Ground-water fundamentals
2.2 Classification of water present in rocks
Water constitutes a significant part of the earth’s crust. Nace (United States of America) estimated that about 0.5 per cent of the total free water of the earth was present as underground water below the land sur€ace and Vernadsky (U.S.S.R.) that 22 per cent (400 million cubic kilometres) of the total volume of the water present within the earth, including chemically bound water molecules, occurs in the upper 16 km of the earth’s crust. Water can be present within soils and rocks in gaseous, liquid and solid phases, the exact form depending upon a number of conditions. Foi example, water can be present within certain crystal lattices or adsorbed on the outer surfaces of certain mole- cules or soil particles. Thus, in the unsaturated zone, one simple system of classification divides types of water into three groups:
1. Hygroscopic water: adsorbed on to the surface of particles.
2. Capillary water: held by surface tension to particles, thus forming a continuous film 3. Gravitational water : representing that additional water which drains downwards Moisture vapour is present in soil air, moving under vapour pressure gradients or as a result of air currents.
Gravitational water moves downwards under gravity from the soil zone, through the intermediate zone and then to the zone of saturation where it becomes classified as
‘ground water’. Immediately above the zone of saturation extends the capillary fringe, where water is held by surface tension to the rock particles or walls of fissures, joints or fractures, the height of the capillary fringe being related to the dimensions (effective pore size) of the void spaces. Theoretically, the capillary fringe may be as high as 50 m above the water table in clays or as low as a few centimetres in sands. The upper surface of the zone of saturation is referred to as the water table in unconfined conditions; at this surface atmospheric pressure is in equilibrium with hydrostatic pressure. In summary, therefore, the void spaces in the zone of saturation are fully saturated with water (other than locally) whereas above this, in the zone of aeration, the void spaces are generally occupied by liquid water and a mixture of gases.
The problem of the physical condition and occurrence o€ water in the zones of aeration and saturation may be approached in a variety of ways by the specialist studying the associated problem. Thus, the agriculturist, the pedologist, the engineering geologist, the ground-water hydrologist and the mineralogist tend to formulate the problem and to describe the occurrence of water somewhat differently.
Moisture in microstructural soils can be subdivided into three categories of mobility;
astatic (from total moisture capacity to field moisture capacity), mid-mobile (from field moisture capacity to capillary rupture moisture content), and slightly mobile (at a moisture content less than in the case of capillary rupture moisture content). The latter is determined by experiment in the field or from observations when soils dry from the surface. Capillary rupture moisture content corresponds to a stable soil moisture after a prolonged drought.
The problem of moisture movement in an unsaturated soil has been studied by Lebedev (1936), Childs and George (1948), Gardner, Gardner and Gardner (1951), Klute (1952), Phillip (1955, 1957), Childs (1956), Youngs (1958), Nielsen, Biggar and Davidson (1962), Sundnitsyn (1964), Childs (1969) and other researchers.
Generalizing the works by these authors, Rode (1952) has expressed the following ideas about the movement of moisture in unsaturated soil. Under isothermal conditions Darcy’s equation is the main one for calculating this movement:
(a few microns in width) around particles in the available void spaces.
under gravity to the zone of saturation.
2.2 Puze 1
V = - k v @ 2.2( 1) where:
V is the volume of water moving per unit of time through a cross-section with a unit area;
k is the water conductivity coefficient depending on soil properties and moisture content :
v
is the symbol of the gradient;@ is the hydraulic potential :
@ = z + M 2.2(2)
where :
z is the height above a selected datum, representing gravitational potential;
M is the pore water pressure, which in unsaturated soil is negative and dependent upon soil-moisture content.
The unsteady flow of moisture is described by the equation:
2.2(3) where: O (x, y, z, t) is volume moisture content. B y introduction of the diffusivity coeffi- cient D (m2/day) equal to D = k (aM/aO), instead of the last equation the following equation is derived when flow is confined to the vertical direction:
2.2(4) The diffusivity coefficient was first proposed by Childs and George (1948). Equation 2.2(4) applies only when hysteresis plays no part. Hence it may be used to treat only problems of moistening or drying. For any boundary conditions one should know the relationship between 8, M and k.
In the case of vertical moisture movement, the solution of Equation 2.2(4) by applying Boltzmann’s function (6 = zt-112) is, according to Phillip (1957):
m
2.2(5)
where :
z is depth of moistening;
fqe)
is a function of 0 obtained as the solution of one of a series of ordinary differen- m is a positive integer;t is time.
tial equations which can be solved by simple, numerical methods;
Other solutions of equations of the movement of moisture under particular conditions are given in the works written by the authors mentioned above.
Four principal types of ground water can be distinguished, namely: (a) unconfined;
(b) karstic; (c) confined (artesian); and (d) vein or fracture.
Unconfnedground waters are characterized by a free surface which exists in equilibrium with atmospheric pressure. This upper surface of the zone of saturation is referred to as the phreatic surface or water table and, may be present either at or near the land surface or down to many hundred metres below the land surface according to the relevant ground-water and topographic conditions. Replenishment to the unconfined ground- water zone is principally from precipitation, and the recharge areas not uncommonly, but not necessarily, coincide with the land-surface topographic catchment. Ground-water flow is laminar in character.
2.2 page 2
Ground-water fundumeiituls
Karstic ground waters are not uncommonly at shallow depth, unconfined, but as they move downwards to depth may be associated with turbulence. These ground waters occur principally in limestones and dolomites, and other strata subject to solution effects and to leaching. They may be characterized by variable temperature and are commonly hard.
ConJined (urtesian) ground waters occur when an aquifer (characterized by unconfined ground-water conditions) passes beneath relatively impermeable strata. Ground water is then confined under pressure so that if the aquifer is pierced by drilling, ground water will rise to a level above the base of the overlying confining bed; the water is subject to hydrostatic pressure. Confined ground waters are associated with a laminar flow régime, other than locally in certain fractured rocks. Such waters do not generally reflect changes in replenishment at the aquifer outcrop area and are commonly fresh, although at great depths mineralized ground waters may be found.
Vein or fracture ground waters may often be found rising from considerable depths due to either hydrostatic or gaseous heads; they are characteiized not uncommonly by turbulent-flow conditions. Such waters tend to occur in zones of tectonic fracture, in which event they tend not to reflect infiltration changes and may be either fresh or mineralized.
Ground waters have been classified in respect of their genetic nature (Ovchinnikov, 1955), the degree of their circulation (Ignatovich, 1950), and the conditions under which their régimes develop (Altovsky, 1962; Konoplyantsev et al., 1963).
2.2 page 3