抄録
An anion index, AI, is proposed as an indicator to estimate the proximity to the center of geothermal activity. AI, defined as following, is calculated from major anion composition of a hot spring water.AI=0.5×(SO4/Cl+SO4+Cl+SO4/Cl+SO4+HCO3) (unit: equivalents) AI is unity at the steaming ground where geothermal activity is highest. It gradually decreases with increasing a distance from the center of activity, with the appearance of Cl type thermal water, and with mixing of HC03 type water. AI is recommended as a practical qualitative indicator presenting degree of closeness to the center of geothermal activity, but should not be applied to thermal water from deeper wells, to volcanic fumaroles emitting HCl, or to springs originated from sea water.
