The Shallow Aquifer in a Plain Area of Northern Italy: Estimation of the Fresh Water Stocked in the Subsoil

It is not easy to estimate how much water is stocked in the subsoil and the size of the volume of potentially available water in order to pump it for different uses. The present paper suggests a solution to this problem: the purpose concerns the estimation of the amount of fresh water, stocked in a shallow aquifer of a plain area, in relation to effective aquifer porosity (the measurement of the amount and size of subsoil pores), and its spatial and temporal changes. This study describes the case of the shallow aquifer of some provinces in Northern Italy, from 1979 to 2005.

The applied method relates to the detailed reconstruction of the aquifer geological structure, of the heterogeneous distribution of textural and hydrogeological characteristics (hydraulic conductivity and effective porosity), beginning with a geostatistical correlation among about 3500 stratigraphic logs, stored in a hydrological database. It also concerns the temporal and spatial reconstruction of the level (m a.s.l.) of the water table.

The 3D reconstruction of effective porosity is combined with piezometric surfaces, resulting in the variability of the zone of saturation (the volume of interstitial water). Then, the final product of this processing provides the volume of potentially available water (about 34 billion m³), according to the variability of percentage classes of effective porosity. However, only the water stocked in coarse and medium size sediments (medium and medium-to-high effective porosity, >17%) is available to be pumped out of the aquifer: for the study area this represents about 14 billion m³. The amount of available water in different geographic areas of the two provinces has also been estimated, according to their geomorphologic characteristics and their recharge behaviour: outwash, mid and fluvial terraces, morainic walls, piedmont plain, hydromorphic plain as well as the area of the city of Milan.