Using the tidal method to develop a conceptual model and for hydraulic characterization at the Argentona research site, NE Spain

Tidal response analysis in coastal aquifers represents a complex but unique low-cost aquifer-scale test to determine hydraulic diffusivity ( D_h ) and connectivity to the sea. Here, a simplified numerical methodology is applied to a well-characterized Mediterranean coastal aquifer—the Argentona experimental research site, NE Spain. First, a harmonic analysis was performed over a 2-month period to identify the main tidal constituents. Then, the amplitude attenuation and phase shift for 16 observation wells, located at different distances from the coastline, were evaluated. It was found that direct application of the tidal method, considering only the hydraulic effect and aquifer homogeneity, leads to D_hvalues estimated from the amplitude attenuation one order of magnitude smaller than those derived from the phase shift. To better understand the aquifer’s response to tidal fluctuations, numerical simulations were performed, considering two different effects: (1) the hydraulic connection between the aquifer layers and the sea, and (2) the mechanical effect generated by the compression of the undersea aquifer portion. The simulations revealed that a specific stratified configuration and the inclusion of mechanical effects are required to accurately reproduce the observed wells’ head responses. A scale effect was observed when calibrating the main constituents separately. Thus, calibrating the short-period components resulted in higher D_h estimates. These numerical results demonstrate that mechanical effects can play a strong role in aquifer response to tides. This study provides the first application of the tidal method to a real aquifer considering both the hydraulic and mechanical effects generated by tides.