Primary enforcement authority, often called primacy, refers to state, territory or tribal responsibilities associated with implementing U.S. Environmental Protection Agency (EPA) approved Underground Injection Control (UIC) programs. A state, territory or tribe with UIC primacy, or primary enforcement authority oversees the UIC program in that state, territory or tribe.

EPA may grant primacy for all or part of the UIC program. As a consequence, in some jurisdictions primacy for certain well classes may be shared with EPA or divided between different state, territory or tribal authorities. 

Primacy for UIC wells in the United States
Primacy map constructed by the U.S. Environmental Protection Agency (EPA) for UIC wells in the United States.1USEPA. (2022, August 18). Primary Enforcement Authority for the Underground Injection Control Program. Retrieved 12/29/2022 from Map is segmented into the 10 EPA Regions.

Thirty-one states, and three territories have primacy for multiple well classes; seven states and two tribes have primacy for Class II wells only. Currently, North Dakota and Wyoming are the only states with primacy for all six well classes.2USEPA. (2022, August 18). Primary Enforcement Authority for the Underground Injection Control Program. Retrieved 12/29/2022 from 

Before getting into the specifics of the different classes of underground injection control wells, one of our experts provides an overview of the program and the technology.


Underground Injection Wells – Mike Parker – Retired, ExxonMobil

To place fluids deep underground, you need an injection well. The Environmental Protection Agency, or EPA, defines an injection well as any bored, drilled, driven shaft or dug hole whose depth is greater than its largest surface dimension. This is a broad definition that covers a wide range of scenarios for injecting fluid into porous and permeable subsurface formations. Injection wells range from deep, technically complex systems that require continuous monitoring, to shallow, on-site drainage systems including septic systems and storm water drainage wells. All injection wells are regulated under the Federal Safe Drinking Water Act’s Underground Injection Control, or UIC, program. This program protects underground sources of drinking water, or USDWs, from potential contamination from injection operations.

The EPA has identified six types, or classes, of injection wells based on function, construction and operating features. Let’s look at each class. Class I wells are used to inject fluid waste into deep, confined rock formations. They’re typically drilled and have steel casing installed and cemented thousands of feet below the lower most USDW in order to protect it. Class I wells are further divided into four subcategories based on the waste type injected. These are hazardous, non-hazardous industrial, municipal and radioactive. Industries such as petroleum refining, metal production, chemical production, pharmaceutical production, commercial disposal, food production and municipal wastewater treatment all use Class I wells.

Class II wells are associated with oil and natural gas production. They’re used to inject fluids for enhanced recovery, disposal and hydrocarbon storage. Class II wells most commonly inject produced water. This is the brine, or saltwater, that’s brought to the surface while producing oil and gas. It’s important to note that production wells and hydrocarbon production, which bring fluids to the surface, are not regulated under the EPA UIC program. Class III wells are used to inject fluids for solution mining. The injected fluids dissolve minerals such as uranium, salt, copper, and sulfur, making them easier to extract from the subsurface. Just like Class II well, production mining wells are not regulated under the EPA UIC program.

Class IV wells are shallow wells in or above a formation that contains a USDW. Formerly used for hazardous or radioactive waste, the EPA banned construction of new Class IV wells in 1984. About 30 Class IV wells remain in the US as of 2016. Those are used in remediating sites contaminated by leaking underground storage tanks. These wells require additional monitoring and water testing to ensure public safety. Both shallow Class IV and many Class V wells inject fluids into or above the uppermost USDW. The difference is in the type of fluid that is allowed to be injected. Only non-hazardous fluids can be injected into a Class V well.

Typically, these are fluids from a variety of municipal, business and industrial activities that receive little or no treatment before injection. Because these fluids are injected directly into or above a USDW, proper management is important. Most Class V wells are shallow disposal systems that depend on gravity to drain the fluids directly into the ground. These might be wells related to storm water drainage, septic systems or agricultural drainage. But, some Class V wells are deep, complex wells used at commercial, scientific and industrial facilities. These could be wells related to aquifer storage and recovery, special scientific purposes, or water injection for geothermal electric power generation. Most of these wells inject well below the lower-most USDW. Finally, Class VI wells are used for geologic carbon storage, or GCS. Carbon storage is a way of reducing carbon dioxide emissions, a greenhouse gas. In this case, carbon dioxide is injected deep underground rather than being released into the atmosphere.

The oil and gas industry primarily deals with Class II injection wells. How do these wells work, and what do we do to protect the environment and human health? First, let’s look at the injection zone itself. Any subsurface formation with adequate pore volume and the ability to accept injection at a reasonable pressure has what we call “good porosity and permeability.” This allows us to maximize both the volume of fluids stored and the rate at which we can inject. Having both porosity and permeability are important when selecting an injection zone. These criteria are most often met by highly porous and permeable sedimentary rocks, which are very common in many areas.

Now, let’s look at how we make sure we keep the fluids in the zone where they’re injected. This involved two key aspects; first, the right geology, and second, good well design and construction practices. Geologically, the ideal injection zone will have a ceiling cap rock above it to prevent any escape of fluids. As the term suggests, this layer acts as a barrier or cap. This is most likely shale, which is a rock made up of dense impermeable layers of clay. A good cap rock prohibits fluids from leaving the injection zone and, ideally, isolates them indefinitely. A good well design involves a two-tiered approach. First, selecting steel casing and tubing and cement that is compatible with the fluids and pressures of injection. Second, selecting points for placing that casing in the well and cementing that casing so the groundwater resources are protected. Robust construction and cementing compliments impermeable cap rock to prevent upward migration of injected fluids.

Like all engineering projects, planning and using Class II UIC wells for the oil and gas industry is based on sound scientific and geologic principles, as well as public participation. When selecting a site for a new injection well, an opportunity for public input should be provided, and operational and public safety should be a prime consideration. The site should provide geologic conditions that protect USDWs, such as a robust cap rock with minimal faulting. The well should be designed to protect USDWs by setting and cementing casing at the proper depths for that location. Finally, the well should be operated safely and in compliance with all applicable rules and regulations. With proper planning, construction, operation, maintenance and monitoring of their UIC wells, the oil and gas industry can ensure well integrity, environmental safety and public health.