Underground injection wells, where water and/or other substances are injected into the Earth, are used for a wide variety of purposes. The U.S. Environmental Protection Agency (EPA) regulates around 850,000 underground injection wells through its Underground Injection Control program under the Safe Drinking Water Act. The EPA defines six classes of wells based on their use:
"Since 1950, the U.S. Geological Survey (USGS) has collected and analyzed water-use data for the United States and Territories [see the USGS Water Use Program webpage]. That data is revised every 5 years."