Groundwater levels on the monitoring wells on Jeju Island were monitored, which were caused by the M 5.4 earthquake in Gyeongju City area, South Korea, at 11:32:54 on 12 September 2016 (UTC time) and the Kumamoto earthquake, Kyushu, Japan, at 16:25:06 on 16 April 2016 (UTC time). The groundwater levels changed after 2–3 min by the generation of the Gyeongju and Kumamoto earthquakes and exhibited spikes or oscillations depending on the magnitude of the earthquakes. The groundwater level change caused by the Gyeongju earthquake (M 5.4) was mostly larger than that caused by the Kumamoto earthquake (M 5.4). The reason is explained by that the energy of the Kumamoto earthquake with high attenuation could not be effectively transmitted to Jeju Island since the earthquake took place in low Q region whereas a higher energy of the Gyeongju earthquake with low attenuation arrived on Jeju Island because the earthquake occurred in the south-eastern part of the Korean peninsula belonging to the high Q crust. Besides the seismic energy from the Kumamoto earthquake was scattered and reflected on the Tsushima-Goto fault zone between Kyushu and the Korean peninsula, with a strike in the ENE-WSW direction that elongates to the east of Jeju Island.

In instances of damage to engineered barriers containing nuclear waste material, surrounding bedrock is a natural barrier that retards radionuclide movement by way of adsorption and delay due to groundwater flow through highly tortuous fractured rock pathways. At the Gyeongju nuclear waste disposal site, groundwater mainly flows through granitic and sedimentary rock fractures. Therefore, to understand the nuclide migration path, it is necessary to understand discrete fracture networks based on heterogeneous fracture orientations, densities, and size characteristics. In this study, detailed heterogeneous fracture distribution, including the density and orientation of the fractures, was considered for a region that has undergone long periods of change from various geological activities at and around the Gyeongju site. A site-scale discrete fracture network (DFN) model was constructed taking into account: (i) regional fracture heterogeneity constrained by a multiple linear regression analysis of fracture intensity on faults and electrical resistivity; and (ii) the connectivity of conductive fractures having fracture hydraulic parameters, using transient flow simulation. Geometric and hydraulic heterogeneity of the DFN was upscaled into equivalent porous media for flow and transport simulation for a large-scale model.

Carbon capture and storage (CCS) is one of several useful strategies for capturing greenhouse gases to counter global climate change. In CCS, greenhouse gases such as CO₂ that are emitted from stacks are isolated in underground geological storage. Natural analogue studies that can provide insights into possible geological CO₂ storage sites, can deliver crucial information about the safety and security of geological sequestration, the long-term impact of CO₂ storage on the environment, and the field operation and monitoring requirements for geological sequestration. This study adopted a probability density function (PDF) approach for CO₂ leakage monitoring by characterizing naturally occurring CO₂-rich groundwater as an analogue that can occur around a CO₂ storage site due to CO₂ dissolving into fresh groundwater. Two quantitative indices, (QI_tail and QI_shift), were estimated from the PDF test and were used to compare CO₂-rich and ordinary groundwaters. Key geochemical parameters (pH, electrical conductance, total dissolved solids, HCO₃⁻, Ca²⁺, Mg²⁺, and SiO₂) in different geological regions of South Korea were determined through a comparison of quantitative indices and the respective distribution patterns of the CO₂-rich and ordinary groundwaters.

Groundwater droughts are one of the natural disasters that raise serious water issues for humans, and are increasing in frequency due to global climate change. In order to identify groundwater droughts, we recorded groundwater level fluctuations upstream at Changnyeong-Haman River barrage from May 2012 to October 2020, based on the groundwater level characteristics and Nakdong River stages. Next, we grouped groundwater levels by K-means clustering, converted groundwater levels to kernel density estimation (KDE), and calculated a standardized groundwater level index (SGLI). Finally, we judged groundwater drought by using the SGLI values corresponding to the opening and closing of the barrage. In the study area, the SGLI criteria for discriminating groundwater drought were −0.674 (caution), −1.282 (severe), and −1.645 (very severe), respectively, corresponding to the 25th, 10th, and 5th percentiles. Based on the SGLI values, groundwater levels on the monitoring wells mostly lie below the 25th percentile during the five opening periods of the barrage. According to cross-correlation analysis, the groundwater level sensitively reacted with the river stage, which influenced groundwater drought. As a result, the SGLI along with the river stages was verified as an efficient tool for evaluating groundwater drought as well as for appropriately operating the barrage.

This study examines the hydrogeological response to the 12 September 2016 Gyeong-Ju earthquake (ML 5.8) in the southeastern region of the Korean Peninsula. Using 2D hydro-mechanical coupled bonded particle modeling, we simulated the dynamic fault rupture process to analyze stress redistribution and its impact on pore pressure and groundwater levels (GWLs). The results indicated that compressional areas correlated strongly with pore pressure increases and GWL rises, while extensional areas showed decreases in both. Observations from the groundwater monitoring Well 5 at Gyeong-Ju San-Nae and Well 8 at Gyeong-Ju Cheon-Buk, located approximately 15 km from the earthquake’s epicenter, aligned well with the model’s predictions and interpretation, providing validation for the simulation. These findings highlight the capability of hydro-mechanical models to capture fault-induced hydrological responses and offer valuable insights into the interplay between seismic activity and groundwater systems.