Processes Influencing Groundwater Level and the Freshwater-Saltwater Interface in a Coastal Aquifer

Few studies have focused on the relationship between the various causes of groundwater level fluctuations and the seawater intrusion process, due to the complexity of this relationship, being the aim of this paper. Piezometric fluctuations in coastal aquifers are determined by a number of processes and their characterization depends on the complexity of the aquifer stratigraphy, as well as many other hydrodynamic factors. The precipitation regime, tides, wave setup and storm surges, as well as atmospheric pressure are the most relevant of these processes. By means of a series of observations made at an experimental site in south-eastern Spain, this study demonstrates the complexity of water table fluctuations in coastal zones. The study employed two piezometers situated very close to the shoreline, excavated in detritic deposits with highly variable hydraulic conductivity (semi-confined aquifer). Continuous measurements were taken of hydraulic head, tide height, electrical conductivity and water temperature. The study concludes that precipitation has the greatest effect on piezometric level, followed by atmospheric pressure and wave action, while the semi-diurnal and fortnightly tidal cycles caused variations of smaller amplitude in the piezometric logs. All these oscillations affect the position of the fresh water-seawater interface. The attenuation of the tidal amplitude observed at the two monitoring points was lower than the value calculated using analytical solutions, and this is due to the semi-confined nature of the aquifer. The calculated tidal efficiency is around 0.4-0.5, giving a t_lag of about 3 h, with a 10–15 min delay between monitoring boreholes, P-II and PI. We also identified that the response of water conductivity and temperature to tidal cycles is not synchronized with the variations in the piezometric level influenced by tidal fluctuations.