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Physical hydrogeology 19

Fig. 2.1 Types of porosity with relation to
rock texture: (a) well-sorted sedimentary
deposit having high porosity; (b) poorly
sorted sedimentary deposit having low
porosity; (c) well-sorted sedimentary
deposit consisting of pebbles that are
themselves porous, so that the whole
deposit has a very high porosity;
(d) well-sorted sedimentary deposit
whose porosity has been reduced
by the deposition of mineral matter
(cementation) in the interstices;
(e) soluble rock made porous by solution;
(f ) crystalline rock made porous by
fracturing. After Meinzer (1923).

In general, unconsolidated sediments such as gravels, Table 2.1 Range of values of hydraulic conductivity and porosity
sands, silts and clays, which are composed of angular for different geological materials. Based on data contained in
Freeze and Cherry (1979) and Back et al. 1988.
and rounded particles, have larger porosities than
indurated, consolidated sediments such as sandstone Geological material Hydraulic Porosity, n
and limestone. Crystalline igneous and metamorphic conductivity,
−1
rocks have especially low porosities because the pores K (m s )
are merely within the intercrystal surfaces. Conver-
−5
Fluvial deposits (alluvium) 10 –10 −2 0.05–0.35
sely, formations rich in platy clay minerals with very
Glacial deposits
fine grain size can achieve high porosity values. Basal till 10 −11 –10 −6 0.30–0.35
As illustrated in Fig. 2.1, porosity is controlled by Lacustrine silt and clay 10 −13 –10 −9 0.35–0.70
−7
the shape and arrangement of constituent grains, the Outwash sand and gravel 10 –10 −3 0.25–0.50
degree of sorting, compaction, cementation, fractur- Loess 10 −11 –10 −5 0.35–0.50
Sandstone 10 −10 –10 −5 0.05–0.35
ing and solutional weathering. Porosity values range
Shales
from negligibly small (0%) for unfractured to 0.1 (10%) Unfractured 10 −13 –10 −9 0–0.10
for weathered crystalline rocks to 0.4–0.7 (40–70%) Fractured 10 –10 −5 0.05–0.50
−9
for unconsolidated clay deposits (Table 2.1). Mudstone 10 −12 –10 −10 0.35–0.45
−9
There is a distinction between primary porosity, Dolomite 10 –10 −5 0.001–0.20
−7
Oolitic limestone 10 –10 −6 0.01–0.25
which is the inherent character of a soil or rock matrix
Chalk
that developed during its formation, and secondary Primary 10 –10 −5 0.15–0.45
−8
porosity. Secondary porosity may develop as a result Secondary 10 –10 −3 0.005–0.02
−5
−3
of secondary physical and chemical weathering along Coral limestones 10 –10 −1 0.30–0.50
−6
the bedding planes and joints of indurated sediments Karstified limestones 10 –10 0 0.05–0.50
−8
Marble, fractured 10 –10 −5 0.001–0.02
such as limestones and sandstones, or as a result of
−7
Volcanic tuff 10 –10 −5 0.15–0.40
structurally controlled regional fracturing and near- Basaltic lava 10 −13 –10 −2 0–0.25
surface weathering in hard rocks such as igneous and Igneous and metamorphic rocks: 10 −13 –10 −5 0–0.10
metamorphic rocks. Where both primary and second- unfractured and fractured
ary porosities are present, a dual-porosity system is
recognized, for example as a result of fracturing and
fissuring in porous sandstone or limestone.
Not all the water contained in the pore space of a of water released from groundwater storage per unit
soil or rock can be viewed as being available to ground- surface area of aquifer per unit decline in the water table
water flow, particularly in fine-grained or fractured is known as the specific yield, S (see Section 2.11.3).
y
aquifers. In an aquifer with a water table, the volume The fraction of water that is retained in the soil or rock