groundwater

Groundwater

Water that fills pores and cracks beneath Earth's surface within soil, sediment, and rock.

Groundwater volume compared to freshwater lakes and streams

About 35 times larger.

Groundwater compared to glaciers and sea ice

Nearly one-third as large.

Depth of most groundwater

More than 50% lies within about 750 meters (~2,500 feet) of the surface.

Unsaturated zone

Zone where spaces are filled mostly with air; also called the vadose zone or zone of aeration.

Saturated zone

Zone where spaces are completely filled with water; also called the phreatic zone.

Water table

The upper surface of the saturated zone — it rises and falls with rainfall and seasons.

Percolation

The slow vertical and lateral movement of groundwater through pore spaces.

Groundwater flow speed

Typically less than 0.5 meters per day.

Aquifer

A body of rock, sand, or gravel that can store and transmit groundwater in usable amounts.

Confined aquifer

An aquifer trapped between impermeable layers; under pressure.

Unconfined aquifer

An aquifer open to surface recharge from rainfall and infiltration.

High Plains (Ogallala) Aquifer

A massive Miocene-Pliocene aquifer beneath the U.S. Great Plains, vital for agriculture.

Threat to the Ogallala Aquifer

Overuse and slow recharge are causing water levels to decline.

Formation time of most aquifers

Aquifers formed over geologic time, as water slowly accumulated in porous rock and sediment over thousands to millions of years.

Replenishment of aquifers

No! The process is so gradual that humans cannot refill them on any meaningful short-term timescale.