On the hydrogeology of the Lower Tagus Basin and its Cenozoic geologic evolution

Abstract

Key-words: Hydrogeology; Lower Tagus Basin; Cenozoic; geological evolution. The chemical features of the ground water in the Lower Tagus Cenozoic deposits are strongly influenced by lithology, by the velocity and direction of the water movement as well as by the localization of the recharge and discharge zones. The mineralization varies between 80 and 900 mg/l. It is minimal in the recharge zones and in the Pliocene sand, and maximum in the Miocene carbonated and along the alluvial valley. Mineralization always reflects the time of permanence, the temperature and the pressure. The natural process of water mineralization is disturbed in agricultural areas because the saline concentration of the infiltration water exceeds that of the infiltrated rainwater. In the discharge zones, the rise ofthe more mineralized, sometimes thermal deep waters related to tectonic accidents give rise to anomalies in the distribution of the aquiferous system mineralization model. The diversity ofthe hydrochemical facies ofthe ground water may be related to several factors whose identification is sometimes difficult.