USING GROUNDWATER RESPONSE TO EARTH AND ATMOSPHERIC TIDES TO ASSESS AQUIFER CONFINEMENT

Earth and atmospheric tides are naturally induced stresses known to influence water levels in open wells completed in the subsurface. Attributes of the groundwater level response to tides have been used to infer the degree of aquifer confinement; these include the phase shift of the groundwater response relative to Earth tide signal and the transfer function (also known as barometric response function) between groundwater level and barometric pressure. However, very few studies have compared the phase shift and barometric response function methods against traditional multi-well aquifer testing methods of assessing the degree of aquifer confinement. This study aims to investigate the accuracy of Earth and atmospheric tide methods for predicting the degree of aquifer confinement. Using a dataset of 40 wells in the Culebra dolomite Member of the Rustler Formation around the Waste Isolation Pilot Plant (Carlsbad, NM), this study explores the spatial correlation between phase shift sign, barometric response function shape, and field-based determination of confinement. For each of the 40 wells, the phase shift was derived from spectral analysis of well pressure and Earth tide time series. Similarly, the barometric response function was estimated through regression deconvolution. Field-based definition of confinement involved comparing the depth to water relative to the top of the Culebra dolomite, mapping the spatial continuity of the underlying and overlying confining units, and analysis of aquifer tests. Results indicate that negative phase shift is a good predictor of confined conditions, whereas positive phase shift is observed regardless of the degree of confinement. Positive phase shifts show a strong correlation with higher transmissivities. The results also show that the barometric response function is accurate at predicting the degree of confinement, except at the transition between confined and semi-confined areas, due to lateral propagation of barometric pressure signal through transmissive fracture networks. Overall, these results show that Earth and atmospheric tide methods are sensitive to the degree of aquifer confinement, however, this sensitivity may be impacted by fracture flow.