Abnormal groundwater levels and microbial communities in the Pohang Enhanced Geothermal System site wells pre- and post-M 5.5 earthquake in Korea.

In this study, two geothermal wells (PX-1 and PX-2) exhibiting abnormal groundwater levels and microbial communities were examined at the Enhanced Geothermal System site before and after the Pohang earthquake (November 2017). Furthermore, the EXP-1 well level, water temperature, microbial communities and their association with earthquakes, as well as the possibility of future earthquakes were explored. The primary objectives of this research were to: (1) perform correlation and cluster analyses of hydrophysical parameters for earthquakes using next-generation sequencing; (2) analyze pre-, co-, and post-seismic changes in groundwater levels, temperatures, and microbial communities; and (3) further assess the analyzed results of the post-earthquake changes in the groundwater levels and temperatures to interpret their implications. Although the pre-earthquake water levels in the three wells were unknown, their depth-to-water levels post-earthquake ranged from 50.33-98.20 m, 570.91-735.00 m, and 47.70-56.04 m for wells PX-1 (depth 4362 m), PX-2 (4348 m), and EXP-1 (180 m), respectively. In particular, the water levels of PX-2 were abnormally low compared with the surrounding area. Moreover, the geothermal wells demonstrated unstable microbial communities prior to the earthquake. However, while the microbial communities of PX-1 recovered relatively quickly post-earthquake, those of PX-2 failed to stabilize even within two years after the earthquake. Thus, it was inferred here that the PX-2 well is more closely related to seismic activity, the effects of which can still be seen. Accordingly, it is important that PX-2 is continuously monitored until June 2024, the minimum period predicted for the water levels to reach stability.