Assessing aquifer responses to earthquakes using temporal variations in groundwater monitoring data in alluvial and sedimentary bedrock aquifers

Water level and hydrochemical parameters from groundwater monitoring wells can be effectively used for interpreting changes in the groundwater system when aquifers are appropriately characterized. Groundwater levels in six paired monitoring wells of alluvial and sedimentary bedrock aquifers in the Gyeongju area, southeastern South Korea were analyzed to evaluate their abrupt changes due to earthquakes, and to identify the differences in responses of groundwater level to the earthquakes and precipitation based on confining conditions of the aquifers, which was verified by principal component analysis (PCA) of water level and major ion concentrations, and lithologic logs. When earthquakes occurred in 2016 near the study area, the different responses of groundwater level to the earthquakes were detected in the two well pairs under confined condition. In particular, wells with single screened interval showed dissimilar patterns in water level fluctuation and hydrochemical water type. The PCA results of groundwater levels and hydrochemical parameters also substantiated the different aquifer types in the two well pairs. These results suggest that groundwater monitoring data should be acquired and analyzed considering the well design and aquifer type to properly evaluate impacts of earthquakes on the aquifers, which can contribute to establishment of earthquake monitoring systems.