Importance of geological setup for the impact of climatic changes on groundwater: the case of Mount Cucco and Gualdo Tadino Mountains (Central Apennines, Italy)

Mount Cucco and Gualdo Tadino Mountains (Central Apennines), are both east-vergent asymmetric anticlines, delimited by a thrust fault on the east side. They are made of carbonate rocks belonging to the Umbria/Marche Sequence, in which carbonate aquifers alternate with marly aquicludes. The main aquiclude is the Marne a Fucoidi formation, which outcrops almost continuously on the west sides of both structures, and represents a no flow boundary. The thrust line on the east side also represents a no-flow boundary. The Marne a Fucoidi and the thrust line define a “belt” around each structure, enclosing an area of about 43 km2 on Mount Cucco and of about 50.4 km2 on the Gualdo Tadino Mountains. Many springs emerge at the contact between the permeable formations and the impervious “belts”. The karst development is higher on the Calcare Massiccio of Mount Cucco, where there are also important normal and strike-slip faults which connect the aquifer located in the Calcare Massiccio with those hosted in the Maiolica limestone. On the Gualdo Tadino Mountains, there are many springs with average discharge of a few hundreds l/s whereas on the Mount Cucco massif the only Scirca spring has such discharge magnitude (yearly average around 210 l/sec). The springs in the two massifs provide high quality water, for an annual average of about 1500 l/s. The yearly water budget of each structure was studied on the basis of springs discharge measurements and temperature-rainfall data. Since rainfall and temperature data at high elevations were almost completely missing, mean annual temperature and rainfall were estimated by defining elevation/temperature and elevation/rainfall relationships. The climatic data indicate similar rainfall and evapotranspiration for the two structures. According to the estimated water budget, groundwater recharge of Mount Cucco is about 30% higher than the total discharge from the springs, whereas in Gualdo Tadino Mountains the recharge is only about 10% higher than the discharge of the springs, such difference being within the possible range of error. Despite of the high degree of uncertainty, the difference between the results of water budget is too high to be considered as a consequence of the data uncertainty. Although the two recharge areas are similar (about 14% difference) the total discharge of the springs is quite lower in Mount Cucco than in the Gualdo Tadino Mountains (19 Mm3/y vs. 33.7 Mm3/y): this corresponds to a specific groundwater yield of 0.44 m3year-1km-2 for Mount Cucco and of 0.67 m3year-1km-2 for the Gualdo Mountains. On both massifs the structural and geological characteristics suggest that the springs could be fed by an overflow of a deeper regional flow. In such situations the recharge areas of springs and regional flow are separated by groundwater divides, which move as the water table lowers or rises. In Mount Cucco the deep flow development is made easier by the presence of deep karst structures and important tectonic discontinuities, which are not so well developed in the Gualdo Tadino Mountains. This seems to be the reason why the two structures behave in different way. Nevertheless, the uncertainty on the input data suggests to carry out further investigations and to collect new data which would help to define, with more confidence, whether or not a consistent deep flow is also present in the Gualdo Tadino Mountains. This is particularly important because, according to most of the studies on climatic change, a decrease of rainfall has to be expected for the next decades. In the hypothesis of a deep flow hydraulically connected to higher springs, a water table lowering, due to decrease in rainfall, would lead the piezometric divides to move towards the springs, whose recharge area would shrink; a way to minimize the impact of climate change would be to exploit the regional flow, which at present is not explicit ally considered in the water inventories of the area.