Vulnerability assessment considering impact of future groundwater exploitation on coastal groundwater resources in northeastern Jeju Island, South Korea

Jeju Island is the largest island in South Korea. Recently, extensive groundwater abstraction has been reported from the shallow aquifer in the northeast region of the island. This study simulated the freshwater resources of the aquifer to estimate the sustainability of groundwater use on Jeju Island in terms of its vulnerability to seawater intrusion. Three-dimensional finite-difference numerical groundwater models were simulated using the MODFLOW-family code SEAWAT. Precise and recent groundwater level and multi-depth salinity data obtained from the study site were used for model calibration; the simulated results showed good agreement with the observed data. SEAWAT was used to delineate the current seawater-freshwater interface to quantitatively estimate the coastal fresh groundwater resources. Future stress scenarios were also simulated in response to increased pumping and various changes in the recharge. The results showed that current groundwater use in the coastal aquifer did not induce seawater intrusion in the coastal aquifer, but seawater intrusion will occur if the dry season continues for the next ten years. The vulnerability assessment based on the predicted groundwater levels and ion concentrations using numerical simulations suggests future vulnerability in the aquifer; therefore, continuous assessment and visualization of the aquifer sustainability is vital. Future projections by the integrated SEAWAT simulation and GALDIT assessment showed that an increase in groundwater pumping may escalate the vulnerability status of coastal groundwater resources from moderate to high in some areas of the study site, by inducing lateral seawater intrusion in deeper areas of the unconfined aquifer.