Hydrogeochemical signatures for sustainable use of shallow groundwater as a thermal resource at groundwater–surface water mixing zone

From the perspective of carbon neutrality, an environmental management tool for reducing the greenhouse effect, groundwater is a sustainable resource. Geothermal energy systems using a groundwater heat pump (GWHP) utilize groundwater as a thermal resource. In this study, the impacts of the GWHP system on groundwater environment were investigated to suggest an environment management plan for sustainable long-term operation, especially at groundwater–surface water mixing zone. It is important to maintain subsurface environment with a high efficiency of the GWHP system. We monitored groundwater temperature and analyzed hydrogeochemical parameters, including radon isotopes, before, during, and after operation of the system. A self-organizing map (SOM), statistical method, and PHREEQC geochemical modeling were also used. The hydrogeochemical analysis indicated that, after operation, the groundwater wells were affected strongly by the operations. Before operation, the hydrogeochemistry was related to surface water intrusion, as the study site is located in groundwater–surface water mixing zone. The key determinants of the water type were Ca, HCO3, and Cl. The clusters derived through SOM, the Stiff diagram, and radon isotope analysis revealed that groundwater wells located near the injection well have distinct anomalies related to the operations. The PHREEQC results also supported this interpretation, with carbonate scaling being predominant. The findings of this study suggest that groundwater wells located near a GWHP system or surface water should be managed more intensively than the wells further from the system. Totally, our results demonstrate that groundwater chemistry reflects the impact of GWHP systems on shallow groundwater, and could inform efficient strategies for managing mixing zones of two water bodies based on SOM and PHREEQC modeling.