Comparison of groundwater levels and hydrochemistry between very deep wells of enhanced geothermal system and surrounding monitoring wells in Pohang, Korea.

Geothermal wells with depths of 4.3 and 4.1 km were drilled into an enhanced geothermal system (EGS) in Pohang, Republic of Korea. After five times of hydraulic fracturing, the groundwater level of the geothermal wells decreased after a Mw 5.5 earthquake in Pohang. The objective of this study was to monitor the groundwater level and temperature and determine the hydrogeological and hydrochemistry of two geothermal wells (PX-1 and PX-2) and one monitoring well (EXP-1) at the EGS site. The hydrochemical data of the EGS wells was also compared with the groundwater characteristics of eight wells at the National Groundwater Monitoring Station (NGMS). First, groundwater samples were collected from a depth of approximately 700 m, and an automatic water level recorder was used to monitor changes in the groundwater level and temperature for two years at the EGS site. Second, groundwater level and temperature data in the NGMS wells were collected from the National Groundwater Information Center's website and annual report on groundwater monitoring for comparisons with the EGS site. The results showed that the rate of groundwater increase was in the order PX-2, PX-1, and EXP-1, and the temperature decrease was the fastest in PX-1 and similar between PX-2 and EXP-1. The eight wells in the NGMS showed only temporary changes. The groundwater in the EGS is the Na-Cl type, but that in the NGMS is a mixture of the Ca-HCO₃ and Na-HCO₃ type. The EGS site has not yet recovered from the earthquake. [ABSTRACT FROM AUTHOR]