Geochemical study for utilization of groundwater heat by open heat pump system in northern Okayama and Akaiwa city areas with low precipitation in Japan

This paper examines geothermal heat from groundwater in Akaiwa City and the northeastern part of Okayama City, where precipitation is the second least in Japan. Isotopes of hydrogen and oxygen and chemical compositions of 53 groundwater samples and 6 river water samples were analyzed to investigate the origin and flow modes. To examine changes in water quality, water samples were taken in Okayama City in July 2015 and 2016 and in Akaiwa City in July 2016 and February 2017. The groundwater samples consist of a mixture of Asahi River and Yoshii River water as well as local precipitation; the chemical composition is Ca²⁺– HCO₃⁻. The amount of precipitation contributing to the groundwater samples increases with an increase in distance from both rivers. To inspect groundwater quality for geothermal heat-pump systems, the calcite and goethite precipitations are examined as scales. The results demonstrate that shallow groundwater can be exploited for heat using an open-type ground-source heat-pump system (GSHP); however, in the context of scaling iron materials, such as goethite, a closed-type GSHP is recommended.