Dug wells (Figure 7 and Figure 8) have large diameters (typically 600 to 1500 mm) and are usually relatively shallow (3 to 10 m deep), extending only a few meters below the water table. They were historically the most common type of domestic well because they could be constructed by hand without the need for heavy equipment. Nowadays, they are less common than drilled wells in developed countries, but there are still many old dug wells in use and new ones continue to be installed in some areas.
Dug wells are still widely used in developing countries where hand dug wells are promoted as practical, low-cost, low-technology domestic water supplies (Collins, 2000). In Sub-Saharan Africa it is estimated that 155 million people rely on dug wells and springs for their domestic water (Sutton and Butterworth, 2021).
Modern dug wells are usually installed using machines, such as excavators or backhoes (Figure 9), although in developing countries they are also still constructed by hand. Dug wells are constructed by excavating a large diameter hole in the ground and installing casing to keep the hole from collapsing. The casing is commonly made from large diameter concrete rings but may also be made of metal, plastic, or fiberglass. Older dug wells are typically lined with rocks or bricks. The backfill around the outside of the casing in a dug well usually includes clean gravel at the bottom of the hole to provide an additional water storage reservoir, followed by a low permeability seal (made of clay, bentonite, or concrete) in the upper portion of the hole to prevent surface contaminants from entering the well.
Dug wells have the advantage that they can be installed at relatively low-cost without heavy equipment. Another advantage of dug wells is that their large diameter creates a significant amount of water storage. This allows dug wells to be installed in lower permeability unconsolidated aquifers because short-term peak water demands can be satisfied from well storage rather than aquifer yield. In such locales, a shallow drilled well would require a storage tank because the well’s flow rate may be insufficient to meet short-term peak water demands.
The main disadvantages of dug wells are their vulnerability to drought and surface contaminants, and that they can only be installed in unconsolidated aquifers with shallow water tables. Dug wells usually extend a few meters or less below the water table, consequently, they have little available drawdown and can go dry when the water table declines during a drought. Drawdown is the difference between the static water level in a well and the water level when the well is pumped. Water level in a well declines in response to pumping. The decline is rapid at first and slows until, in most settings, it reaches a stable level for a given pumping rate. During the drought of 2016 in Nova Scotia, Canada, 93 percent of the wells that went dry were dug wells (Kennedy et al., 2017).
Dug wells draw water from shallow unconfined aquifers so they are vulnerable to shallow groundwater contaminants originating from human activities, such as road salt from de-icing operations, or nitrate from fertilizers and septic systems. Groundwater in dug wells often contains microbial contaminants because the short groundwater flow paths associated with these wells do not allow microbes to be removed by natural filtration within the aquifer. Dug wells are also prone to naturally occurring water quality problems that can occur in shallow groundwater, such as elevated concentrations of humic substances (decomposed plant matter within soil) or organic carbon.