Missouri has approximately 133,462 registered private domestic water wells with roughly 3,100 new private wells drilled each year. However, as wells installed prior to 1987 were not required to be registered with the state, the total number of private wells in the state is unknown. According to the U.S. Census (1990), 15.3% of households in the state use private wells as their source of drinking water. The U.S. Census stopped asking citizens their primary source of water supply following the 1990 Census; however, a U.S. Geological Survey (USGS) report entitled Estimated Use of Water in the United States in 2010 (USGS Circular 1405), estimates the population of self-supplied water users in Missouri to be 883,000 (15% of the state population).