Groundwater mixing and pollution induced by the geothermal system operations in shallow groundwater

Groundwater, one of the environmental tools to achieve carbon neutrality, can be a sustainable thermal resource for a geothermal system. This study aims to investigate the groundwater system characteristics related to the thermal use of groundwater from the view of groundwater mixing and pollution for sustainable water resource management. Investigations have been conducted using hydrogeochemistry, multiple isotopes (O, H, Sr, and Rn), and microbial community structure data, around an open loop groundwater heat pump (GWHP) system. Continuous data of groundwater level and temperature showed the thermal plume propagation characteristics depending on the system operations. Multiple isotopes also revealed the specific characteristics accompanying the thermal use of groundwater. Especially, radon tracer quantitatively showed that the horizontal and vertical mixing occurred along main groundwater flow direction by mixing ratio calculations. In contrast, the clogging effects were observed in the wells located near the main flow direction by PHREEQC geochemical modeling and microbial diversity data, suggesting intensive management in these wells. A lot of time for recovery is needed in these wells. Overall results confirmed that combined analysis of hydrogeochemistry, multiple isotopes, and microbial community structure data can be effectively used to identify the impacts of geothermal system on shallow groundwater and to suggest effective management plan.