in rainwater affected by Mt. Unzen eruption is higher than other rainwaters.