This illustration shows the two most common types of aquifers, confined aquifers and unconfined aquifers. An unconfined aquifer can receive water directly from the surface, while a confined aquifer is trapped between two layers of rock.

Illustration by Tim Gunther

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  • An aquifer is an underground layer of rock that holds groundwater. Groundwater is rain or melted snow that has seeped into the ground and is held there. Aquifers are filled slowly. For this reason, aquifers can dry up when people drain them faster than they can be refilled—a process called aquifer depletion. Aquifers can be drained by man-made wells or they can flow out naturally in springs.

  • Term Part of Speech Definition Encyclopedic Entry
    aquifer Noun

    an underground layer of rock or earth which holds groundwater.

    Encyclopedic Entry: aquifer
    aquifer depletion Noun

    process by which people pump more water out of aquifers than can be replaced by rain or snow.

    drain Verb

    to empty.

    groundwater Noun

    water found in an aquifer.

    Encyclopedic Entry: groundwater
    rain Noun

    liquid precipitation.

    Encyclopedic Entry: rain
    rock Noun

    natural substance composed of solid mineral matter.

    seep Verb

    to slowly flow through a border.

    snow Noun

    precipitation made of ice crystals.

    spring Noun

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

    well Noun

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