Geology and Water

Geology is the study of rocks, and we tend to think of rocks in terms of the solid mineral constituents. This is natural because almost the rocks we examine, in outcrop or in hand speciment, are dry. We are apt to forget that, at depths below only a few metres from the surface, almost all rocks in nature are saturated with water (exceptionally and locally, also oil or gas). Some of this water is important to us as a source of fresh water for drinking, agriculture and industry: much of it is not, being too salty. But whether we can use it or not, it is there as an integral part of the rocks.

Just as it is our experience that the upper surface of a liquid at rest is horizontal, so is it our experience that the upper surface of a liquid in motion is inclined from the horizontal, and inclined in the direction of flow. Hydrostatics, the science of fluids at rest, is a special case of hydrodynamics, the science of fluids in motion

We have considered so far those aspects of fluid statics and fluid dynamics that are essential for an understanding of the main theme of this book — pore water and geology — and we have seen that the topics can barely be considered without some mathematics, but that the mathematics could be done by a High School student. Mathematics is a language that more geologists understand nowadays, but it is worth remembering that much of applied mathematics in the natural sciences is either trivial or intractable. It is not our purpose here to derive the laws of flow through porous solids from the Navier-Stokes equation, but rather to express them in terms of the measurable and useful parameters of sedimentary rocks. Our purpose is to understand the processes rather than to develop predictive equations or formulae. Those who seek a higher goal may find the note at the end of the chapter a useful starting point

It is one thing to make experiments to determine, as Darcy did, the amount of water that can be passed through a sand filter: it is quite another to apply these results to the geological materials of an aquifer. Consider an artesian aquifer, as in Figure 4-1, with water entering it in the intake area on high ground and leaving it by leakage or extraction where the ground is lower (to the left of the figure). Any well drilled into this aquifer will encounter water in it that will flow at the surface unless it is restrained by wellhead equipment. The pressure of the water and its density can be measured at the surface, and so the pressure head computed (if the density varies well to well, mean or unit density is taken)

Let us remind ourselves at the outset that rivers are not merely channels that carry rain-water runoff to the sea. Perennial rivers are fed by ground water, streams, and occasionally by rain-water runoff during and after storms: perennial streams are fed by ground water, smaller streams, and occasional runoff: the source is a spring or a zone of seepage, and is the highest intersection of the ground surface with the water- table. Common causes of springs are illustrated in Figure 5–1. When there is a depression in the ground that penetrates the water-table, there is a lake: but a lake may also be fed by a stream, and it may overflow as a stream. In these matters we are nearly always concerned with unconfined aquifers, which are recharged by rainfall that percolates downwards through the soil.

The distinction between ground water and pore water may not be very logical, but it has the merit of distinguishing the readily-exploitable fresher pore water near the surface from the brackish to salty water in the pore spaces of most sedimentary rocks at greater depth. As always, such distinctions recognize tendencies only, for there are areas (such as the Niger delta; see Dailly, 1976, p. 96, fig. 3) where fresh water is found in sands to depths of two or three kilometres. The distinction is also seen as a distinction between meteoric water and what is called connate, the former being derived ‘recently’ from rainfall, the latter being defined as water that was trapped in the pore spaces when the sediment accumulated

We must make the distinction between pre-orogenic deformation and orogenic deformation of sedimentary basins because there is a great deal of evidence that important deformation occurs in sedimentary basins while they are accumulating sediment on a subsiding sedimentary column — particularly in the terminal regressive sequence. The regressive sequence itself is due to a neighbouring orogeny outside the sedimentary basin: the creation of mountains creates sediment in increasing amounts until the supply of sediment to a sedimentary basin exceeds the volume created by subsidence of the sedimentary basin. The sea then tends to become shallower, and the sediments prograde away from the orogeny.

Large-scale sliding of geological sequences, with little internal deformation, is not a new idea. During the second half of the 19th Century, as the geology of the Alps, north-west Scotland, and Scandinavia was being unravelled, evidence emerged of lateral displacements of blocks many tens of kilometres long in the direction of movement. For example, Tornebohm (1896, p. 194) postulated movement of blocks at least 130 km long on Caledonian thrusts. The main difficulty in these ideas was in understanding the mechanics. The paradox was this: the strength of the rock limits the length of the block that can be pushed along a horizontal surface because, if the force applied to the end exceeds the strength of the material, the block will fail by internal shear at the end being pushed. The strength of rocks is quite inadequate to support the push required to move blocks longer than a few kilometres. On the other hand, if the block slides down a slope under the force of gravity, the previous difficulty is replaced by two others: the coefficient of friction of rock on rock suggests that an angle of about 30° would be required for gravitational sliding — and that also implies a vertical relief of about half the length of the block. The restrictions on relief limit the length of blocks that slide under gravity to a few kilometres.

We revert in conclusion to the central theme of this book — the movement of water in the subsurface. The principles we have developed and discussed can be applied to a range of geological problems either as qualitative or as semi-qualitative arguments. We take one of each for illustrative purposes.