Multi-depth monitoring of electrical conductivity and temperature of groundwater at a multilayered coastal aquifer: Jeju Island, Korea

To supplement conventional geophysical log data, this study presents temporal variations in electrical conductivity (EC) and temperature with depth in a multilayered coastal aquifer, on the eastern part of Jeju Island, Korea. One-month time-series data obtained at eight points from a multi-depth monitoring system showed that semidiurnal and semimonthly tidal variations induced dynamic fluctuations in EC and temperature. At some depths, EC ranged from 1483 to 26822 μS cm^-1, while some points showed no significant variations. The results of EC log and time-series data revealed that a sharp fresh-saltwater interface occurred at low fide, but the diffusion zone broadened to 20 m at high tide. EC, temperature, and tide level data were used for the cross-correlation analysis. The response time of EC and temperature to tide appears to range from less than 30 min to 11 h. Using end-member mixing analysis (EMMA), the fraction of variations of chloride concentration in the multilayered aquifer was explained, and a conceptual model was developed which subdivided the coastal aquifer into four vertical zones. The percentage of water derived from seawater varied from 2 to 48 at specific depth, owing to tidal fluctuations. Continuous observations of EC and temperature at multiple depths are powerful tools for quantifying the transport of saline water by tidal variations in multilayered coastal aquifers.