Groundwater Tidal Fluctuations

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In most coastal environments, there is a physical continuum and hydraulic connection between seawater and groundwater. This usually results in the formation of a fresh-saline water interface, but also in tidal fluctuations of the groundwater table close to the sea, whether because of direct tidal ingression and regression of seawater in the aquifer or because of tidal loading, which results in a fluctuating pressure signal in the aquifer. The size (i.e. amplitude) of fluctuation, as well as its timing (i.e. lag after ocean tide), could imply about the physical connection between seawater and (confined) aquifer units and where does it occur (distance from shore), as well as about the aquifer hydraulic characteristics.

We conducted a detailed study of two confined units (B & C) in Dor Bay, at different distances from the shore. Unit B is apparently exposed to seawater right by to shore, therefore allowed the determination of hydraulic properties of the aquifer. On the other hand, the top of unit C is 25-30 m deep just by the coast, and the distance offshore of its connection with the sea is not clear. The very nice tidal fluctuations in this unit, with amplitudes even slightly larger than in B suggest that it is exposed not very far from shore. Using the solution of Li & Jiao (2001), Yulia Narovliansky calculated in her M.Sc. thesis that the connection with the sea occurs at a distance of 500 m offshore.

We also studied different boreholes in the coastal Pleistocene aquifer, as well as in the much deeper Cretaceous aquifer. We found out that ocean tides may cause tidal fluctuations in the aquifer up to > 2 km from the sea (dependent on aquifer storativity), while farther inshore fluctuations are mainly due to earth tides.