Hydrogeological and Hydrogeochemical characteristics of the ground water in the Northern Sector of the Narnese-Amerina Chain (Southern Umbria, Italy)

The goal of this study is to contribute to the hydrogeological and hydrogeochemical characterization of the groundwater in the northern section of the Narnese-Amerina carbonatic ridge (southern Umbria, Italy). In particular a summary is given of the results obtained from a series of hydrogeological and hydrogeochemical investigations carried out after the drilling of the new well field, completed in 2004 by the Umbra Acque S.p.A company, at “Fosso delle Pasquarella”, in the province of Terni, near Corbara lake. The well field provides water for the new water supply system in the middle Tiber valley district; this made it possible to abandon the insufficient and poor quality local water resources used previously. Furthermore, the well field can supply water to Perugia when necessary. The hydrogeological, hydrogeochemical and isotopic investigation referred to in this work were carried out on the well field from 1996, the year in which the first well was drilled, to 2005. This investigation gave some indication on the groundwater circulation on the area and allowed to obtain a general evaluation of the quality and the quantity of the water resources available. The study made it evident the presence of two overlapping aquifers with substantially different hydraulic and hydrogeochemical characteristics. The aquifers closer to the surface lies within middle Jurassic formations (Maiolica and Scaglia) hosts a typical Ca-HCO3 water with low salinity and has a productivity lower than the underlying aquifer. The deeper aquifer hosts a Ca-Mg-HCO3 water, with higher total salinity. The deeper aquifer lies within the lower Jurassic Corniola and Calcare Massiccio formations. The analysis of the piezometric surface variations showed that the annual oscillation of the deeper aquifer is about 8 meters, with the minimum values recorded in May 2002 and in February 2004. Several pumping tests carried out in the well field showed that the trasmissivity of the lower aquifer is about 10-1 m2/sec. Step drawdown tests carried out on the wells were interpreted by means of the Jacob equation (s = BQ + CQ2); according to this equation all of the wells have a low value of parameter B, which resulted at most 28 s/m2. This indicates that the flow’s laminar component is not very significant, as can logically be expected in wells drilled in fractured aquifers. Thus in this case the calculation of the efficiency, which in any case would be low, has no meaning. On the other end, the specific capacity of the wells examined is always rather high, since drawdowns always remain limited even with high discharge. Pumping tests have shown that the well field can supply water with a total discharge of about 300 l/s.