A joint geophysical-hydrogeological experiment was performed in 2011 to monitor groundwater level changes in a shallow aquifer caused by tidal loading. Electrical resistance tomographies (ERT) were recorded in time-lapse mode for two days over a 5 hour period and consisted of a 1-m spacing profile of 41 electrodes. Eleven piezometers were installed with an average separation of 10m; pressure and temperature data was recorded at 2 min interval. Other hydrogeological data included water electrical conductivity, and porosity estimations from laboratory and in-situ measurements. The ERT monitoring experiment has shown that it was possible to monitor cyclic seawater intrusion into the shallow coastal aquifer driven by tide loading, and the subsequent hydrodynamic regime of the aquifer. Salinity changes with times caused by the telescoping salt water and freshwater flows and tide loading generate lateral subsurface resistivity changes that can then be monitored. The hydrogeology study has shown that water head data displayed minute changes up to 30 m inland; however, ERT is not sensitive enough to monitor those small vertical changes. The combination of spatially coarsely sampled piezometer data combined with geophysical monitoring allows yielding a more complete picture of the hydrodynamic system occurring in this shallow coastal aquifer.


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