- BORED WELLS:
- An earth auger rotated, by hand or power, bores the hole and carries the earth to the surface. Casing is usually steel, concrete or plastic pipe. Bore hole diameter ranges from 2 to 30 inches. Bored wells can be up to 1000 feet deep.
- DUG WELLS:
- Historically, dug wells were excavated by hand shovel to below the watertable 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. Modern large-diameter dug wells are dug or bored by power equipment and typically are lined with concrete tile. Because of the type of construction large-diameter bored wells can go deeper beneath the watertable than can hand-dug wells.
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 watertable drops below the well bottom.
- DRIVEN-POINT (sand point) WELLS:
- constructed by driving assembled lengths of pipe into the ground with percussion equipment or by hand. Normally 2 inches or less in diameter and less than 50 feet deep. Can only be installed in areas having relatively loose soils, such as sand or gravel. Usually a screened well point is attached to the bottom of the casing before driving. Relatively simple and economical to construct. Poses a moderate to high risk. Easily contaminated from nearby surface sources.
- DRILLED WELLS:
- are constructed by either percussion or rotary-drilling machines. Drilled wells that penetrate unconsolidated material require installation of casing and a screen to prevent inflow of sediment and collapse. They can be drilled more than 1,000 feet deep. To prevent contamination by water draining from the surface downward around the outside of the casing, the space around the casing must be sealed.
The two common types of drilled wells are:
The most common DRY WELL problems occur in dug wells. Most dug wells are shallow and excavated in poorly permeable material; consequently they are readily affected by drought and by seasonal declines in the watertable.
Many dug wells extend only to the bedrock surface and tap the perched watertable (unconfined groundwater separated from an underlying main body of groundwater (aquifer) by an unsaturated (impermeable) zone) on top of the bedrock. These wells cannot be easily deepened. In such cases a new drilled well is the only long-term solution.
Some drilled wells that tap shallow bedrock will yield only 1 or 2 gallons of water per minute. These wells are not deep enough to provide adequate storage of water for short-term pumping cycles - they may contain only 50 feet of water above the pump intake. As an example, if the watertable drops 10 ft due to drought, only 40 ft of water is available in the well for one pumping cycle, and the well seems to "go dry". In this situation, deepening the well may solve the problem as long as the deeper water is of good quality. If usable water is not available at a greater depth, the pumping rate must be reduced so that less water is pumped during each cycle.
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