Salt-Water Encroachment in Shirahama Hot Spring. II. Changes in Issuing States and Chemical Components and Mechanism of Sea Water Intrusion

Abstract

Characteristics of salt-water encroachment in Shirahama hot spring were clarified from the analyses of changes in both issuing state and in chemical components, and the mechanism of the intrusion of sea water were discussed from the analyses of changes in major component in the process of intrusion. The progress of salt-water encroachment was traced from the change of chlorine ion. The first step of the progress, which was shown by the marked increase of chlorine ion in some springs distributed on the coast since about 1937, is closely connected with the degassing caused by the discharge of a large amount of thermal water and carbon dioxide gas through many bored wells. The second step, which may be characterized as the after effect of the degassing in the first step, was the gradual progress of the intrusion of sea water since about 1950. Behavior of major components in the process of the intrusion was clarified as the increased potassium, calcium and bicarbonate ions, decreased magnesium and sulfate ions, and no marked change of sodium ion. It was deduced that the following reactions between sea water and surrounding rocks play an important role in changes of above components in the process of sea water intrusion : (a) Hydrothermal alterations with the fixation of magnesium ion and the release of potassium and calcium ions, (b) sulfate reduction with the emergence of hydrogen sulfide and carbon dioxide, (c) formation of calcium sulfate (anhydrite), and (d) dissolution of calcium carbonate deposited in the strata.