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Water: Underground Injection Control

Basic Information about Injection Wells

This page answers the following questions about injection wells and the UIC Program:


What is an injection well?
An Injection well is a device that places fluid deep underground into porous rock formations, such as sandstone or limestone, or into or below the shallow soil layer. These fluids may be water, wastewater, brine (salt water), or water mixed with chemicals.

The UIC Program defines an injection well as:

  • A bored, drilled, or driven shaft, or a dug hole that is deeper than it is wide,
  • An improved sinkhole, or
  • A subsurface fluid distribution system.

How an injection well looks (is constructed) depends on the fluid injected and the depth of the injection zone. For example, deep wells that inject hazardous wastes or carbon dioxide (CO2) into isolated formations far below the Earth's surface are designed to provide multiple layers of protective casing and cement. Shallow wells that inject into or above drinking water sources are usually of simple construction and inject non-hazardous fluids.

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What are injection wells used for?
Injection wells have a range of uses that include long term (CO2) storage, waste disposal, enhancing oil production, mining, and preventing salt water intrusion. Widespread use of injection wells began in the 1930s to dispose of brine generated during oil production. Injection effectively disposed of unwanted brine, preserved surface waters, and in some formations, enhanced the recovery of oil. In the 1950s, chemical companies began injecting industrial wastes into deep wells. As chemical manufacturing increased, so did the use of deep injection. Injection was a safe and inexpensive option for the disposal of unwanted and often hazardous industrial byproducts.  In 2010, the EPA finalized regulations for geologic sequestration of CO2. This final rule created a new class of wells, Class VI. Class VI wells are used solely for the purpose of long term storage of CO2.

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How does the UIC Program categorize the different types of injection?
EPA’s regulations group injection wells into six groups or “classes.” Classes I - IV and VI include wells with similar functions, construction, and operating features. This allows consistent technical requirements to be applied to each well class.  Class V wells are those that do not meet the description of any other well class. Wells in Class V do not necessarily have similar function, construction, or operating features.  

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Why does EPA regulate injection wells?
In 1974, Congress passed the Safe Drinking Water Act (SDWA). Part of SDWA required EPA to report back to Congress on waste disposal practices, and develop minimum federal requirements for injection practices that protect public health by preventing injection wells from contaminating underground sources of drinking water (USDWs).

  • Visit the Regulations page for more information on SDWA and the UIC requirements for injection wells.

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What is a USDW?
An underground source of drinking water (USDW) is an aquifer or a part of an aquifer that is currently used as a drinking water source or may be needed as a drinking water source in the future.  Specifically, a USDW:

  • Supplies any public water system, or
  • Contains a sufficient quantity of ground water to supply a public water system, and
    • currently supplies drinking water for human consumption, or
    • contains fewer than 10,000 mg/l total dissolved solids (TDS), and
  • Is not an exempted aquifer

The UIC Program implements this protective mandate through the UIC regulations.

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How do the UIC regulations protect ground water?
The UIC Program protects USDWs from endangerment by setting minimum requirements for injection wells. All injection must be authorized under either general rules or specific permits. Injection well owners and operators may not site, construct, operate, maintain, convert, plug, abandon, or conduct any other injection activity that endangers USDWs. The purpose of the UIC requirements is to:

  • Ensure that injected fluids stay within the well and the intended injection zone, or
  • Mandate that fluids that are directly or indirectly injected into a USDW do not cause a public water system to violate drinking water standards or otherwise adversely affect public health.

For more information about how the UIC regulations protect ground water:

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Who regulates injection wells in my state or tribe?
Injection wells are overseen by either a state or Tribal Agency or one of EPA's regional offices. States and tribes may apply for primary enforcement responsibility, or primacy, to implement the UIC Program within their borders. In general, state and tribal programs must meet minimum federal UIC requirements to gain primacy. If a state or tribe does not obtain primacy, EPA implements the program directly through one of its regional offices.

EPA has delegated primacy for all well classes to 33 states and 3 territories; it shares responsibility in 7 states, and implements a program for all well classes in 10 states, 2 territories, the District of Columbia, and most Tribes.

  • Visit the Where You Live page.
  • page to find the contacts for your state.
  • Visit the page on UIC Program Primacy for more information on how states, tribes, and territories can gain primacy for the UIC program.

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