How Ground Water Occurs--Continued

An aquifer may be only a few or tens of feet thick to hundreds of feet thick. It may lie a few feet below the land surface to thousands of feet below. It may underlie thousands of square miles to just a few acres. The Dakota Sandstone, for example, carries water over great distances beneath many States, including parts of North Dakota, South Dakota, Montana, Wyoming, Colorado, Nebraska, Kansas, New Mexico, and Oklahoma. On the other hand, deposits of sand and gravel along many streams form aquifers of only local extent.

The quantity of water a given type of rock will hold depends on the rock's porosity--a measure of pore space between the grains of the rock or of cracks in the rock that can fill with water. For example, if the grains of a sand or gravel aquifer are all about the same size, or "well sorted," the water-filled spaces between the grains account for a large proportion of the volume of the aquifer. If the grains, however, are poorly sorted, the spaces between larger grains may be filled with smaller grains instead of water. Sand and gravel aquifers having well-sorted grains, therefore, hold and transmit larger quantities of water than such aquifers with poorly sorted grains.

Natural and artificial recharge of an aquifer.

Artesian aquifer. Both wells are artesian wells, although only one flows.

If water is to move through rock, the pores must be connected to one another. If the pore spaces are connected and large enough that water can move freely through them, the rock is said to be permeable. A rock that will yield large volumes of water to wells or springs must have many interconnected pore spaces or cracks. A compact rock almost without pore spaces, such as granite, may be permeable if it contains enough sizable and interconnected cracks or fractures. Nearly all consolidated rock formations are broken by parallel systems of cracks, called joints. These joints are caused by stresses in the Earth's crust. At first many joints are hairline cracks, but they tend to enlarge through the action of many physical and chemical processes. Ice crystals formed by water that freezes in rock crevices near the land surface will cause the rocks to split open. Heating by the Sun and cooling at night cause expansion and contraction that produce the same result. Water will enter the joints and may gradually dissolve the rock or erode weathered rock and thereby enlarge the openings.

next page