Hydrogeology and recharge periods of the unconfined and confined alluvial aquifers in Friuli Venezia Giulia
|Location||International Geological Congress,oslo 2008|
|Author||Franceschini, Giuliana; Cucchi, Franco; Zini, Luca; Treu, Francesco|
|Holding Date||03 September 2008|
The basins of many European rivers expand over highly populated areas that are subjected to intensive exploitation. Considering that groundwater hosted in these alluvial basins supplies the majority of the water consumed by the European Community, the need to understand hydrodynamics and recharge periods of these aquifers is extremely important. The Friuli Venezia Giulia plain, located in the northeastern sector of Italy, hosts well developed Plio-Quaternary unconfined and confined alluvial aquifers. The main surface drainage of the Plain is the Tagliamento River. The regional hydrogeological situation is characterized in the north by an extensive alluvial unconfined aquifer mostly contained in carbonate gravels. This area extends from the Pre-Alps to the resurgence belt. The resurgence belt is 2 to 8 km wide and 80 kilometres long. In this area the water table intersects the topographic surface forming numerous plain springs and rivers. The resurgence belt sets a geohydrological boundary between the Upper and Lower Friulian plain. In this strip the unconfined aquifer changes into a multi-layered confined that reach a thickness of up to 500 m with a progressive increase in a westward direction towards the Adriatic Sea. The multi-layered confined aquifers have been subdivided in shallow (2 layers from -10 to -100m depth) and deep confined (4 layers from -110 to -350m depth) aquifers, separated by a 10 m thick impermeable layer found at 110 m depth.
Rainfall samples, surface river waters and groundwater samples were collected in the period 2004-2007 for physical/chemical analyses such as water temperature, electrical conductivity, pH, anion (HCO3, NO3, Cl, SO4) and cation (K, Na, Mg, Ca) concentrations as well as stable isotope and radiocarbon analyses. The obtained data suggests that the unconfined and shallow confined groundwaters are recharged by river water infiltrations and winter rainfall. This fast recharge process makes these groundwaters susceptible to contamination by discharge from urban areas and small-scale industries. Four hydrogeological provinces have been recognised for these subsurface groundwaters. Radiocarbon dating as well as stable isotope data indicates that there is very little continuity between these aquifers and the deeper aquifers that have more complex groundwater circulations or with a very low permeability. The deep confined groundwaters have probably recharged during the Late Pleistocene (23 to 27 ka). Significant late Quaternary sea-level fluctuations, associated with alternating cooler and wetter periods, would have changed the hydraulic gradients and partially or completely disconnected the deeper parts of the aquifer systems from the more active surface circulations. Comparison with deep confined aquifers in other regions of the Padain Plain indicates that the recharge rates of these deep confined aquifers are low and that, consequently, the deep groundwaters are impacted by over abstraction.