Groundwater Impacts from the M5.8 Earthquake in Korea as Determined by Integrated Monitoring Systems

• Published in: Ground water 2020-11, Vol.58 (6), p.951-961
• Main Authors: Lee, Soo‐Hyoung, Lee, Jae Min, Yoon, Heesung, Kim, Yongcheol, Hwang, Seho, Ha, Kyoochul, Kim, Yongje
• Format: Article
• Language: English
• Subjects: Aquifer systems; Aquifers; Basement rock; Boreholes; Brackish water; Coastal aquifers; Data logging; Earthquakes; Electrical conductivity; Electrical resistivity; Environmental Monitoring; Fractures; Freshwater; Groundwater; Groundwater data; Groundwater flow; Groundwater levels; Inland water environment; Joints (timber); Logging; Low temperature; Monitoring; Monitoring systems; P-waves; Permeability; Piezometric head; Precipitates; Rain; Rainfall; Republic of Korea; Research Paper; Research Papers; Rocks; Saline water; Saline water intrusion; Saline-freshwater interfaces; Salt water intrusion; Saltwater intrusion; Seawater; Seismic activity; Seismic waves; Temperature measurement; Temperature profiles; Tidal effects; Vertical profiles; Water circulation; Water column; Water levels
• ISSN: 0017-467X; 1745-6584
• DOI: 10.1111/gwat.12993

This paper describes the impacts of the M5.8(5.1) Gyeongju earthquakes on groundwater levels using data obtained from a unique coastal monitoring well. The monitoring strategy integrates conventional water level monitoring with periodic, continuous measurements of temperature and electrical conductivity (EC) within the water column of the well. Another important component of the monitoring system is a new instrument, the InterfacEGG, which is capable of dynamically tracking the freshwater‐saltwater interface. Although the system was set up to monitor seawater intrusion related to over‐pumping, as well as rainfall and tidal effects, it recorded impacts associated with a large earthquake and aftershocks approximately 241 km away. Seismic energies associated with the M5.8(5.1) Gyeongju earthquakes induced groundwater flows to the monitoring well through fractures and joints in the crystalline basement rocks. Temperature and EC logging data showed that the EC vertical profile declined from an average of approximately 5300 to 4800 μS/cm following the earthquakes. The temperature profile showed a trend toward lower temperatures as the depth increased, a feature not commonly observed in previous studies. Data from the InterfacEGG suggested that the rise in EC was not due to the saltwater intrusion, but from the tendency for brackish water entering the borehole to induce convective mixing at deeper depths as the seismic waves travel through the well‐aquifer system. The increase in groundwater levels was caused by pulse of colder, less brackish water flowing into the well because of the earthquake. This behavior reflects an enhancement in rock permeability by removing precipitates and colloidal particles from clogged fractures, which improve the hydraulic connection with a nearby unit with a higher hydraulic head. This study suggests there is value added with a more aggressive monitoring strategy. Article impact statement: This paper describes the impacts of the M5.8(5.1) earthquakes on groundwater using data obtained from a novel integrated monitoring system. The study illustrates the value added by using continuous well logging rather than single‐depth measurements of water level change. In addition, a new instrument, InterfacEGG, proved to be useful in monitoring the freshwater‐saltwater interface. This new and innovative approach provided convincing evidence of the impact of the main earthquake on the groundwater system. Although our application herein was spec