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Aquifers, Groundwater, and Wells – An Introduction
In many rural communities, domestic water is not an option – people rely on domestic wells and irrigation water (if available). One of the many great questions I have received from the public over the last few months was: “How do our Wells Work?” Excellent question and worth delving into!
Wells tap into underground aquifers.
Groundwater held in aquifers is one of the most important sources of water on Earth. Approximately 30% of our liquid freshwater is groundwater. The rest of Earth’s liquid freshwater is held in streams, lakes, rivers, and wetlands. Most of the world’s freshwater – 69%! – has, historically, been locked away in glaciers and ice caps.
An aquifer is essentially an underground repository of water. Groundwater can be found in a variety of rock types, but the most productive aquifers are found in porous, permeable rock (e.g. sandstone), or the open cavities and caves of limestone aquifers. Groundwater (and gases) move more readily through permeable rock, which allows faster methods of extracting water. Aquifers can also occur where rock is made of denser material — such as granite or basalt — if that rock has cracks and fractures. The top of the water level in an aquifer is called the water table.
Aquifers fill with water that drains into the ground – the water can come rain or melted snow. In some areas, the water passes through the soil on top of the aquifer, but in other places, 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.
Dense, impermeable material like clay or shale can act as an “aquitard,” i.e., a layer of rock or other material that is almost impenetrable to water. A confined aquifer lies between two layers of less permeable rocks and is filled with water. Water trickles down in a couple of ways: through the upper layer of less permeable rock, or a nearby water source, such as an underground river or lake, or a nearby unconfined aquifer.
An aquitard traps groundwater in an aquifer – these are called artesian aquifers. As the groundwater flows beneath an aquitard from a higher to a lower elevation, the pressure on the groundwater can be enough to force the water out of any well that’s drilled into that aquifer without the need for pumping. These “artesian” wells sit below the water table at the bottom of U-shaped aquifers and pressure from water in the aquifer pushes the water up the well shaft.
The groundwater supply is “recharged” when new surface water enters an aquifer. Surface water can come from numerous sources including: precipitation, snowmelt flowing downhill from higher elevations, or it can come from as innocuous a source as earthen irrigation ditches. Groundwater can move downward as slowly as a few inches in a century, or – if the rock and soil are looser and more permeable, groundwater can move several feet in a single day.
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. The oldest groundwater ever found was discovered 2 miles deep in a Canadian mine and was trapped there between 1.5 and 2.6 billion years ago.
But the deeper one digs for water, the saltier the liquid becomes; groundwater can be very, very deep, but eventually it’s a brine.
Much of the world’s drinking water is contained in shallow aquifers. The Ogallala Aquifer — a vast, 174,000 square-mile groundwater reservoir — supplies almost one-third of America’s agricultural groundwater, and more than 1.8 million people rely on the Ogallala Aquifer for their drinking water. Texas gets almost 60% of its water from groundwater; Florida, 90%. But these important sources of freshwater are increasingly endangered.
The groundwater system as a whole was recharged 10,000 years ago during the most recent glacial period, and what we’re doing now is mining the water without replenishing it. Rapidly growing populations are placing greater demands on limited aquifer resources — pumping can exceed the aquifer’s ability to recharge its groundwater supplies.
When groundwater pumping results in a lowering of the water table (overdrafting), then the water table can drop so low that it’s below the depth of a well and the well “runs dry”. No water can be removed until the groundwater is recharged — which, in some cases, can take hundreds or thousands of years.
Sources
https://www.livescience.com/39625-aquifers.html
https://www.livescience.com/32028-oldest-water-found-underground.html