Hydraulic head fluctuations in an intermediate depth coastal surface aquifer

Abstract

Tides and storm surges are important drivers of groundwater circulation and fluxes in coastal aquifers. However, few field studies have characterized the response of the coastal aquifer to both forcings. Here, three years of hydraulic head observations in a roughly 20-m deep barrier island surface aquifer are used to investigate the inland propagation of fluctuations driven by ocean water-level changes owing to diurnal and semi-diurnal tides and storm-induced surge and wave-driven setup. Similar to prior observations, the observed rate of amplitude attenuation of the hydraulic head fluctuations is higher than the inland increase in phase lag for the tidally-driven head fluctuations. Additionally, tidal hydraulic head fluctuations deeper in the aquifer lead those nearer the surface of the aquifer. In contrast, storm surges with periods of several days have similar rates of amplitude attenuation and inland increase in phase lags, and hydraulic head fluctuations are roughly depth uniform. A nonlinear, intermediate aquifer-depth theory (not previously compared with field observations) describes the inland and vertical changes of tidal amplitudes and phase lags, and collapses to the linear solution for long period fluctuations, consistent with the storm surge observations. The diffusivity estimated by fitting intermediate depth solutions to the observed tidal amplitude attenuation and rate of inland phase lag is consistent with the aquifer properties, providing a method to characterize aquifers using coastal head observations. Numerical model simulations neglecting capillary effects, hysteresis, and vertical layering (which can cause discrepancies between amplitude attenuation and phase lag evolution) are similar to the observations and support the importance of the depth of the aquifer relative to the wavelength of the hydraulic head fluctuations.