Underground Injection Control


An injection well is used to place fluid underground into porous geologic formations. These underground formations may range from deep sandstone or limestone, to a shallow soil layer. Injected fluids may include water, wastewater, brine (salt water), or water mixed with chemicals.

The Underground Injection Control program consists of six classes of injection wells. Each well class is based on the type and depth of the injection activity, and the potential for that injection activity to result in endangerment of a underground sources of drinking water (USDW).

  • Class I wells are used to inject hazardous and non-hazardous wastes into deep, isolated rock formations.
  • Class II wells are used exclusively to inject fluids associated with oil and natural gas production.
  • Class III wells are used to inject fluids to dissolve and extract minerals.
  • Class IV wells are shallow wells used to inject hazardous or radioactive wastes into or above a geologic formation that contains a USDW. 
  • Class V wells are used to inject non-hazardous fluids underground. Most Class V wells are used to dispose of wastes into or above underground sources of drinking water.
  • Class VI wells are wells used for injection of carbon dioxide (CO2) into underground subsurface rock formations for long-term storage, or geologic sequestration.

The success of the deep well Underground Injection Control (UIC) program in isolating massive volumes of pollutants from underground sources of drinking water and other parts of the ecosystem has led some national policy makers to assume that no additional funding is needed, even though new challenges and responsibilities continue to be added to the program.


Underground injection refers to the placement of fluids into the subsurface through a well bore. The federal UIC Program, designed to prevent contamination of underground sources of drinking water (USDWs), covers wells used to inject a wide range of fluids, including oilfield brines; industrial, manufacturing, pharmaceutical, and municipal wastes; and water for solution mining. A “mature” regulatory” program suggests that the major processes are working smoothly, the principal issues are well understood, and significant problems encountered have been solved. While this is the case for Class I, II, III, and IV UIC well types, the Class V part of the UIC program has not kept pace with the rest of the program.

From the Ground Water Report to the Nation

Underground Injection Control – Summary Sheet : Full Chapter

Other Resources

Class V Informational Video