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2

Physical hydrogeology

2.1 Introduction volume of material that is occupied by void space,
or interstices. Porosity, indicated by the symbol n, is
The occurrence of groundwater within the Earth’s usually expressed as the ratio of the volume of voids,
crust and the emergence of springs at the ground V , to the total unit volume, V , of a soil or rock, such
v t
surface are determined by the lithology of geological that n = V /V . Porosity can be determined in the
v t
materials, regional geological structure, geomor- laboratory from knowledge of the bulk mass density,
phology of landforms and the availability of recharge ρ , and particle mass density, ρ , of the porous mater-
b s
sources. The infiltration of rainfall to the water table ial (see Section 5.4.1) using the relationship:
and the flow of groundwater in an aquifer towards a
discharge area are governed by physical laws that ρ b
=−
1
n eq. 2.1
describe changes in energy of the groundwater. In ρ
s
this chapter, the physical properties of aquifer storage
and permeability are discussed in relation to different
In fractured rocks, secondary or fracture porosity
rock types and hydrogeological conditions. Then,
can be estimated by the method of scan lines using
starting with Darcy’s law, the fundamental law of
the relation n = Fa where F is the number of joints
f
groundwater flow, the equations of steady-state
per unit distance intersecting a straight scan line
and transient groundwater flow are derived for the
across a rock outcrop, and a is the mean aperture of
hydraulic conditions encountered above and below
the fractures.
the water table. Next, examples of analytical solu-
Porosity is closely associated with the void ratio, e,
tions to simple one-dimensional groundwater flow
the ratio of the volume of voids to the volume of the
problems are presented and this is followed by an
solid material, V , such that e = V /V . The relation
v
s
s
explanation of the influence of topography in produc-
between porosity and void ratio can be expressed as:
ing various scales of groundwater movement, includ-
ing patterns of local, intermediate and regional flow. e
=
The last section of this chapter deals with the occur- n + eq. 2.2
1
rence of groundwater resources. The wide range of ( e)
aquifer types and expected borehole yields associated
or
with sedimentary, metamorphic and igneous rock
types are described with reference to the hydrogeo-
logical units that occur in the United Kingdom. = n
e eq. 2.3
−
1
( n)
2.2 Porosity Void ratio displays a wide range of values. In soils and
rocks with a total porosity ranging from 0.001 to 0.7,
The porosity of a soil or rock is that fraction of a given the corresponding void ratio range is from 0.001 to 2.3.