Isotope and chemical studies of groundwater in the Kurobe River alluvial fan, Toyama, Japan, for the efficient utilization of groundwater heat

Abstract

The source of groundwater and its flow pattern were investigated by analyzing hydrogen and oxygen isotope ratios and chemical compositions for efficient heat utilization of groundwater in the Kurobe River alluvial fan area in eastern Toyama Prefecture for indoor heating and cooling and greenhouse cultivation. Forty-five groundwater and five river water samples were taken in July 2012, and additional samples were also taken in August and September 2013 to analyze annual changes in the water's chemical composition. The isotope composition analyses revealed that the contribution of river water was large near the Kurobe River, but precipitation was also mixed in. The contribution of river water decreases from the Kurobe River to the central part of the alluvium fan, and the contribution of precipitation increases. The dissolved chemical constituents in the groundwater did not change significantly over time. The electric conductivity (EC) of the groundwater tended to increase with distance from the Kurobe River, which was attributed to the increase in HCO3− and Ca2+ concentrations. When groundwater is used as a heat source in a geothermal heat pump system, it is necessary to investigate the behavior of its precipitates (scales), which can reduce the heat exchange rate. The saturation indices of common scale components—calcite/aragonite (CaCO3) and iron (FeOOH, Fe(OH)3) compounds—were calculated based on the chemical composition of the groundwater. All groundwater was undersaturated for CaCO3, and FeOOH and Fe(OH)3 precipitated in all regions except for some areas; however, the amount of precipitation was not expected to be significant due to the low iron concentration.