Variation in Fluoride and Chloride Ion Contents in the Shirakawa River Water

Abstract

The Shirakawa River which has Aso caldera as the drainage basin is a representatible of intermediate scale river which flows through a volcanic rock region.The present study is the elucidation of the variation in fluorid e and chloride ion contents in the river water at the single point of the Shirakawa River, based on the chemical analysis of the river water samples taken from 1975 to 1984. Especially daily samples were collected at Meigo-bashi in Kumamoto City, day by day from May in 1975 to April in 1976.The measured values vary with time (Fig.2 and 3). The contents of fluoride and chloride ion were found to be in the range of 0.36∼1.33 mg/l (arithmetic mean 0.70 mg/l) and 2.5∼16.6 me (arithmetic mean 12.5 mg/l) for daily 366 samples, respectively. The correlation coefficient between fluoride and chloride ion contents was +0.437; thus there was a slight positive correlation. The mean value of fluoride ion contents in the Shirakawa River water was abnormally high and became about 10 times larger than that in the entire Kyusyu region as reported by Onishi (1979).The fluoride ion contents in the river wa ter of main and tributaries in the Shirakawa River which flow through Aso central cones and Aso caldera wall were found to be in the range of 0.35∼3.26 mg/l and less than O.16 mg/l, respectively.The fluoride and chloride ion contents decreased with increasing the flow rate and rainfall in Aso region. However, the extent of the reduction of fluoride ion content was not so pronounced as that of chloride ion content.Ten-year variation in fluoride and chloride ion contents was small, except for 1979 when the Shirakawa River drainage basin was affected by Aso volcanic activities.As the results, it is considered that the origin of fluoride ion in the Shirakawa River water is attributed to volcanic activities including those in the past.