• aquifer
    The study of aquifers and the water flows in them is called hydrogeology.

    Illustration by Tim Gunther

    Great Artesian Basin
    The world's largest known aquifer is the Great Artesian Basin, in Australia, at more than 1.7 million square kilometers (661,000 square miles).

    An aquifer is an underground layer of water-bearing rock. Water-bearing rocks are permeable, meaning that they have openings that liquids and gases can pass through. Sedimentary rock such as sandstone, as well as sand and gravel, are examples of water-bearing rock. The top of the water level in an aquifer is called the water table.

    An aquifer fills with water from rain or melted snow that drains into the ground. In some areas, the water passes through the soil on top of the aquifer; in others, it enters through joints and cracks in rocks. The water moves downward until it meets less permeable rock.

    Aquifers act as reservoirs for groundwater. Water from aquifers sometimes flows out in springs. Wells drilled into aquifers provide water for drinking, agriculture, and industrial uses. Aquifers can dry up when people drain them faster than nature can refill them. Because aquifers fill with water that drains from the surface of the Earth, they can be contaminated by any chemical or toxic substance found on the surface.

    There are two types of aquifers. An unconfined aquifer is covered by permeable rock and can receive water from the surface. The water table of an unconfined aquifer rises or falls depending on the amount of water entering and leaving the aquifer. It is only partly filled with water.

    In contrast, a confined aquifer lies between two layers of less permeable rocks and is filled with water. Water trickles down through cracks in the upper layer of less permeable rock, a nearby water source, such as an underground river or lake, or a nearby unconfined aquifer.

    An artesian well is a type of confined aquifer that flows upward to the Earth's surface without the need for pumping. The artesian well sits below the water table at the bottom of U-shaped aquifers. Pressure from water in the long sides of the aquifer pushes the water up the well shaft.

  • Term Part of Speech Definition Encyclopedic Entry
    agriculture Noun

    the art and science of cultivating the land for growing crops (farming) or raising livestock (ranching).

    Encyclopedic Entry: agriculture
    aquifer Noun

    an underground layer of rock or earth which holds groundwater.

    Encyclopedic Entry: aquifer
    artesian well Noun

    type of confined aquifer that flows to the Earth's surface without the need for pumping.

    confined aquifer Noun

    layer of water-bearing rock between two layers of less permeable rock.

    contaminate Verb

    to poison or make hazardous.

    gravel Noun

    small stones or pebbles.

    groundwater Noun

    water found in an aquifer.

    Encyclopedic Entry: groundwater
    industrial Adjective

    having to do with factories or mechanical production.

    permeable Adjective

    allowing liquid and gases to pass through.

    rain Noun

    liquid precipitation.

    Encyclopedic Entry: rain
    reservoir Noun

    natural or man-made lake.

    Encyclopedic Entry: reservoir
    sand Noun

    small, loose grains of disintegrated rocks.

    sandstone Noun

    rock formed by grains of sand.

    sedimentary rock Noun

    rock formed from fragments of other rocks or the remains of plants or animals.

    snow Noun

    precipitation made of ice crystals.

    soil Noun

    top layer of the Earth's surface where plants can grow.

    spring Noun

    small flow of water flowing naturally from an underground water source.

    toxic Adjective

    poisonous.

    unconfined aquifer Noun

    layer of water-bearing rock covered by permeable rock.

    water-bearing rock Noun

    rock that can hold water in tiny pores.

    water table Noun

    underground area where the Earth's surface is saturated with water. Also called water level.

    Encyclopedic Entry: water table
    well Noun

    a hole drilled in the Earth to obtain a liquid or gaseous substance.

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