Hydrological and Thermal Relationship Between Geothermal Reservoir and Nearby Aquifer Identified by Geochemical Characteristic of the Aquifer

Abstract

To estimate possible influence of geothermal development onto nearby hot spring aquifer, a criterion based on geochemical characteristic of the aquifer was developed. Hydrological and thermal connection of a geothermal reservoir to nearby aquifer may be expressed by five types of simplified structure models, which are, identical system, water supply system, steam supply system, heat conduction system and independent system. Recommended monitoring items for protection of hot spring resource are different for each type because possible interferences by geothermal development are different for each type. For volcanic resources, the relationship between a specific geothermal reservoir and a nearby aquifer may be roughly estimated by temperature and anion characteristics of the hot spring. In a former study, a criteria to judge such types was developed based on geochemical data. However, categorization into five types was not quite successful because geochemical characteristics of hot springs are not well represented by only five types although hydrological relationship is well described by five types. Consideration in geometrical setting of the aquifer and the geothermal reservoir is needed as well. Therefore in this study, a new criteria was developed by adding two new types, volcanic gas supply and CO2 supply from rock, resulting in seven types in total. Geometrical settings is also taken in consideration. The threshold values in this criteria were set based on geochemical data taken from hot spring wells and natural manifestations all over Japan. Then a data set of hot springs in Kuju region, Oita, Japan was applied to the criteria, for which all seven types were identified. The new criteria, which enables protection of aquifer and geothermal development as well, may be applied for other volcanic regions.