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Underground Injection Control (UIC)

General Information About Injection Wells

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Definition of injection well

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 definition of a well is codified in the UIC regulations at 40 CFR 144.3.

Well means: A bored, drilled, or driven shaft whose depth is greater than the largest surface dimension; or, a dug hole whose depth is greater than the largest surface dimension; or, an improved sinkholeHelpimproved sinkholeA naturally occurring karst depression or other natural crevice found in volcanic terrain and other geologic settings which have been modified for the purpose of directing and emplacing fluids into the subsurface.; or, a subsurface fluid distribution system.

Injection well construction is based on the type and depth of the fluid injected. For example, wells that inject hazardous wastes or carbon dioxide (CO2) into deep isolated formations have sophisticated construction.  These wells are designed to provide multiple layers of protective casing and cement. In contrast, shallow wells are usually of simple construction.

Use of injection wells

Injection wells have a range of uses including:

  • Storing CO2
  • Disposing of waste 
  • Enhancing oil production
  • Mining
  • 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 and preserved surface waters. In some formations injection 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 proved to be a safe and inexpensive option for the disposal of unwanted and often hazardous industrial byproducts.

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Categories of injection wells

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 these well classes. 

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 functions, construction, or operating features. 

In 2010, 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.

UIC fact sheet

The UIC program has created a fact sheet that provides basic information and data about injection wells, and how the program protects underground sources of drinking water and public health.

EPA regulation of injection wells

In 1974, Congress passed the Safe Drinking Water Act (SDWA). The SDWA required EPA to:

  • Report back to Congress on waste disposal practices 
  • Develop minimum federal requirements for injection practices that protect public health and prevent contamination of underground sources of drinking water (USDWs)

Technical and program guidance

Technical memoranda, manuals, and guidance documents have developed by the UIC program since its inception.  The documents and technical resources are used to inform decision-making, clarify issues and questions, and guide UIC program implementation consistent with the requirements of the Safe Drinking Water Act and the federal UIC regulations.  

Review a list of technical and program guidance documents.

Definition of underground sources of drinking water

An underground source of drinking water (USDW) is an aquiferHelpaquiferAn aquifer is a geological formation or group of formations or part of a formation that is capable of yielding a significant amount of water to a drinking water well or spring. or a part of an aquifer that is currently used as a drinking water source. A USDW may also be ground water needed as a drinking water source in the future.  A USDW is defined in the Code of Federal Regulations (40 CFR 144.3) as:

an aquifer or its portion: (a)(1) Which supplies any public water system; or (2) Which contains a sufficient quantity of ground water to supply a public water system; and (i) Currently supplies drinking water for human consumption; or (ii) Contains fewer than 10,000 mg/l total dissolved solids; and (b) Which is not an exempted aquifer. ​

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

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Protecting drinking water resources

The UIC program protects USDWs from endangermentHelpendangermentThe construction, operation, maintenance, conversion, plugging, or abandonment of an injection well, or the performance of other injection activities, by an owner or operator in a manner that allows the movement of fluid containing any contaminant into a USDW, if the presence of that contaminant may cause a violation of any primary drinking water regulations or may adversely affect the health of persons. by setting minimum requirements for injection wells. All injection must be authorized under either:

  • General rules
  • Specific permits

Injection well owners and operators may not:

  • Site, construct, operate, maintain, convert, plug, or abandon wells
  • Conduct any other injection activity that endangers USDWs

The purpose of the UIC requirements is to ensure that either:

  • Injected fluids stay within the well and the intended injection zone
  • 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.

Review the UIC regulations for more infomation about the protection of USDWs.

Review web pages for individual well classes for brief descriptions of well class-specific requirements. 

Regulating agency

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 to implement the UIC program. This is called primacy.

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 three territories. EPA shares responsibility in seven states. EPA implements a program for all well classes in 10 states, two territories, and the District of Columbia, and for most tribes.

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