Cenozoic Aquifer System in Sedimentary Basin of the Lower Tagus River, Portugal.

The Lower Tagus Basin occupies a large area of Portugal, from the coastal region of Lisbon and the Setúbal Peninsula, to beyond the Spanish border. It is a symmetrical, western counterpart of the large Upper Tagus Basin in the west which is centred in Madrid (Spain). Palaeogeographical reconstruction for the Basin, in Portugal, proposes the existence of a large, tectonic, closed depression filled by coarse deposits coming from the surrounding mountains. In the sedimentary rocks a complex heterogeneous and anisotropic aquifer system has been formed. The presence and the properties of this aquifer system are strongly influenced by the internal geological structure and evolution of the basin during the Cenozoic. This system is made up of an alternating sequence of confined, semiconfined and unconfined aquifers located in the conglomerates, sands, sandstones and limestones, as well as, aquitards and aquicludes in the silts, clays and marls. Sediment heterogeneity in the basin strongly influences groundwater flow path, velocity, transmissivity and yield. Regional changes in groundwater chemistry reveal the direction of flow paths and give an indication of the residence time in the flow system. This aquifer system has along been a source of concern because of the high level of extraction over the last few decades, as well as the progressive degradation of the water quality. Available groundwater resources have been affected by intensive agricultural and industrial activity. This research on aquifers formation in the Tagus Sedimentary Basin was based on geological, hydrogeological, hydrochemical, geophysical and climatic data, which resulted in a vast amount of information that was included in specific reports gathered from Portugal Geological Survey (LNEG), National Water Institute (INAG), Meteorological Institute (INMG) and Oil Research Department (GPEPE), and from an inventory of springs and wells in the area.