Geochemistry of the formation waters in the Po plain (Northern Italy): an overview.

The Po Valley brines represent the base level of the Quaternary aquifer loc ated in a thick clay-sands sedimentary sequence. Geochemistry indicates tha t these are marine waters, evaporated to the stage of gypsum precipitation and trapped at the bottom of the basin in the late Messinian. Most of the g roundwater samples collected from different springs and wells in the plain result from a mixture of these Na-Cl brines and shallow groundwaters latera lly recharged by the Alpine and Apennine chains. Natural outflows of brackish waters are associated with major tectonic feat ures. Mud volcanoes, located in the eastern sector of the Po plain, are con stantly monitored as sudden chemical changes are significant precursors of seismic activity. In the western sector, calcite-filled veins isotopically record different degrees of water-rock interaction. These are outcropping f ossil conduits, where mixing between shallow groundwaters and deep seated b rines has occurred. The temporal continuity of the hydrological circuits al lows the reconstruction of past and present groundwater circulation pattern s. This paper summarises and integrates the geochemical data produced over man y years in order to obtain a regional picture of brine origins and the natu ral mechanisms of groundwater how. (C) 1999 Elsevier Science Ltd. All right s reserved.