Seawater intrusion (SWI) has been traditionally described as a seawater wedge that penetrates inland beneath freshwater that flows seawards. We were told that it can be characterized by conventional hydrogeological tools, such as piezometric surfaces (being careful with density variations) and hydraulic tests. We were also told that specific tools for characterizing SWI include salinity profiles and tidal response tests. What we have found in Argentona, as part of project MEDISTRAES, is that none of the above is accurate and some is simply untrue. We have observed that minor heterogeneities, coupled to seasonal fluctuations in flux, cause the SW wedge to be penetrated by discharging fresh water, which leads to complex salinity distributions. Subtle and extensive salinization makes heads hard to measure and often meaningless, so that piezometric surfaces are not only hard to plot but also uninformative. Hydraulic tests are extremely hard to perform due to the impact of sea level fluctuation, which are often more marked than fluctuations due to pumping and require finely tuned filtering. We have also explained the everpresent “anomalies” in salinity profiles measured in fully slotted wells (sharp changes in salinity rarely reaching SW salinity at the bottom): they reflect mixing within the well caused by the complex salinity distributions in the aquifer mentioned above. We have also found that tidal response tests, the landmark hydraulic test of coastal aquifers, are affected by ubiquitous hydro-mechanical effects, which makes their interpretation hard but opens new opportunities. And this is the overall conclusion. The traditional SWI paradigm is not appropriate. A new one is needed that includes new characterization. At Argentona, thermal and geophysical methods have proven very useful.
Universidad Politécnica de Cataluña