Salt-Water Encroachment in Shirahama Hot Spring. I. Present Status of Hot Springs and Chemical Characteristics of Thermal Water

Abstract

At present, there are about 90 thermal springs in Shirahama town and surrounding area. Springs of high temperature above 50°are distributed in the western beach of the town. The majority of springs, which have been found by boring since 1925, belong to the spouting type with a large amount of carbon dioxide gas. Continual natural issuing of thermal water is maintained by the spouting of a mixture of the gas and water due to the bubbling and expansion of dissolved carbon dioxide through rising of water from the underground. A large amount of calcareous sinter is formed after emergence of the water. The present status of thermal springs and chemical characteristics were examined, and the relationship between partial pressure (PCO2) of carbon dioxide gas and spouting power of thermal water is discussed. PCO2 can be calculated from pH, temperature, and bicarbonate ion content by using Eq. (7). It was found from this calculation that changes in PCO2 or pH during ascent of thermal water in the spring well are closely related to the amount of flow or precipitation of calcareous sinter. Origin of carbon dioxide and bicarbonate ion is discussed from the analyses of chemical nature and the distribution of thermal water of bicarbonate type in southern Kii district.