Hydrogeochemical and microbial signatures of deep and shallow groundwater near the enhanced geothermal system in response to the earthquake

Earthquakes can potentially change deep and shallow groundwater systems, but groundwater responses to the triggering events are rarely investigated. Therefore, we analyzed the hydrogeochemistry and microbial community of two geothermal wells in the Pohang Enhanced Geothermal System (EGS) site and proximal groundwater monitoring wells, and monitored their water levels. These analyses and PHREEQC geochemical modeling for the two geothermal wells of the EGS site indicated that the 4.3-km-deep geothermal wells may not recover their original groundwater quality after the mainshock despite a recovery in groundwater level data. The mixing ratios calculated showed an increase in surface water or stagnant water in the two geothermal wells. Microbial analysis results revealed the effect of the earthquake and similar recovery patterns in the geothermal wells. Conversely, most of the shallow groundwater monitoring wells in the Pohang area near the EGS site recovered their original hydrochemistry values during the four years following the mainshock. Radon concentrations, in particular, recovered their original values within two years, suggesting that radon can be a helpful tracer for seismic responses in groundwater systems. This might be due to the short half-life of radon and the associated rapid response characteristics. This study demonstrated that the groundwater systems showed a certain resilience to the earthquake; however, very deep geothermal wells near the hypocenter of the mainshock failed to fully recover the original water level values within four years. The study results emphasize the significance of post-earthquake monitoring and management of the deep groundwater system from the perspective of groundwater recovery.