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Water: Underground Injection Control


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Abandoned well – A well that is no longer being used or cannot be used because of its poor condition.

Agricultural Drainage Wells – Class V wells that receive agricultural runoff. This well type includes improved sinkholes and abandoned drinking water wells that receive agricultural runoff, wells that recharge aquifers with agricultural tail waters, and wells that drain flood irrigation.

Annulus – Either the space between the casing of a well and the well bore or the space between the tubing and casing of a well. The annulus is also referred to as the "annular space."

Aquaculture Wells – Class V wells that dispose of water used for the cultivation of marine and freshwater animals and plants.

Aquifer – An underground geological formation or group of formations, containing water. Aquifers are sources of groundwater for wells and springs.

Aquifer Remediation Wells – Class V wells that are used to clean up, treat, or prevent contamination of underground sources of drinking water (USDWs). Treated ground water (pump and treat), bioremediation agents, or other recovery enhancement materials may be injected into the subsurface via Class V wells. These wells may be associated with Resource Conservation and Recovery Act (RCRA) or Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) projects.

Aquifer Recharge/Recovery Wells – Class V wells that are used to inject fluids to recharge an aquifer. These wells may have secondary purposes such as saline intrusion prevention, subsidence control, or aquifer storage and recovery (ASR).

Area of Review (AoR) – The area around a deep injection well that must be checked for artificial penetrations, such as other wells, before a permit is issued. Well operators must identify all wells within the AoR that penetrate the injection or confining zone, and repair all wells that are improperly completed or plugged. The AoR is either a circle with a radius of at least ¼ mile around the well or an area determined by calculating the zone of endangering influence, where pressure due to injection may cause the migration of injected or formation fluid into a USDW.

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Brine – Water that has a quantity of salt, especially sodium chloride, dissolved in it. Large quantities of brine are often produced along with oil and gas.

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Capillary force – Adhesive force that holds a fluid in a capillary or a pore space. Capillary force is a function of the properties of the fluid, and surface and dimensions of the space. If the attraction between the fluid and surface is greater than the interaction of fluid molecules, the fluid will be held in place. 

Caprock – See confining zone.

Carbon Capture and Storage (CCS) – The process of capturing carbon dioxide (CO2) from an emission source, (typically) converting it to a supercritical state, transporting it to an injection site, and injecting it into deep subsurface rock formations for long-term storage.  Carbon Dioxide Plume – The extent underground, in three dimensions, of an injected CO2 stream.

Carbon Dioxide Stream - Carbon dioxide that has been captured from an emission source (e.g., a power plant), plus incidental associated substances derived from the source materials and the capture process, and any substances added to the stream to enable or improve the injection process. This does not apply to any carbon dioxide stream that meets the definition of a hazardous waste under 40 CFR Part 261.

Casing – The pipe material placed inside a drilled hole to prevent the hole from collapsing. The two types of casing in most injection wells are surface casing, the outer most casing that extends from the surface to the base of the lowermost USDW, and long-string casing, which extends from the surface to or through the injection zone.

Cement – Material used to support and seal the well casing to the rock formations exposed in the borehole. Cement also protects the casing from corrosion and prevents movement of injectate up the borehole. The composition of the cement may vary based on the well type and purpose; cement may contain latex, mineral blends, or epoxy.

Class I wells – Technologically sophisticated wells that inject wastes into deep, isolated rock formations below the lowermost USDW. Class I wells may inject hazardous waste, non-hazardous industrial waste, or municipal wastewater.

Class II wells – Wells that inject brines and other fluids associated with oil and gas production, or storage of hydrocarbons. Class II well types include salt water disposal wells, enhanced recovery wells, and hydrocarbon storage wells.

Class III wells – Wells that inject fluids associated with solution mining of minerals. Mining practices that use Class III wells include salt solution mining, in-situ leaching of uranium, and sulfur mining using the Frasch process.

Class IV wells – Wells that inject hazardous or radioactive wastes into or above a USDW. These wells are banned unless authorized under a federal or state groundwater remediation project.

Class V wells – Wells not included in Classes I to IV and Class VI. Class V wells inject non-hazardous fluids into or above a USDW and are typically shallow, on-site disposal systems; however, this class also includes some deeper injection operations. There are approximately 20 subtypes of Class V wells.

Class VI wells – Wells that inject carbon dioxide for the purposes of long-term storage, also known as CO2 geologic sequestration.

Code of Federal Regulations (CFR) – A compilation of all federal rules currently in effect. The CFR is updated annually. The requirements for owners and operators of injection wells appear in Title 40 of the CFR, Parts 144 through 148.

Confining zone – A geological formation (or group or part of a formation) capable of limiting fluid movement out of an injection zone. The ideal confining zone has low permeability and sufficient thickness to prevent upward movement of injected waste.

Corrective Action – The use of Director-approved methods to assure that wells within the area of review do not serve as conduits for the movement of fluids into USDWs.

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Dip – The angle between a planar feature, such as a sedimentary bed or a fault, and the horizontal plane. The dip of subsurface rock layers can provide clues as to whether injected fluids may be contained.

Direct Implementation (DI) – The oversight/implementation of a UIC program by an EPA Regional office. EPA assumes DI responsibilities for the UIC Program if a state, tribe, or territory has not obtained primacy.

Director – The person responsible for permitting, implementation, and compliance of the UIC Program. For UIC Programs administered by the EPA, the Director is the EPA Regional Administrator; for UIC Programs in Primary States, the Director is the person responsible for permitting, implementation, and compliance of the State, Territorial, or Tribal UIC Program.

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Endangerment – The 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.

Exempted aquifer – An aquifer, or a portion of an aquifer, that meets the criteria for a USDW, for which protection under the SDWA has been waived by the UIC Program. Under 40 CFR Part 146.4 [broken link], an aquifer may be exempted if it is not currently being used — and will not be used in the future — as a drinking water source, or it is not reasonably expected to supply a public water system due to a high total dissolved solids content. Without an aquifer exemption, certain types of energy production, mining, or waste disposal into USDWs would be prohibited. EPA makes the final determination on granting all exemptions.

Experimental Wells – Class V wells that are used to test new technologies. Wells will not be classified as experimental if the technology can be considered under an established well subclass. For example, a well used for bioremediation will be classified as an aquifer remediation well.

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Formation or geological formation - A  layer of rock that is made up of a certain type of rock or a combination of  types.

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Geologic sequestration (geosequestration) – The process of injecting carbon dioxide (CO2), which has been compressed into a liquid state, into the deep subsurface.

Geothermal Direct Heat Return Flow Wells – Class V wells that dispose of spent geothermal fluids following the extraction of heat used directly (without conversion to electricity or passage through a heat exchanger) to heat homes, swimming pools, etc.

Geothermal Electric Power Wells – Class V wells that dispose of spent geothermal fluids following the extraction of heat for the production of electricity.

Geologic sequestration project - An injection well or wells used to emplace a CO2 stream beneath the lowermost formation containing a USDW; or, wells used for geologic sequestration of carbon dioxide that have been granted a waiver of the injection depth requirements pursuant to requirements at 40 CFR 146.95.

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Heat Pump/Air Conditioning Return Flow Wells – Class V wells that reinject ground water that has been passed through a heat exchanger to heat or cool buildings. A heat pump takes thermal energy from the ground water and transfers it to the space being heated. When cooling is required, the heat pump removes heat from a building and transfers it to the ground water.

Hydraulic fracturing – The creation of fractures within a reservoir that contains oil or natural gas to increase flow and maximize production. A hydraulic fracture is formed when a fluid is pumped down the well at pressures that exceed the rock strength, causing open fractures to form in the rock.

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Improved sinkhole - A 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.

Injectate - The fluids injected into an underground injection well.

Injection – The subsurface discharge of fluids through a well.

Injection zone – A geological formation, group of formations, or part of a formation that receives fluids through a well.

In-Situ Fossil Fuel Recovery Wells – Class V wells that are used to recover lignite, coal, tar sands, and oil shale. The wells inject water, air, oxygen, solvents, combustibles, or explosives into underground or oil shale beds to free fossil fuels so they can be extracted.

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Large-Capacity Septic Systems - Class V wells that dispose of sanitary waste through a septic tank and are used by multiple dwellings, business establishments, communities, and regional business establishments for the injection of wastes. Systems serving single families and non-residential systems serving less than 20 persons are not included.

Lithology - The description of rocks, based on color, mineral composition and grain size.

Lixiviant – A solution of water or steam, possibly mixed with other chemicals, that is injected through an injection well into a formation to extract minerals in solution mining operations.

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Mechanical integrity – The absence of significant leakage within the injection tubing, casing, or packer (known as internal mechanical integrity), or outside of the casing (known as external mechanical integrity). Periodic mechanical integrity tests (MITs) are performed to confirm that a well maintains internal and external mechanical integrity. MITs are a means of measuring the adequacy of the construction of an injection well.

Mine Backfill Wells – Class V wells that inject water, sand, mill tailings, or other mining byproducts in order to control subsidence caused by mining, to dispose of mining byproducts, or to fill sections of a mine.

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No-migration petitions – Demonstrations that fluids injected into a Class I hazardous waste well will remain in the injection zone for 10,000 years or as long as the fluid remains hazardous or until the waste decomposes or otherwise is attenuated to non-hazardous levels before migrating from the injection zone. No-migration petitions involve sophisticated hydrogeological and geochemical modeling.

Noncontact Cooling Water Wells – Class V wells that are used to inject noncontact cooling water that contains no additives and has not been chemically altered.

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Owner/operator – The owner or operator of any facility or activity subject to the UIC regulations.

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Packer – A mechanical device set immediately above the injection zone that seals the outside of the tubing to the inside of the long string casing. A packer may be a simple mechanically set rubber device or a complex concentric seal assembly.

Permit – An authorization, license, or equivalent control document issued by EPA or a State to operate an injection well.  Permits may be individual permits (covering a single well) or area permits (covering multiple wells in one area).

Pinch-out - The location where a porous, permeable formation that is located between overlying and underlying confining formations thins to a zero thickness, and the confining formations are in contact with each other.

Plugging – The act or process of stopping the flow of water, oil or gas into or out of a formation through a borehole or well penetrating that formation.

Point of injection –The last accessible sampling point prior to waste fluids being released into the subsurface environment through a Class V injection well.

Pore space - Open spaces in rock or soil. These are filled with water or other fluids such as brine (i.e., salty fluid).

Post-injection site care - Appropriate monitoring and other actions (including corrective action) needed following cessation of injection to assure that USDWs are not endangered

Primacy – or primary enforcement authority – The authority to implement the UIC Program. To receive primacy, a state, territory, or tribe must demonstrate to EPA that its UIC program is at least as stringent as the federal standards; the state, territory, or tribal UIC requirements may be more stringent than the federal requirements. (For Class II, states must demonstrate that their programs are effective in preventing pollution of USDWs.) EPA may grant primacy for all or part of the UIC program, e.g., for certain classes of injection wells.

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Rule authorization – Allowing a well owner or operator to construct or operate an injection well as long as the requirements outlined in 40 CFR Part 144 and 146 are followed. Wells authorized by rule do not require an individual or area permit. Rule authorization may be granted to Class II enhanced oil recovery wells or Class V wells (except for motor vehicle waste disposal wells and large-capacity cesspools).

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Safe Drinking Water Act (SDWA) – The main federal law that ensures the quality of Americans' drinking water. The SDWA sets the framework for the UIC Program to control the injection of fluids. EPA and states implement the UIC Program, which sets standards for safe injection practices and bans certain types of injection.

Saline formations - Deep  and geographically extensive sedimentary rock layers saturated with waters or  brines that have a high total dissolved solids (TDS) content (i.e., over 10,000  mg/L TDS).

Saline Intrusion Barrier Wells – Class V wells that are used to inject fluids to prevent the intrusion of salt water into an aquifer. These wells may have secondary purposes such as aquifer recharge.

Sanitary waste – Liquid or solid wastes originating solely from humans and human activities, such as wastes collected from toilets, showers, wash basins, sinks used for cleaning domestic areas, sinks used for food preparation, clothes washing operations, and sinks or washing machines where food and beverage serving dishes, glasses, and utensils are cleaned.

Sewage Treatment Effluent Wells – Class V wells that are used by privately or publicly owned treatment works (POTWs) to inject treated or untreated domestic sewage through a vertical well or a leachfield. Aquifer recharge wells, aquifer storage and recovery wells, subsidence control wells, and saline intrusion barrier wells injecting treated or untreated wastewater are considered sewage treatment effluent wells.

Shut-off device - A valve coupled with a control device which closes the valve when a set pressure or flow value is exceeded. Shut-off devices in injection wells can automatically shut down injection activities when operating parameters unacceptably diverge from permitted values.

Site closure - The point/time, as determined by the Director, at which the owner or operator of a well site has completed their post-injection site care responsibilities.

Solution Mining Wells – Class V wells that inject leaching solutions (lixiviants) to remove mineral ores from their original geological settings. The saturated solution is extracted by a production well, and the target mineral is harvested for processing. This process is used to mine copper, gold, salt, silver, and uranium.

Special Drainage Wells – Class V wells that include potable water tank overflow, construction dewatering, swimming pool drainage, and mine dewatering wells. Special Drainage Wells receive fluids that cannot be classified as agricultural, industrial, or storm water.

Spent Brine Return Flow Wells – Class V wells that dispose of the spent brine that results from the extraction of minerals, halogens, and other compounds from fluids. These wells are commonly associated with manufacturing facilities that produce specialty chemicals such as boron, bromine, magnesia, or their derivatives.

Storm Water Drainage Wells – Shallow Class V wells that are designed for the disposal of rain water and melted snow. These wells typically drain paved areas such as streets and parking lots as well as roofs. Improved sinkholes and abandoned drinking water wells that receive storm water runoff are considered storm water drainage wells.

Subsidence Control Wells – Class V wells that are used to control land subsidence caused by ground water withdrawal or over pumping of oil and gas. These wells may have secondary purposes such as aquifer recharge.

Subsurface fluid distribution system – An assemblage of perforated pipes, drain tiles, or other similar mechanisms intended to distribute fluids below the surface of the ground.

Supercritical fluid - A fluid above its critical temperature (31.1oC for CO2) and critical pressure (73.8 bar for CO2). Supercritical fluids have physical properties intermediate to those of gases and liquids.

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TDS –Total dissolved solids.

Tubing – A small-diameter pipe installed inside the casing of a well. Tubing conducts injected fluids from the wellhead at the surface to the injection zone and protects the long-string casing of a well from corrosion or damage by the injected fluids.

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UIC Program Director – The chief administrative officer of any state or tribal agency or EPA Region that has been delegated to operate an approved UIC program.

Underground Source of Drinking Water (USDW) – An aquifer or portion of an aquifer that supplies any public water system or that contains a sufficient quantity of ground water to supply a public water system, and currently supplies drinking water for human consumption, or that contains fewer than 10,000 mg/l total dissolved solids and is not an exempted aquifer.

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Well – A bored, drilled, or driven shaft or a dug hole whose depth is greater than the largest surface dimension; an improved sinkhole; or a subsurface fluid distribution system.

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