UNDERGROUND WATER

Where it Comes From

Have you ever thought of what happens to the rain or snow after it has fallen? Most people know that ditches are filled and rivers rise higher in their valleys after rains but seldom think directly of the fate of falling rain or melting snow. There are five places it may go. Some of it evaporates, perhaps as it is falling or later when it is on the ground. Engineers who are estimating the power potential from the rainfall of an area allow for evaporation. Some of the water becomes part of the vegetation, either as water or bound up chemically in organic substances. Some of the water becomes permanently taken out of circulation in the hydration of minerals. In most parts of the world a very large portion passes into the runoff—the rivers and streams, and lakes. The fraction we are interested in now has still another fate—it sinks into the ground to become part of the underground water or ground-water system.

Just what proportion of the rainfall of any area becomes ground water depends on several things. The amount of rainfall affects it for in regions of heavy and steady rainfall the ground becomes saturated so that most of what falls goes into the runoff system. The climate of the region will affect evaporation and whether the precipitation of the region is rain or snow and, thus again, how much water goes into the underground system. A variety of characteristics of the ground that the rain or snow falls on will affect underground water supplies too. The steeper the country the quicker will be the runoff. The penetrability of the ground will be important, for a waterproof clay or an impenetrable, dense rock will allow little or no water through. Man's activities affect greatly the underground water system and how much water comes into it. By removal of forests and overcultivation he increases the runoff fraction and decreases the underground water fraction. The situation is reversed where he builds great dams, irrigates his fields, or practises reforestation. Now, what happens to the water that does sink into the ground?

Almost all rocks and the superficial cover of sand, clay, mud and soil have some pore space between the particles which make them. This may vary from close to none at all in some dense igneous rocks to as much as 35 percent in some very porous materials. If the pores connect with one another then water will be able to trickle through them and the rock is said to be permeable. If we go down a deep mine, in a region where there is lots of water in the surface rocks, we will find that as we travel deeper into the mine the amount of water gradually drops off. This is probably because pore spaces cannot stay open because of the great pressures on the rock at depth. So ground water is a feature of the rocks in the region of the surface and down for only a few thousand feet.

Springs and Wells

In any region where the rocks are porous and permeable the water from rain or melting snow moves downward. Eventually it comes to a surface or a zone below which the pores are filled with water. This surface is called the ground water table and a hole or a well drilled below this level will fill with the water from surrounding pores. Most wells near the surface are exactly like this. This explains too why wells drilled or dug will encounter water almost anywhere.

Springs are a little different, but may originate in a variety of ways. If water penetrates downward from the surface and encounters a non-porous or non-permeable layer it may be forced to run along the top of that layer. If the layer comes to the surface of the ground somewhere the water may come gushing out as a spring. Many springs originate in clays and gravels where the gravels are water-bearing and the clays are non-permeable. In other places springs originate in fracture systems in rocks. If breaks or fractures in a rocky hillside receive water from rainfall, the water may flow downward along the cracks and issue where there is a connection to the surface somewhere down the hill.

In some places such waters, which penetrate deep below the surface, come into contact with hot rocks and themselves get heated. When these waters issue from the ground hot springs are the result. You can swim in naturally warmed water with this kind of history at Banff, Jasper, and Kootenay National Parks.

In regions where sedimentary rocks are tilted up on edge unusual underground water conditions prevail and are sometimes very useful to man. Suppose sandstone and shale, the former porous and permeable and the latter impermeable, are interlayered and lie in a basin-shaped structure with upturned edges. The upturned sandstones would receive water which would percolate downward along the beds where it would be held in by the waterproof shales. Water in the centre of the basin would actually be under pressure and, if a hole were to be bored through the waterproof shales which hold it in, the water would push up towards and perhaps beyond the surface. This type of well is the true artesian well and any such water-bearing layer is called an aquifer.

Many parts of the world would be desolate, semi-deserts were it not for supplies of water which man has been able to tap with drilled wells. Very large areas of northern Queensland in Australia are fed by artesian water, as are vast areas in the western part of Canada and the United States, where upturned sandstones gather water in the mountains and conduct it out under the flat plains where it appears in wells.

Caves and Sinks

When water travels through a rock in pores or in fractures it will dissolve anything which is soluble. Limestone or calcium carbonate, gypsum or calcium sulphate, rock salt or sodium chloride are all highly soluble. If the water becomes channeled in cracks or fissures in such rocks it is common for the percolating waters to gradually enlarge the openings. This tends to increase the amount of water which travels through them. Large openings made this way eventually become caves. In some limestone regions vast caverns with connecting passageways form great mazes underground. This is how the Nakimu Caves in Glacier National Park were formed.

Underground water may dissolve large openings in soluble rocks like limestone during its travels. Nakimu Caves in Glacier National Park consist of several thousand feet of underground caves and connecting passageways. Here, at the end of Cave 4, you can see the dimpled solution surfaces on the rock and the layering or stratification which dips down to the right. © Canadian Government Travel Bureau, 1963

Water which penetrates the rocks above such caverns may pick up material in solution from the rocks it is passing through. When it emerges on the ceiling of an open cavern it may begin to evaporate and deposit some of its dissolved load. When a little lump of this stuff is deposited on the ceiling of the cave, it makes a projection which is a little lower than the surroundings so that the next drop of water will naturally run down on it. In evaporating before it drops to the floor of the cave the water may deposit some more of the lime or whatever it has in solution. Thus, stone "icicles" are built from the roofs of caverns and are called stalactites. The drops of water hitting the floor may build up small masses of stony material there, and these are called stalagmites. They would do this because as they are dropping through the air from ceiling to floor they would be evaporating and would be instantly ready to precipitate on hitting any object. Stalactites and stalagmites are sometimes of great beauty, for small amounts of impurities colour the tall icicles and graceful fluted columns so commonly formed.

When underground water circulation is long continued caves get dissolved out larger and larger. It often happens that large caves near the surface get so large that their roofs collapse and great conical holes filled with rocky rubble are thus formed. In some areas the surface of the ground is pitted with these "sinks" and travel becomes very difficult with closely spaced, steep-walled holes. In gypsum areas the solution is very fast and sinks are filled with twisted and tangled forest undergrowth. In 1959, a farmer and his team of horses were plowing a field in Cape Breton when the ground fell away under them as a hidden gypsum cave suddenly collapsed and a new sink formed. Gypsum areas with sink holes occur at Dingwall, Cheticamp, Pleasant Bay and Ingonish all of which are on the edges of Cape Breton Highlands National Park.

Summary

We have seen, then, that part of the precipitation which falls on the surface of the land evaporates, part becomes immobilized in vegetation and in minerals, part goes into the runoff system, and some of it percolates through pores and openings in the rocks to form an underground water system. We have traced the path of the underground water to find out how springs and wells are formed, how caves and sinks are made and how various conditions affect the distribution of underground water.