Groundwater Flow Modelling of the Aosta Plain in Northern Italy

The work describes the construction of a 3D finite difference groundwater flow model of the Aosta plain (northern Italy), both in steady state and in transient solution. The model takes into account the relation between groundwater and the Dora Baltea river, which crosses the valley from west to east. It has been designed to provide the Regional Agency for Environmental Protection of the Valle d'Aosta Region (ARPA VdA) with tools able to assess quantitative groundwater management. The studied aquifer system is composed by quaternary gravelly and sandy deposits 80 m thick (western side) and 50 m thick (eastern side), overlaying silty lacustrine deposits. Locally silty layers separate the unconfined aquifer from a semi confined aquifer. The mathematical model, implemented with USGS finite-difference code Mod-flow2005, consists in 27 layers, 655 columns and 243 rows, with 20 x 20 m(2) cell size. Stratigraphic logs have been coded and stored in TANGRAM(C) (www.tangram.samit.unimib.it) well database; it has linked to hydraulic conductivity and effective porosity values interpolated within a 3D geostatistical approach to rebuild the complex heterogeneity of the system. The surface-water network has been modelled by means of the StreamFlow-Routing Package (SFR2). The model considers all the items of the groundwater balance, such as recharge, evapotranspiration, snowmelt and well withdrawals. A total number of 26 groundwater head targets and one river flux target were used. Following a successful calibration of the model in a steady state simulation (January 2009), a transient simulation has been developed, on a period of 2 years (2009-2010) with 24 monthly time-step.