Factors controlling groundwater response to earth and atmospheric tides

• Correlations between tidal and hydrological parameters were investigated. • Above 1.6 × 10 -5 m 2 /s, amplitude ratio becomes negatively correlated with transmissivity. • The phase shift becomes positive when transmissivity > 1.6 × 10 -5 m 2 /s. • Storativity is less correlated to amplitude ratio than transmissivity. • Semi-confined BRFs can be observed in fractured, high-diffusivity aquifers whether confined or unconfined. Earth and atmospheric tides are natural stresses known to influence subsurface pore pressure. However, the influence of hydrologic parameters, state of confinement, and flow conditions on the groundwater level response to these natural stresses is the subject of ongoing research. Using a dataset derived from 40 wells in an extensively characterized dolomite formation, this study explores the relationships between five hydrologic parameters and the amplitude and phase of the groundwater response to tides. Predictions of confinement from the phase shift sign and barometric response function are also compared with field-based understanding of confinement. Results indicate that the amplitude ratio of the groundwater response relative to Earth tides increases with transmissivity, diffusivity, and storativity. Specific gravity and aquifer depth are negatively correlated with amplitude ratio. Similar correlation trends are observed between the phase shift and hydrologic parameters. The correlations between phase shift and hydrologic parameters are stronger than the correlations between amplitude ratio and hydrologic parameters. Above a certain threshold (1.6 × 10 -5 m 2 /s) the correlation between amplitude ratio and transmissivity becomes negative, while phase shifts tend to be positive. Predicted amplitude ratios using Hsieh et al. (1987) solution are ≈1 above the threshold. The phase shift sign and the shape of the barometric response function are strongly influenced by diffusivity. Negative phase shift is observed only in unfractured confined regions, where diffusivity is low, whereas positive phase shift is observed regardless of the state of confinement, but only where diffusivity is high (i.e., fractures are present). The barometric response function is accurate at predicting the state of confinement, however, even a confined aquifer can show a semi-confined response function if lateral hydraulic connection to adjacent semi-confined/unconfined areas is significant.