An aquifer is an underground layer of rock or gravel that is capable of holding water and supplying it to wells or springs. An aquifer may be a small strip supplying one or two mountain pools that go unnoticed except in spring, or it may be a huge underground reservoir like the Ogallala aquifer, that supplies eight parched states with water.
At road-cuts through hills or mountains, or along canyon sides, nearly everyone has seen rock piled in distinct layers. These layers may be tilted or bent, or partly eroded away, but it is still easy to imagine how the rocks must have been piled up over the centuries, shale over sandstone over differing shale.
Some kinds of rock are more permeable than others, with pores between the grains of mineral where water can pass or be held. Other kinds of rock, like shale, are relatively impermeable, and will not allow water through. Rock that will not let water pass is caprock, or aquaclude. A layer of rock, sand, or gravel that can hold water may form an aquifer.
Different kinds of rock have differing amounts of porosity. Some sedimentary rocks like sandstone and conglomerate (which is made of pebbles and boulders mixed with sand) have high porosity because of the large spaces between the deposited grains. For similar reasons, buried beaches and gravel beds have high porosity because there is room for water between the grains. When fine silt is mixed into the sand though, porosity declines, as the pores fill with fine particles. Limestone may have cavernous openings dissolved out of the mass through chemical action, and hold a lot of water.
Mud and clay begin as formations that carry water. But when they are compacted into slate, mudstone, they become impenetrable. Many igneous rocks are impenetrable too, because they have melted together and lack pores. Heat and pressure have also altered metamorphic rocks to a state in which their component crystals have grown together without pores. Any of these rocks can be fractured or faulted through, however, to a point that water finds passage.
In general, an aquifer sits above an aquaclude, and perhaps below one. A perched water table happens when a raised aquifer is separated form the main water table below by aquaclude. The water of the upper aquifer may flow along the top of the aquaclude until it finds its way out at a spring line along a cliff. One California
example of this is Moses Spring at Pinnacles National Monument, where, in late winter, water rains down on delighted hikers.
Another interesting type of discharge is the artesian spring. Here, a down-sloping aquifer is charged at a high elevation, and the water flows downhill until it finds an opening, possibly miles away. Then the water pressure makes natural fountains where water leaves the ground at the low point of the valley.
Problems with aquifers arise when water is taken from the aquifer at a greater rate than it can reasonably sustain. Eventually the mineral water, deposited over the centuries, will be gone. Another problem can arise when industrial or agricultural chemicals pollute the aquifer. Overbuilding, too, can prevent the aquifer from recharging, as water slides off asphalt and concrete, and flows uselessly to sea.
