ABSTRACT
In this chapter, we present from a pedagogical point of view the theory behind in situ determination of two aquifer characteristics: the transmissivity and the storativity of the aquifer. We shall first define these terms for confined aquifers and later extend the definition to unconfined aquifers. For a confined aquifer, the transmissivity is defined as T = KH, where T denotes the transmissivity, K the coefficient of permeability of a homogeneous, isotropic aquifer, and H the uniform depth of a horizontal confined aquifer. The storativity, S, of a horizontal confined aquifer represents the amount of water released or stored per aquifer area per unit change in the piezometric head. Storativity, S, is a dimensionless quantity, and in the case of confined aquifers, its numerical value is rather low-generally less than 0.001. These characteristics are further shown graphically in Figure 7.1, which illustrates the physical meaning of transmissivity in a confined aquifer and that of storativity in an unconfined aquifer. The transmissivity can be interpreted as the discharge per width through the entire aquifer depth under a unit hydraulic gradient (Figure 7.1a). The storativity on the other hand depends on two separate physical properties: the compressibility of water and the squeezing (consolidating) property of the aquifer. The amount of water released due to the decline of pressure depends on the compressibility of water as well as the consolidation of the aquifer. Thus, the storativity of an aquifer, in principle, can be calculated in terms of compressibility of water and the consolidation characteristics of the aquifer.
