Geochemical criteria to evaluate hydraulic and thermal relationship between geothermal reservoir and nearby hot spring aquifer

Abstract

A criterion to evaluate possible influence of geothermal development onto nearby hot spring aquifer, based on geochemical characteristic of the aquifer, was developed in a form of decision tree. Hydraulic and thermal connection of a geothermal reservoir to nearby hot spring aquifer may be expressed by five types of simplified structure models, which are, identical thermal water-, seepage thermal water-, steam heated-, heat conduction- and independent- systems. Since interference of geothermal fluid production differs depend on such types, required monitoring items for protection of hot spring aquifers also differ for each type. The type of relationship between a specific geothermal reservoir and a nearby hot spring aquifer can be roughly estimated by temperature and anion characteristics of the hot spring waters for volcanic geothermal systems. In a former study, a criterion was developed based on five types of geochemical characteristics of hot spring waters. However, such a categorization was not quite successful when volcanic gas or CO₂ gas was supplied to the aquifer or geometrical setting was not appropriate. Although hydraulic relationship is well described by five types, geochemical characteristics of hot springs are not well represented by five types only. Therefore in this study, a new criterion was developed by adding two new types, CO₂ gas supply and volcanic gas supply types, resulting in seven types in total. Spatial arrangement of the target aquifer and geothermal reservoir is also taken into consideration. The threshold values in this criterion are based on data sets obtained from hot spring wells and natural geothermal manifestations all over Japan. Then the new criterion was applied to hot spring waters in Kuju region, Oita, Japan where three geothermal power plants are in operation. As a result, all seven types were identified for this region. It was found that hot springs of seepage thermal water type and steam heated type densely exist near geothermal power plants, suggesting that such hot spring type identification may be an effective tool for geothermal exploration. The new criterion, which helps to estimate effects of geothermal development onto each nearby hot spring aquifer and enables to protect hot springs against excess geothermal development, may be applied for other volcanic geothermal regions.