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Physical hydrogeology 47

Fig. 2.19 Types of springs. After Ford and
Williams (1989).

abstraction decreased, but with no apparent effect on supported by an upward effective stress, σ , on the
e
groundwater levels in the recharge zone), such that aquifer material and the water pressure contained in
the water-bearing formation was demonstrating elas- the pore space, P , such that:
w
tic behaviour in releasing water from storage. Later,
in deriving the general partial differential equation σ = σ + P eq. 2.25
T e w
describing transient groundwater flow, Jacob (1940)
formally described the elastic behaviour of porous If the porewater pressure is decreased by groundwa-
rocks. There are two mechanisms that explain how ter pumping or by natural groundwater outflow, the
water is produced by confined aquifers: the porosity stress on the aquifer material will increase causing it
of the aquifer is reduced by compaction and ground- to undergo compression.
water is released; and the water itself expands since Compressibility is a material property that
water is slightly compressible. describes the change in volume (strain) induced in a
As shown in Fig. 2.23, the total downward stress, material under an applied stress and is the inverse of
σ , applied at the top of a confined aquifer is the modulus of elasticity (equal to a change in stress
T