1. Basic Concepts

The largest source of human drinking water is stored and flows in the subsurface. Geological formations saturated in mobile groundwater that can be exploited for human use are called aquifers. This chapter introduces basic notions that set the ground for the understanding and description of subsurface water flow. First, the main properties of water are illustrated, with a particular focus on the forces it establishes with the solid matrix of a porous medium and on how these affect its mobility. Then, broad aquifer classifications are provided, based on their geographical location, their permeability characteristics as a function of the type of porosity (i.e., intergranular, fracture or karst), and their degree of confinement. The latter, which categorizes aquifers as unconfined, leaky or confined, has crucial implications on both their storage capacity and hydrodynamic behavior. The key parameters that characterize an aquifer’s storage capacity are porosity and storativity. While the former is indicative of the total amount of water that can be stored within a porous medium, the latter indicates the fraction that can be released. Both these notions apply to any aquifer type although the mechanism of water release is distinct in unconfined and confined aquifers: in the former, water is released under the effect of gravity alone, and storativity is called specific yield; in the latter, water is released as a result of water expansion that follows a pressure drop. Subsurface water transport, instead, is driven by the existence of a hydraulic gradient (i.e., a drop in hydraulic head, or piezometric level). Under specific hypotheses, groundwater flow can be described by Darcy’s law, which establishes a proportionality relationship between flow rate and hydraulic gradient, and can be used to map an aquifer’s flow field. The relation defined by Darcy’s law is measured by an aquifer-specific parameter called hydraulic conductivity. This parameter is crucial not only in the description of the transport capacity of a porous medium, but also in the calculation of its productivity, which is a function of the hydraulic conductivity and the thickness of an aquifer.