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Much of the drinking water on which society depends is contained in shallow aquifers. "For freshwater, the depths are very limited." "Groundwater can be very, very deep, but eventually it's a brine," he said. The oldest groundwater ever found was discovered 2 miles (2.4 km) deep in a Canadian mine and trapped there between 1.5 and 2.64 billion years ago.īut the deeper one digs for water, the saltier the liquid becomes, Phillips said. The water in an aquifer can be held beneath the Earth's surface for many centuries: Hydrologists estimate that the water in some aquifers is more than 10,000 years old (meaning that it fell to the Earth's surface as rain or snow roughly 6,000 years before Egypt's Great Pyramid of Giza was built).
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In other areas, where the rock and soil are looser and more permeable, groundwater can move several feet in a day. Depending on the density of the rock and soil through which groundwater moves, it can creep along as slowly as a few centimeters in a century, according to Environment Canada. Recharge primarily happens near mountains, and groundwater usually flows downward from mountain slopes toward streams and rivers by the force of gravity, Phillips said. When new surface water enters an aquifer, it "recharges" the groundwater supply. Such wells are known as artesian wells, and the aquifers they tap into are called artesian aquifers or confined aquifers. When groundwater flows beneath an aquitard from a higher elevation area to a lower elevation, such as from a mountain slope to a valley floor, the pressure on the groundwater can be enough to force the water out of any well that's drilled into that aquifer. An aquitard can trap groundwater in an aquifer and create an artesian well.