Fig. 27. Diagram of Well which goes dry in Drought, a, and of of Unfailing Well, b
Redraw the diagram, showing by dotted line the normal ground-water surface and by broken line the ground-water surface at times of drought
Fig. 28. Diagram of Wet Weather Stream, a, and of Permanent Stream, b
Redraw the diagram, showing ground-water surface by dotted line
A city in a region of copious rains, built on the narrow flood plain of a river, overlooked by hills, depends for its water supply on driven wells, within the city limits, sunk in the sand a few yards from the edge of the stream. Are these wells fed by water from the river percolating through the sand, or by ground water on its way to the stream and possibly contaminated with the sewage of the town?
At what height does underground water stand in the wells of your region? Does it vary with the season? Have you ever known wells to go dry? It may be possible to get data from different wells and to draw a diagram showing the ground-water surface as compared with the surface of the ground.
Fissure springs and artesian wells. The deeper zones of flow lie in pervious strata which are overlain by some impervious stratum. Such layers are often carried by their dip to great depths, and water may circulate in them to far below the level of the surface streams and even of the sea. When a fissure crosses a water- bearing stratum, or aquifer, water is forced upward by the pressure of the weight of the water contained in the higher parts of the stratum, and may reach the surface as a fissure spring. A boring which taps such an aquifer is known as an artesian well, a name derived from a province in France where wells of this kind have been long in use. The rise of the water in artesian wells, and in fissure springs also, depends on the following conditions illustrated in [Figure 29]. The aquifer dips toward the region of the wells from higher ground, where it outcrops and receives its water. It is inclosed between an impervious layer above and water- tight or water-logged layers beneath. The weight of the column of water thus inclosed in the aquifer causes water to rise in the well, precisely as the weight of the water in a standpipe forces it in connected pipes to the upper stories of buildings.
Fig. 29. Section across South Dakota from the Black Hills to Sioux Falls (S), Illustrating the Conditions of Artesian Wells
a, crystalline impervious rocks; b, sedimentary rocks, shales, limestones, and sandstones; c, pervious sandstone, the aquifer; d, impervious shales; w, w, w, artesian wells.