Finally, water may be an aid or a hindrance to excavation and to a great variety of structural operations, both in war and in peace; and in this relation it again affords geologic problems.
The part played by water in geologic processes, such as that of mineral segregation, is more or less incidentally discussed in other chapters. We may consider more fully in this chapter the application of geology to the general subject of water supplies.
From the geological point of view, water is a mineral,—one of the most important of minerals,—as well as a constituent of other minerals. It becomes a mineral resource when directly used by man. It is ordinarily listed as a mineral resource when shipped and sold as "mineral water," but there is obviously no satisfactory line between waters so named and water supplies in general, for most of them are used for the same purposes and none of them are free from mineral matter. Water which is pumped and piped for municipal water supply is as much a mineral resource as water which is bottled and sold under a trade name. Likewise water which is used for irrigation, water power, and a wide variety of other purposes may logically be considered a mineral resource.
Notwithstanding the immense economic importance of water as a mineral resource its value is more or less taken for granted, and considerations of valuation and taxation are much less in evidence than in the case of other mineral resources. Water must be had, regardless of value, and market considerations are to a much less extent a limiting factor. Economic applications of geology to this resource are rather more confined to matters of exploration, development, total supply, and conservation, than to attempts to fix money value.
DISTRIBUTION OF UNDERGROUND WATER
Free water exists in the openings in rocks where it is sometimes called hygroscopic water. There is also a large amount of water combined molecularly with many of the minerals of rocks, in which form it is called water of constitution. This water is fixed in the rock so that it is not available for use, though some of the processes of rock alteration liberate it and contribute it to the free water. The immediate source of underground water, both free and combined, is mainly the surface or rain waters. A subordinate amount may come directly from igneous emanations or from destruction of certain hydrous minerals. Ultimately, as already indicated, even the surface water originates from such sources.
The openings in rocks consist of joints and many other fractures, small spaces between the grains of rocks (pore space), and amygdaloidal and other openings characteristic of surface volcanic rocks. Many of these openings are capillary and sub-capillary in size. Most rocks, even dense igneous rocks, are porous in some degree, and certain rocks are porous in a very high degree. The voids in some surface materials may amount to 84 per cent of the total volume. In general the largest and most continuous openings are near the surface,—where rocks on the whole are more largely of the sedimentary type and are more fractured, disintegrated, and decomposed, than they are deep within the earth. The largest supplies of water are in the unconsolidated sediments. The water in igneous and other dense rocks is ordinarily in more limited quantity.
Approximate Quantity of Water which will be Absorbed by Soils and Rocks1
| Material | Volume of water absorbed per 100 of material | |
| Sandy soil2 | 45.4 | |
| Chalk soil2 | 49.5 | |
| Clay2 | 50-52.7 | |
| Loam2 | 45.1-60.1 | |
| Garden earth2 | 69.0 | |
| Coarse sand2 | 39.4 | |
| Peat subsoil2 | 84.0 | |
| Sand | 30-40 | |
| Sandstone | 5-20 | |
| Limestone and dolomite | 1-8 | |
| Chalk | 6-27 | |
| Granite | 03.-.8 | |
| 1 Mead, Daniel W., Hydrology: McGraw-Hill Book Co., New York, 1919, p. 393. | ||
| 2 Woodward, H. B., Geology of soils and substrata: Edward Arnold, London, 1912. | ||
Immediately at the surface, the openings of rocks may not be filled with water; but below the surface, at distances varying with climatic and topographic conditions, the water saturates the openings of the rocks and forms what is sometimes called the zone of saturation or the sea of underground water. The top surface of this zone is called the water table, or the ground-water level. The space between the water table and the earth's surface is sometimes referred to as the vadose zone or the zone of weathering, since it is the belt in which weathering processes are most active. The zone of weathering is not necessarily dry. Water from the surface enters and sinks through it and water also rises through it from below; it may contain suspended pockets of water surrounded by dry rocks; it is not continuously and fully saturated.