Frequency-based detection of tidal influence in groundwater

This technical note introduces a frequency-based framework to detect tidal influences in groundwater using monitoring data. The method combines Butterworth high-pass filtering with fast Fourier transform (FFT) to classify wells based on semidiurnal tidal components, particularly M2 (1.93 cpd). A percentile-based amplitude threshold provides a reproducible classification of ‘Sea Tide’ and ‘Earth Tide’ wells, while dominant S2 (2.00 cpd) signals exceeding M2 are interpreted as pumping influences. Validation was achieved through a complementary time-domain estimate of tidal magnitude (AMT). Applied to 66 wells in the Choushui River Alluvial Fan, the approach identified five coastal stations influenced by sea tides, consistent with spatial AMT patterns. Unlike traditional harmonic analysis, this framework requires no concurrent tide records, offering a scalable and objective tool for coastal aquifer management and seawater intrusion assessment.