Material adapted from: Vandas, S.J., Winter, T.C., and Battaglin, W.A. 2002. Water and the Environment, p. 28-29. Published by the American Geosciences Institute Environmental Awareness Series. Click here to download the full handbook.

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^[1]:

U.S. Geological Survey FAQs:

U.S. Geological Survey FAQ and U.S. Geological Survey page on sinkholes:

"A sinkhole is a depression in the ground that has no natural external surface drainage. Basically this means that when it rains, all of the water stays inside the sinkhole and typically drains into the subsurface.