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Underground Injection Control (UIC) program: Chapter 173-218 WAC

We administer the statewide Underground Injection Control (UIC) program to protect groundwater by regulating the discharge of fluids from UIC injection wells. The UIC program — authorized by the Safe Drinking Water Act — is administered under Title 40 Code of Federal Regulations (CFR) part 144.

All injection wells must either receive a program rule authorization or a state discharge permit in order to operate.

What are injection wells?

Underground Injection Control (UIC) wells — or injection wells — are structures built to allow fluids to flow into the ground under the force of gravity. These are also known as drywells. 

An injection well is designed:

  • Deeper than the largest surface dimension.
  • To contain a perforated pipe.
  • As an improved sinkhole.
Examples include sump pump, drywell, drainfield, and an infiltration trench containing perforated pipe.

The many uses of injection wells

Injection wells are used to manage stormwater, remediate groundwater contamination, replenish aquifers, and as return-flow wells for heat pumps.

We protect groundwater by regulating injection wells

We minimize the potential for groundwater contamination from UIC wells by regulating:

  • The well construction and location.
  • The volume and quality of the fluids injected.
  • The hydrogeologic setting of the well.

It is illegal to dispose of industrial or municipal waste into an injection well — unless it is done under a state discharge permit.

Injection well classifications

The U.S. Environmental Protection Agency (EPA) groups UIC wells into six classes, depending on the type of waste received by the well. Class 1, Class 3, and Class 4 wells are prohibited in Washington.

Class 1

Receives industrial, commercial, or municipal waste fluids injected beneath the lowermost formation containing an underground source of drinking water within 1/4 mile of the drilled well.

Class 2

Receives fluids that are brought to the surface as part of oil or natural gas exploration, recovery, or production.

Class 3

Used for mineral extraction through solution mining or in-situ leaching.

Class 4

Receives radioactive or hazardous waste injected into or above underground sources of drinking water.

Class 5

All other injection practices not included in the other classes.

Class 5 injection wells are the most common injection well in Washington. These are generally shallow wells used to discharge fluids into or above a groundwater aquifer. In many cases, these aquifers are shallow, unconfined, or surficial.

Large onsite septic systems, serving 20 people or more per day or having a capacity of 3,500 gallons per day, are considered Class 5 wells. Read EPA's clarification on which stormwater infiltration practices/technologies we may regulate as a Class 5 UIC well.

Class 6

Wells used for injection of carbon dioxide (CO2) into underground subsurface rock formations for long-term storage, or geologic sequestration. Geologic sequestration refers to a suite of technologies that may be deployed to reduce CO2 emissions to the atmosphere to help mitigate climate change.


Class 4 wells may be used at approved Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) or Resource Conservation and Recovery Act (RCRA) facilities that re-inject treated groundwater into the same formation.

Stormwater management using injection wells

In Washington, the majority of injection wells — mainly dry wells — are used to manage stormwater from roads, parking areas, and roofs.

Stormwater management guidance

Our stormwater management guidance for UIC wells provides technical guidance for stormwater facilities regulated under the Underground Injection Control program. It provides the design and best management practices (BMPs) to reduce solids, metals, and oil from injection wells used along roads and parking areas, or used to collect roof runoff at non-industrial settings.

Infiltration trenches with perforated pipe

Infiltration trenches with perforated pipe must be registered, except for trenches that receive only roof runoff from single family homes or duplexes, or used to control basement flooding.

Infiltration trenches with perforated pipe are designed, constructed, operated, and maintained according to the current stormwater management manual guidance, or an equivalent approved manual.

Note: Section 5.6, Subsurface Infiltration of the Stormwater Management Manual for Eastern Washington Manual does not apply to infiltration trenches. View a summary of the infiltration trench design requirements for eastern and western Washington. If an infiltration trench will be located in Western Washington, and is in a National Pollutant Discharge Elimination System (NPDES) stormwater-permitted community, use the manual that is required in the stormwater permit. For trenches located in Western Washington and not in an NPDES permitted community, use the 2012 Stormwater Management Manual for Western Washington to design and maintain the infiltration trench.

Best Management Practices (BMPs) for injection wells

We must approve Best Management Practices (BMPs) for treatment for injection wells that require rule authorization.

Other types of injection well uses

Floor drains

Do you know where your floor drain discharges? Many floor drains send untreated waste fluids directly to injection wells, such as septic systems, drywells, or to a pit or cesspool. The waste fluids could reach the groundwater, or go to a lake or stream.

UIC wells that are used at cleanup sites

Groundwater remediation projects — cleanup sites — sometimes use injection wells. UIC registration and approval is required for all types of remediation sites.

Cleanup sites typically include:

  • Independent sites

  • Those enrolled in the Voluntary Cleanup Program

  • Sites managed under a Model Toxics Control Act (MTCA)

  • Sites managed under an EPA legal agreement

Aquifer storage and recover wells

Aquifer storage and recover wells increase existing groundwater supplies by artificially recharging groundwater. The recharged water is later recovered and used. This practice can help communities be more prepared for drought.

Subsidence control wells

Subsidence control wells are used to reduce or eliminate subsidence associated with the removal of groundwater due to construction activities. This well type is not used for oil or natural gas production.

Vehicle maintenance waste disposal wells

Vehicle maintenance waste disposal wells can be found in repair bays at service stations, garages, or at car dealerships or shops that repair or maintain vehicles. This type of discharge is prohibited in Washington.