Groundwater Wells | U.S. Geological Survey (2024)

•Water Science School HOME•Groundwater topics •

Groundwater Wells

There's a good chance that the average Joe who had to dig a well in ancient Egypt probably did the work with his hands, a shovel, and a bucket. He would have kept digging until he reached the water table, where all the spaces between the rock and dirt particles are filled with water,and water filled the bottom of the hole. Some wells are still dug by hand today, but more modern methods are available.

Wells are extremely important to all societies. In many places wells provide a reliable and ample supply of water for home uses, irrigation, and industries. Where surface water is scarce, such as in deserts,people couldn't survive and thrive without groundwater.

Types of wells

Digging a well by hand is becoming outdated today as automated drilling methods replace manual-labor methods. Modern wells are more often drilled by a truck-mounted drill rig. Still, there are many ways to put in a well — here are some of the common methods.

DUG WELLS

Hacking at the ground with a pick and shovel is one way to dig a well. If the ground is soft and the water table is shallow,then dug wells can work. Historically, dug wells were excavated by hand shovel to below the water table until incoming water exceeded the digger's bailing rate. The well was lined with stones, brick, tile, or other material to prevent collapse, and was covered with a cap of wood, stone, or concrete. They cannot be dug much deeper than the water table— just as you cannot dig a hole very deep when you are at the beach... it keeps filling up with water!

Dug and bored wells have a large diameter and expose a large area to the aquifer. These wells are able to obtain water from less-permeable materials such as very fine sand, silt, or clay. Some disadvantages of this type of well are that they are shallow and lack continuous casing, making them subject to contamination from nearby surface sources, and they go dry during periods of drought if the water table drops below the well bottom.

DRIVEN WELLS

Driven wells are still common today. They are built by driving a small-diameter pipe into soft earth, such as sand or gravel. A screen is usually attached to the bottom of the pipe to filter out sand and other particles. Problems? They can only tap shallow water, and because the source of the water is so close to the surface, contamination from surface pollutants can occur.

DRILLED WELLS

Most modern wells are drilled, which requires a fairly complicated and expensive drill rig. Drill rigs are often mounted on big trucks. They use rotary drill bits that chew away at the rock, percussion bits that smash the rock, or, if the ground is soft, large auger bits. Drilled wells can be drilled more than 1,000 feet deep. Often a pump is placed in the well at some depth to push the water up to the surface..Wells and Pumpage

Water Levels in Wells

Groundwater users would find life easier if the water level in the aquiferthat supplied their well always stayed the same. Seasonal variations in rainfall and the occasional drought affect the "height" of the underground water level. Withdrawing water from a well causes the water levels around the well to lower. The water level in a well can also be lowered if other wells near it are withdrawing water. When water levels drop below the levels of the pump intakes, then wells will begin to pump air - they will "go dry."

Pumping a well lowers the water level around the well to form a cone of depression in the water table. If the cone of depression extends to other nearby wells, the water level in those wells will be lowered. The cone develops in both shallow water-table and deeper confined-aquifer systems. In the deeper confined-aquifer system, the cone of depression is indicated by a decline in the pressure and the cone spreads over a much larger area than in a water-table system. For a given rate of withdrawal, the cone of depression extends deeper in low-yielding aquifers than in high-yielding ones.

Even though water is present at some depth at almost any location, the success of obtaining an adequate domestic supply (usually 5 gallons per minute) of water from a well depends upon the permeability of the rock. Where permeable materials are near land surface, a shallow well may be adequate. Elsewhere, such as where clayey material directly overlies bedrock, a deep well extending into bedrock may be needed.

Private Wells

Many people in the United States and worldwide supply their own water for their homes, often in more rural locations that don't have large public-supply water systems to supply water. Here is a basic diagram showing how these wells function. Although this diagram shows a single home, large wells that supply more customers work generally the same.

WELL COMPONENTS

Below are descriptions of the basic components found in a private water well. (Source: National Ground Water Association)

• Well Casingis the tube-shapedstructureplaced in the well to maintain the well opening from the target ground water to the surface. Along with grout, the casing keeps dirt andexcess waterout of the well. This helps preventcontaminants from less desirable groundwater from entering the well and mixing with the drinking water. Some states andlocal governing agencies have laws that require minimum lengths for casing.The most common materials for well casing are carbon steel, plastic,and stainless steel. Local geology often dictates what type of casing can be used.
• Well Capsare placed on top of the well casing to prevent debris, insects, or small animals from gettinginto the well. Well caps are usually made of aluminum or plastic. They include a vent to control pressureduring well pumping.
• Well Screensareattached to the bottom of the casing to prevent too much sediment from entering the well.The most common well screens are continuous slot, slotted pipe, and perforated pipe.
• Pitless Adapterisa connector that allows the pipe carrying water to the surface to remain below the frost line.Itensures thata sanitary andfrost-proof sealis maintained.
• Jet Pumpsare the most commonly used pumps for shallow wells (depth of 25 feet or less). Jet pumps are mounted above ground and use suction to draw water from the well.
• Submersible Pumpsare the most commonly used pumps for deepprivate wells. The pumping unit is placed inside the well casing and connected to a power source on the surface.

Information on this page is from Ground Water and the Rural Homeowner, Pamphlet, U.S. Geological Survey, 1982, by Roger M. Waller.

_{Quiz icon made by mynamepong fromwww.flaticon.com}

Want to learn more about groundwater wells?Follow me to the Groundwater Basics andDomestic (Private) Supply Wellswebsites!