2009 Volume 31 Issue 2 Pages 95-106
In the Uenotai geothermal power station, hydrogen and oxygen isotopic ratios of geothermal water from production wells have been observed since the start of operation in 1994. The Uenotai geothermal water has hydrogen isotopic ratio (δD, SMOW) in a range of -70∼-73‰, and oxygen isotopic ratio (δ18O, SMOW) in a range of -10.2∼-10.8‰. Although the δD is about 10‰ lower than the local meteoric water, the relationship of δD and δ18O suggests meteoric water origin of the geothermal water. The Uenotai geothermal water is classified into three kinds different in isotopic character. Two of them are minor, and one of those two has little higher δ18O, while the other little higher δD than the major one. The occurrence of three different geothermal waters may be explained by a possible case that they are individually ascending through separate paths, or by an alternative case that a single geothermal water is isotopically modified through reaction with rock, mixing with other water, boiling and other processes little before flowing into the production wells. The former case seems more likely, because the latter processes of isotopic modification may be fairly difficult to happen for rather short period before flowing of the geothermal water into the wells. If the former case is a fact, the original meteoric waters are expected to circulate in fairly large and deep part of underground, and infiltration area and time(age) of the original meteoric water require to explain. Residual hot water separated from steam and also cooling tower drainage are reinjected to depth almost the same as those of production wells. A considerable portion of the reinjected water comes out quickly from production wells together with the original geothermal water. During flowing of several hundreds meters from the reinjection wells to the production wells, the reinjected water is extremely heated by surrounding rock, and the amount of heat received is estimated to be as much as 2000 kJ/kg.
