Integrated approach in groundwater model building: a case study from a fissured aquifer of central Apennine (Italy)

The study area is located in the north-eastern portion of Sibillini Mountains on the Tennacola river basin, between the Mt. Valvasseto (1526 m a.s.l.) to the north and the Mt. Castel Manardo (1917 m a.s.l) to the south. The geological framework is characterized by the Umbria-Marche succession composed by a multilayer of pelagic and emipelagic formations, with alternating calcareous, marly calcareous, calcareous-siliceous and siliceous rocks; in particular, the study area is characterized by the formations of Maiolica, Marne a Fucoidi and Scaglia. These formations are influenced by the presence of the Sibillini Thrust, the most important tectonic-structural element of the area. From the hydrogeological point of view the study area is located in the Umbria-Marche carbonatic domain, and it is characterized by the presence of the Maiolica and Scaglia aquifers, separated by the Marne a Fucoidi aquiclude; the last one is negligible because of particular structural setting (formations folded and overthrusted). The only known data is the observed spring hydrograph measured at the springs, for the period 2004 - 2014; no data are available concerning the hydrogeological parameters or the hydraulic head. Some hydrogeological parameters were derived from the spring hydrograph analysis and an infiltration value was evaluated using different methodologies and bibliographic data. For the first time a preliminary spatial-temporal analysis has been applied to the study area in order to evaluate the real extension of the aquifer studied. The analysis was based on four models that were characterized by an increasing degree of complexity through the use of a minimum of two zones and a maximum of five zones, which consequently increased the number of adjustable parameters from a minimum of 10 to a maximum of 22, calibrated using PEST. Statistical index and information criteria were calculated for each model, which showed comparable results; the information criteria indicated that the model with the low number of adjustable parameters was the optimal model and had a likelihood of 100%. A comparison of the simulated and observed spring hydrographs showed a good shape correspondence but a general overestimation of the discharge, which indicated a good fit with the rainfall time series, and consequently with the estimated water budget and a probably incorrect extension of the aquifer structure. The numerical analysis was closed to the recharge area estimated by the geological and geomorphological analyses, and the study confirmed that the recharge contributes more than half of the total outflow at the springs but is not able to completely feed the springs. It is necessary to consider an external contributor, which is probably flow from the nearby Basal aquifer on the western side of the study area.