Abstract
Tamagawa hot spring is a volcanic gas-bearing hydrothermal system derived from Quaternary andesitic to dacitic magma beneath Yakeyama volcano. Thermal waters and precipitates of Tamagawa Hot Spring in 2002 and 2007 were investigated to clarify the geothermal structure of the area. Thermal waters in Tamagawa Hot Spring area are divided into three types: Cl-SO4type (Ohbuki Hot Spring: pH 1.2), SO4type (pH 1.8 to 2.9) and neutral-type (pH 6.1). Concentrations such as F, Cl, SO4, Na, K, Mg and Ca in Cl-SO4type Ohbuki thermal water are ten to several hundred-times higher than those of elements in SO4type thermal waters. The concentrations of elements in SO4 type thermal waters also varied according to changes in geothermal structures supplying steam and the flow system of groundwater and vapor in Tamagawa geothermal system from 2002 to 2007.
The oxygen and hydrogen isotopic ratios of the thermal waters in the system suggest that SO4 type thermal waters are formed by heating of shallow groundwater by the steam separated from Ohbuki thermal water below Tamagawa Hot Spring area. On the other hand, the small variation of chemistry and hydrogen and oxygen isotopic ratios of Cl-SO4 type Ohbuki thermal water from 2002 to 2007 suggests that the Cl-SO4 type Ohbuki thermal water originates from a deeper part of Yakeyama Volcano. The flow rate of Cl-SO4 type Ohbuki thermal water below Tamagawa hot spring area was estimated to be 13,000 l/min.
