Water and Heat Budgets and Chemical Components of a Hot Pool, Oyunuma, Goshogake Geothermal Area, Northern Japan

Abstract

The Goshogake geothermal area shows intensive surface geothermal manifestations, such as hot pools, fumaroles, hot springs, boiling springs and mud volcanoes. Hot grounds and thermally altered zones exist extensively around the above manifestations. The detailed distribution of ground temperatures at 1 m depth was clarified. By using the ground temperature data, conductive heat discharge was estimated to be 4.3×10⁶ cal/sec, which is much larger than those from ordinary geothermal areas. Pattern of the temperature distribution obtained in this study is similar to that obtained before about 30 years. The most characteristic feature of the geothermal area is an existence of a hot pool called Oyunuma. Water and heat budgets of the hot pool were investigated based on various measurements such as water depth, surface and deep water temperature, evaporation and radiation etc. The area of the pool is about 3000m². The western half of the hot pool is generally deeper than 10 m, but the eastern half of it is rather shallower than 5 m. Mean surface temperature is 83.4°C in 1982 and 81.0°C in 1983. Surface temperature in the eastern half of the hot pool are higher than those in the western half. Vertical temperature profiles show that the temperature increases gradually from the surface to 1 or 2 m depth, then to about 10 m depth is constant and below the depth increases rapidly to about 120°C at the right above the bottom. Evapora-tion rates were estimated to be 2.2×10⁵ cc/min and 7.4×10⁴ cc/min by the direct and indirect (micro-meteorological) methods, respectively. Heat discharges are also estimated to be 1.2×10⁸ cal/min and 4.1×10⁷ cal/min, depending on the direct and indirect methods, respectively. The values obtained by the direct method are a little larger than those by the indirect one. In this paper by using the data obtained by the direct method, the following water and heat budgets model was proposed: The hot water of about 90 to 95°C is supplied to the hot pool at a rate of 4 t/h, from the shallow bottom in the eastern half of it and the saturated vapor of about 120°C is supplied to the hot pool at a rate of 9 t/h, from the deep bottom in the western half of it. The supplied water is lost only by evaporation at the water surface. Chemical analyses were made for hot springs in the geothermal area and surface and deep hot water in the hot pool. The hot springs are sulphate springs and the pH of them is about 2.5 on the average. Ac-companying with the suppled hot water and vapor, SO₄-S of 0.6 kg/h and H₂S-S of 5.1 kg/h are brought into the hot pool. These sulphur compounds were deposited at the bottom of the hot pool. The shifts of the geothermal activity in the hot pool may be caused by such a deposition process.