Tidal fluctuations in a multi-unit coastal aquifer

Groundwater tidal fluctuations were studied in a multi-layered sandy coastal aquifer at Dor Bay, Israel, 35 m from shore. The aquifer at the site is ca. 50 m thick, and it consists of a superficial phreatic unit, underlain by two confined units, separated by 1–3 m thick clay layers. While the top of the deep unit is at 30 m below sea level, the top of the intermediate unit is irregular, and it is exposed in a nearby onshore ridge, as well as in the bay. The phreatic unit showed negligible tidal amplitudes, while the two deeper units showed relatively strong amplitudes, with those in the deeper unit exceeding those in the shallow unit (4-month average A_gw/A_sw of 0.72 and 0.53). Using Li and Jiao's (2001) solution, we showed that the deeper unit is in hydraulic contact with the sea at 100–400 m from shore, which should be due to the termination or breaching of the deeper clay. This is probably the result of a buried ridge, similar to the submerged calcareous sandstone ridge that exists today in the same location. The weaker amplitudes in the intermediate unit are not in accordance with its closer to shore exposure. We suggest that this is a consequence of the nearby onshore exposure of this unit, which suppresses the tidal signal.