Hydrogeologic Heterogeneity Impacts on Fresh-Saltwater Interaction in Jeju Volcanic Island, Korea.

Jeju volcanic island of South Korea is characterized by hydrogeological heterogeneity, which has resulted in complex environments in a coastal aquifer system. The shape of the fresh-saltwater transition zone (FSTZ) and depth-dependent tidal influences on fresh-saltwater interaction in the eastern part of Jeju Island were examined by assessing geological logs from drilling cores, vertical profiles of specific conductance (SC) and temperature from geophysical logging, and performing time series analysis of groundwater level and multi-depth SC (collected from multiple sensors installed at various borehole depths). A sharp interface and step-like FSTZ were developed in the hyaloclastite and lava layers, respectively. The tidal influences on groundwater levels were highly associated with the distance from the coastline; however, SC data revealed different responses to tidal changes according to depth. Based on these data, we propose a conceptual hydrogeological model that incorporates different volcanic structures, including hyaloclastite and lava layers. Conduit flow through the highly permeable hyaloclastite layers led to the development of a sharp interface of FSTZ and disturbed the tidal signals on SC by acting as a preferential pathway for fast and abundant fresh groundwater discharge. Conversely, in the lava layers characterized by the successive formation of high- and low-permeability layers, boundary flows in the geological boundaries created a step-like FSTZ and showed a relatively high association between the tide and SC. This study highlights the crucial role of hydrogeological heterogeneity in determining the complex behaviors of fresh-saltwater interactions in the coastal aquifers of volcanic regions.