Dug wells are holes in the ground dug by a shovel or backhoe. Historically, a dug well was excavated below the groundwater table until incoming water exceeded the digger’s bailing rate. The well was then lined or cased with stones, brick, tile, or other material to prevent collapse. It was covered with a cap of wood, stone, or concrete.
Since it is so difficult to dig beneath the groundwater table, dug wells are typically only 10 to 30 feet deep. Being so shallow, dug wells have the highest risk of becoming contaminated. To minimize the likelihood of contamination, a dug well should have certain features that can help prevent contaminants from traveling along the outside of the casing (or through it) and into the well.
Dug wells usually have a diameter of about 3 feet, which is considerably wider than a drilled well. Thus, more water can be stored in reserve at a dug well. However, this water has a greater surface area, which can contribute to the well running dry.
Construction Features of a Dug Well
Land activities around a dug well can also contaminate it.
While dug wells are usually cheaper than drilled wells and have been used as a household water supply source for many years, most are considered relics of older homes, dug before drilling equipment was readily available or when drilling was considered too expensive. If there is a dug well on a property and it's being used for drinking water, it should be checked to make sure that it's properly covered and sealed.
Another problem relating to the shallowness of a dug well is that it may go dry during a drought when the groundwater table drops.
Bored wells are similar to true dug wells. Both types can be built in clay, silt, sand and gravel. It is even possible to build one into sandstone and limestone if the rock is soft or fractured. The only difference is that bored wells can reach a much greater depth -- up to 100 feet -- and can have a diameter ranging from 3 to 30 inches, as opposed to dug wells, which reach a depth of only 10 to 30 feet.
Driven Well Construction Features
Like dug wells, driven wells pull water from the water-saturated zone above the bedrock.
Driven wells can be deeper than dug wells. They are typically 30 to 50 feet deep and are usually located in areas with thick sand and gravel deposits where the groundwater table is within 15 feet of the ground’s surface. In the proper geologic setting, driven wells can be easy and relatively inexpensive to install, although some of the cost saved up front may be offset by costly repairs down the road.
Due to their relatively shallow depth, driven wells have a moderate-to-high risk of contamination from nearby land activities. They are unable to provide a high quantity of water and are the most likely of all well types to run dry.
The cover should be a tight-fitting concrete curb and cap with no cracks and should sit about a foot above the ground. The ground should be sloped away from the well so that surface water will not pond around the well. If there’s a pit above the well (to hold the pump or to access the fitting), it may be possible to pour a grout sealant along the outside of the well pipe.
Protecting the water quality requires proper well construction and regular monitoring of activities around the well. It's also important that land use precautions are followed, which are the same for driven wells as those described for dug wells.
Drilled wells penetrate between 100 to 400 feet into the bedrock. Bedrock at the surface is commonly called ledge. To serve as a water supply, a drilled well must intersect bedrock fractures containing groundwater. Drilled wells do not run out of water as easily as dug wells because of their exceptional depth. Drilled wells are often considered the most dependable source of water. They typically have diameters of around 6 inches and are surrounded by a steel casing. Drilled wells have the capability of reaching depths of water that other types of wells cannot. The one disadvantage of drilled wells is that they are more expensive; there is an increased cost with increased dependability. Drilled wells can cost thousands of dollars more than dug wells, but drilled wells are less prone to becoming contaminated. However, drilled wells are not always the best option. The most appropriate type of well is often dictated by the geographic location and geological makeup of that location. For example, dug wells tend to be more common in Virginia, while drilled wells are more common in Maine.
Construction Features of Drilled Wells
Hydrofracting a Drilled Well
Hydrofracting (or fracking) is a process that applies water or air under pressure into the well to open up existing fractures near the well. It can even create new ones. This can often increase the yield of the well. This process can be applied to new wells with insufficient yield and to improve the quantity of older wells.
The depth of a drilled well can vary, but they are usually at least 100 feet deep and can reach depths of more than 500 feet. There are various types of drilling equipment, and each has its advantages. The two different types of basic drilling equipment are rotary rigs and percussion cable tool rigs. There are several other types of drilling rigs in addition to these, but these two are the most common.