This study quantitatively evaluates the effects of tidal fluctuations on groundwater level and electrical conductivity (EC) in a coastal fractured-rock aquifer. The study was conducted at a monitoring well approximately 180 m from the shoreline of Yongho Bay, Busan. Groundwater level, tidal level, and rainfall were continuously measured at 10 min intervals for approximately six months, from 21 August 2008 to 5 February 2009. Vertical EC profiles were measured sequentially during low tide, rising tide, and high tide on 17 February 2009. Following rainfall events exceeding 20 mm, groundwaterlevel peaks occurred after time lags of 16–44 h; the differences were associated with rainfall intensity and duration, antecedent moisture conditions, initial groundwater level, and fracture-network response. The correlation coefficient between tidal and groundwater levels increased from 0.242 for the 10 min records to 0.722 for daily means, showing that the apparent relation is strongly scale dependent after semi-diurnal oscillations are filtered. EC remained below 1,000 μS/ cm above approximately 19 m below ground level and increased sharply below approximately 25 m. Within the single tidal sequence, the maximum EC increased from 47,300 μS/cm at low tide to 67,900 μS/cm at high tide. This stageordered increase is consistent with tide-associated redistribution of saline water in the open borehole below the 19 m casing; however, because the profiles represent one tidal cycle and depth-discrete fracture and borehole-flow data are unavailable, they are interpreted as borehole-scale evidence rather than definitive aquifer-wide interface displacement. The legacy dataset provides a rare high-frequency baseline for scale-dependent tidal–groundwater analysis and informs the design of depth-discrete coastal groundwater monitoring.

Abstract
1. Introduction
2. Materials and Methods
    2.1. Study area
    2.2. Groundwater and tidal level monitoring
    2.3. Vertical electrical-conductivity profiling
3. Results and Discussion
    3.1. Groundwater-level variations caused byrainfall
    3.2. Groundwater-level variations related totidal level
    3.3. Electrical-conductivity variations relatedto tidal level
    3.4. Scope and limitations of the legacydataset
4. Conclusions
References

• Dong-hwan Kang(Institute of Sustainable Earth and Environmental Dynamics (SEED), Pukyong National University, Busan 48513, Republic of Korea) Corresponding author