Groundwater Salinity and Environmental Change Over the Last 20,000 Years: Isotopic Evidences in the Lower Sado Aquifer Recharge, Portugal

Environmental stable isotopes (δ¹⁸O, δ²H), tritium (³H) and carbon (¹⁴C and ¹³C), together with geochemical data were used in order to identify the origin of salinization in the Lower Sado aquifer (Portugal). Groundwater salinization occurs, which may reach values of several grams of total dissolved salts per liter and above permissible limits of drinking water. The source of this high mineralization could be: (a) seawater intrusion (ancient vs. modern); (b) brine dissolution at depth. Stable and environmental radioactive isotopes together with geochemical signatures of waters have provided an effective label for seawater and freshwater to enable tracing of seawater intrusion, as well as identifying other processes that may be responsible for water salinization, such as salt minerals dissolution and ion exchange processes. The apparent groundwater ages estimated for the Miocene aquifer present an average around 20 ka Before Present indicating the presence of paleowaters. However, in the northern part of the basin modern seawater intrusion seems to play the major role in the groundwater mineralization in the shallow aquifer, where the increase of salinization is not followed by a groundwater ageing increase.