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Chemical hydrogeology

3.1 Introduction known as hydrogen bonding. The size of the clusters
increases with decreasing temperature reaching a
The study of groundwater chemistry, or hydrochem- maximum at 4°C. When water is cooled from 4°C to
istry, is useful in hydrogeology in a number of ways. 0°C the size of the clusters creates a more open struc-
Interpretation of the distribution of hydrochemical ture and the water becomes less dense, with further
parameters in groundwater can help in the under- expansion on freezing. Hence, ice has a lower density
standing of hydrogeological conditions and can also than liquid water. Values for water density, viscosity,
aid decisions relating to the quality of water intended vapour pressure and surface tension over a temper-
for drinking water. Hydrochemical processes are also ature range of 0–100°C are given in Appendix 2.
significant in attenuating groundwater contaminants. As illustrated in Table 3.1, water is not simply H O
2
In this chapter, the major hydrochemical processes but rather a mixture of six molecules depending on
of importance in groundwater are introduced. Inter- the hydrogen and oxygen isotopes that combine to
pretation techniques for combining data and defining form the water molecule. Eighteen combinations are
16
1
hydrochemical types are also discussed as part of an possible, the most common of which is H O. Pure
2
integrated approach to understanding groundwater water contains hydrogen and oxygen in ionic form as
flow mechanisms. well as in the combined molecular form. The ions are
formed when water dissociates as follows:
+ −
3.2 Properties of water H O j H + OH eq. 3.1
2
+ +
The chemical structure of water is illustrated in The H ion is normally in the form H O (the hydro-
3
Fig. 3.1, which shows one oxygen atom bonded nium ion) and in rock–water interactions, the transfer
+
asymmetrically to two hydrogen ions with a bond of protons (H ions) between the liquid and solid
angle of 105°. The shape results from the geometry
of the electron orbits involved in the bonding.
Oxygen has a much higher electronegativity (a meas-
ure of the tendency of an atom to attract an addi-
tional electron) than hydrogen and pulls the bonding
electrons towards itself and away from the hydrogen
atom. The oxygen thus carries a partial negative
charge (usually expressed as δ−), and each hydrogen a
partial positive charge (δ+), creating a dipole, or elec-
trical charges of equal magnitude and opposite sign a
small distance apart. As a consequence, the opposite
Fig. 3.1 The structure of the water molecule showing the dipole
charges of water molecules attract each other to form created by the partial negative charge on the oxygen atom and
clusters of molecules, through a type of interaction partial positive charge on each hydrogen atom.