Census data can be used to estimate domestic well use if the type of water supply used by each household is included in the census. In the United States, the 1990 census was the last census to collect this information in a nationally consistent manner. However, the United States Geological Survey (USGS) has estimated the number and location of domestic wells across the country using household density per square kilometer. After a threshold is reached, the number of people using domestic wells declines as household density increases. This is consistent with the observation that there are fewer domestic wells in urban areas with high household densities because public water supplies are usually available. Using this approach, the USGS estimated that in 2010 approximately 37 million people in the United States used domestic wells (Johnson et al., 2019). The distribution of people using domestic wells in the United States is shown in Figure 3.
Public water supply information, combined with other data, can also be used to estimate how many people use domestic wells. Because public water supplies are usually regulated, information about the population they serve, and the location of their distribution system is often available. An estimate of the number and location of domestic well users can be made by subtracting the population served by public water supplies from the total population of a given area. This approach assumes that anyone who is not served by a public water supply is using a domestic well. Given that domestic wells are the most common type of water supply in areas without public water supplies, this method provides a reasonable estimate of domestic well use. However, it does not account for people using other types of private water supplies, such as surface water and rainwater cisterns. This method was used in Nova Scotia, Canada, where public-water distribution-zone information was combined with residential address data and household density data in a geographic information system to investigate the sources of domestic water across the province (Kennedy and Polegato, 2017). The study concluded that 42 percent of the population used domestic wells for their water supply.
Many jurisdictions have water well record databases that are a potential source of information about the number and location of domestic wells for a given area. In practice, however, water well record databases are typically not used for this purpose because they do not usually contain a complete list of all water wells or keep track of wells that have been abandoned. For example, the previously referenced Nova Scotia study used civic address points and public water supply zone information to determine that 197,000 households in the province relied on domestic wells. The Nova Scotia provincial water-well database contained 113,000 records of domestic wells at the time of the study, indicating that the database did not account for all domestic wells. Another book in the Groundwater Project series on domestic wells provides more information on (Kennedy, 2022).