Page 67 - Hydrogeology Principles and Practice

HYDC02 12/5/05 5:38 PM Page 50

50 Chapter Two

(a)

(b)

Fig. 2.20 (a) Geological section at Gale Common, South
Yorkshire showing a spring line at the position of a normal fault
that acts as a barrier to groundwater movement. The existence of
the fault causes groundwater flow through the overlying Upper
Permian Marl via a zone of enhanced permeability. (b) Springs
from Permian Magnesian Limestones appearing through
Quaternary deposits at Gale Common, South Yorkshire. For scale,
a camera lens cap is shown to the right of the spring pool.

hydraulic head (the specific storage, S ) is the sum of water expansion. As a consequence, for an equivalent
s
the volumes of water produced by the two mech- unit decline in hydraulic head, yields from confined
anisms of compaction of the aquifer (eq. 2.30) and aquifers are much less than from unconfined aquifers.
expansion of the water (eq. 2.32) thus: Hence, storage coefficient values of confined aquifers
are much smaller than for unconfined aquifers.
S = αρg + βnρg = ρg(α + nβ) eq. 2.33 Values of material compressibility, α, range from
s
−9
−6
−8
−1
2
10 to 10 m N for clay and sand and from 10 to
−1
2
In other words, groundwater pumped from a con- 10 −10 m N for gravel and jointed rock (Table 2.3).
fined aquifer does not represent a dewatering of the These values indicate that a greater, largely irre-
physical pore space in the aquifer but, instead, results coverable compaction is expected in a previously
from the secondary effects of aquifer compaction and unconsolidated clay aquitard, while smaller, elastic