The Uenotai geothermal field, currently developed by the Akita Geothermal Energy Co. (AGECO) which succeeded the extensive exploration by Dowa Mining Co. (DOWA) in 1986, is located approximately 25 kilometers north of the Onikobe geothermal power plant, on the northern heights of the Quaternary volcanic mountains(Oyasu-dake and Takamatsudake)at the southeastern corner of Akita Prefecture, Japan. No surface manifestation is found in the project area itself which is totally covered with the Quaternary acidic pyroclastics but several hot springs and fumaroles are known in the surrounding area.
The field is one of the most promising areas which had been found from the surface and subsurface exploration by DOWA in the area covering 200 equare kilometers since 1971. Based on the data obtained during the exploration and because of the geographic advantages, the exploitation had been concentrated to the Uenotai field since 1975 as the first priority area to be developed, including the drilling of the large exploratory holes and the joint investigation on the potential for the commercial power generation with the Tohoku Electric Power Co.. After the simultaneous flow test using 8 discharge wells in 1988, the geothermal reservoir modeling and simulation studies on a significant amount of the data which were provided by DOWA, AGECO and the government, it was concluded that the field had enough potential for a long-term 27.5 MW power generation. The project is scheduled to be on the commercial operation in 1994 and the construction of the steam supply system has already been started.
The subsurface geology which was revealed by a large number of wells shows that the Uenotai project area is underlain by a NW trending horst block which is considered to be a northern part of the major Oyasu-dake uplift, bounded by a NW-SE fault system. It has also been found that the production in the area is primarily from the microfractures in the lower Doroyu and pre-Tertiary formations, which were created by the uplifting movements, a large number of the intrusive rocks and the faults.
The temperatures of the Uenotai geothermal reservoir at the elevation of 500m below sea level are more than 220°C with a very similar trend of the contours to the geological structure of the horst block and the highest temperatures(>300°C)which are found at the SE corner of the project area are still extending toward Oyasu-dake. From the subsurface temperature distribution and the geochemical studies on the discharged fluids, the heat source of the Uenotai geothermal reservoir is suggested to be located beneath the Oyasu-dake uplift.
During the simultaneous production test in 1988, it was observed that the stabilized flow rate of the steam from 8 wells was 272tons/hour (WHP=6KSCA) and the most of the wells are the steam dominated with a few exceptions from which a minor amount of a typical NaCl-type hot water were produced. The overall steam/hot water ratio is at 3/1.

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The dissolution of limestone collected from Akiyoshidai, Yamaguchi Pref., Japan, in dilute hydrochloric acid solution has been followed by measurements of calcium concentration and pH as a function of time, under various conditions of initial pH, temperature and stirring rate. The initial dissolution rate increases as the stirring rate increasing. The Arrhenius plot shows that the apparent activation energy at 700rpm of stirring rate is 15kJ/mole for initial pH=4 and 25kJ/ mole for initial pH=5.
The viscosity of water depends on temperature and differs about two times within the range from 5°Cto 4°C in this experimental conditions, so stirring rate cannot express the real conditions of bulk fluid. In spite of stirring rate, Stirring Reynolds Number Re_S, which is one of non-dimension numbers possible to express fluid condition, defined as ρd²n/μ where ρ is the density of water, d characteristic length of stirring rate, n the revolutions per second, μ the viscosity of water, is proposed.
The initial dissolution rate at initial pH=4 is almost proportional to Re_S and is independent from temperature. These facts suggest that the dissolution reaction process of calcite is much faster than the mass transport process of species. In the case of initial pH=5, the initial dissolution rate increases as Re_S increasing and as temperature rasing. The apparent activation energy of initial dissolution rate at Re_S =7×10⁴ for initial pH=4is 1kJ/mole and for initial pH=5 is 12kJ/ mole. These values are smaller than those obtained from stirring rate, because the previous studies have not considered fluid condition in estimation of the activation energy.
The facts mentioned above suggest that dissolution of calcite in weak acid region is mass transport controlled reaction.

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Phosphorite nodules were newly found in laminated lacustrine diatomaceous sediments of the Middle Pleistocene Nogami Member in Kusu Formation northern Kyusyu (Japan) which deposited in palaeo-Kusu lake. The phosphorous content of these nodules is about 25% in P₂O₅. These nodules resemble marine phosphorite nodules in composition, consisting primarily of carbonate-fluorapatite.
Phosphorite nodules were formed from phosphate originally contained in diatom during early diagenesis. These nodules show compositional zoning and color banding due to dissolution by meteoric or hydrothermal water. The fixation of phosphorous is likely to have been controlled by concentration of Ca.

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白金の溶解を調べるため,150°C,120日間にわたって各種濃度の塩酸溶液と白金を反応させた.その結果,白金は,1Nと3Nの塩酸溶液に溶けた.特に,3N塩酸溶液(調整時)には,白金試料が全て溶け,約6600ppmの濃度を示した.
白金のこのような高濃度の溶解には,H₂Oの分解により,H₂ガスの発生と逃散のため,O₂ガス分圧が高くなったことに起因すると思われる.
これらの結果は,明らかに,酸性条件と高いfO₂の条件のもとで,白金が高い溶解と移動性のあることを示している.
このことは,白金の溶解度に関する実験的研究(MOUN-TAIN and WOOD,1988),および地熱水(ソルトンシィー地熱地帯)中の高白金濃度に関する研究結果(MCKIBBEN et al., 1990)による指摘と一致している.

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