Time series models based on an artificial neural network (ANN) and support vector machine (SVM) were designed to predict the temporal variation of the upper and lower freshwater-saltwater interface level (FSL) at a groundwater observatory on Jeju Island, South Korea. Input variables included past measurement data of tide level (T), rainfall (R), groundwater level (G) and interface level (F). The T-R-G-F type ANN and SVM models were selected as the best performance model for the direct prediction of the upper and lower FSL, respectively. The recursive prediction ability of the T-R-G type SVM model was best for both upper and lower FSL. The average values of the performance criteria and the analysis of error ratio of recursive prediction to direct prediction (RP-DP ratio) show that the SVM-based time series model of the FSL prediction is more accurate and stable than the ANN at the study site.

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.

This study showed the hydrogeological characteristics of an alluvial aquifer that is composed of sand, silt, and clay layers in a small domain. It can be classified into a lower high-salinity layer and an upper freshwater layer and contains shells and remnant paleo-seawater (average 5000 μS/cm) due to sea level fluctuation. Geological and electrical conductivity logging, a long-term pumping test, and multi-depth water quality measurements were conducted at pumping, injection, and observational wells to evaluate the hydrogeologic properties, identify the optimal recharge rate, and assess artificial recharge. Using a hydraulic test, a large difference in drawdown and salinity appeared at the radially located observational wells because of the difference in hydraulic connectivity between the wells in the small study area. It was concluded that the hydraulic anisotropy and heterogeneity of the alluvial aquifer should be carefully examined when locating an injection well and considering the efficient design of artificial recharge.

Seismic waves caused by earthquakes can lead to the movement of fresh groundwater and saltwater in coastal aquifers. The groundwater level, temperature, and electrical conductivity in coastal monitoring wells on the volcanic island of Jeju all responded to the 2011 M 9.0 Tohoku-Oki earthquake. As a result of the earthquake, groundwater temperature and electrical conductivity patterns demonstrated freshwater outflow and saltwater inflow through the monitoring wells in multi-layered coastal aquifers. The seismicity also affected the behavior of ocean tides occurring at depth along the multi-layered coastal aquifers. These observations prove that the use of multi-depth systems for monitoring groundwater level, temperature, and electrical conductivity are more effective than single monitoring systems for understanding the exact behavior of multi-layered aquifers as well as efficiently detecting earthquake-induced or anthropogenic impacts on aquifers in coastal, karstic, or volcanic areas.

Coastal aquifers are complex systems governed by fresh-saline water interactions and ocean tidal effects. The vertical electrical conductivity (EC) and temperature (T) are general indicators for detecting the fresh-saline water interface (FSI) and sea water intrusion in groundwater wells located in coastal aquifers. In this method brief, we developed a cost-effective Arduino-based automatic-vertical profile monitoring system (A-VPMS) to continuously record vertical EC and T in groundwater wells, with the aim of testing its effectiveness in spatiotemporal monitoring of the FSI in a coastal aquifer located in eastern Korea. By analyzing the high-density EC and T data obtained by the A-VPMS, we evaluated the characteristics of the FSI, such as depth and spatial distribution. Our established EC and T data collection method using the A-VPMS proved to be efficient and reliable, providing an excellent tool for fine-scale temporal and spatial understanding of sea water intrusion. The results of this study demonstrate the potential of the A-VPMS for continuous monitoring of the FSI in coastal aquifers, which is crucial for sustainable management of groundwater resources.

During the cold dry season, a large amount of groundwater is pumped for water-curtain cultivation in agricultural fields mostly located near the four major rivers of South Korea. Several weirs, installed previously to secure sufficient water resources from these rivers, were opened in 2017 for restoring the riverine environment. Weir-controlled river level fluctuations have altered stream–aquifer interactions, leading to groundwater drawdown. In this study, changes in stream–aquifer interaction caused by seasonal pumping and weir opening were investigated, using Visual MODFLOW. Two indexes that considered the pump type and aquifer characteristics—the pumping available thickness (PAT) and aquifer productivity index (API)—were used to analyze the effects of weir opening on seasonal groundwater use. Our simulations indicated that weir opening had a large impact on seasonal groundwater use due to groundwater drawdown, especially with respect to centrifugal pumps located in low transmissivity areas where the API decreased after the weir was opened. The resulting decrease in groundwater pumping rates would lower the thermal effect of the water-curtain greenhouse process. This indicates that water supply alternatives are needed so that the weirs can be operated efficiently and field requirements can be met, while the deleterious effects on groundwater are avoided.