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

What are injection wells?

Underground Injection Control (UIC) wells — or injection wells — are structures built to allow fluids to flow into the ground (usually) under the force of gravity. The most common in Washington are known as drywells. 

An injection well is designed and built:

• Deeper than the largest surface dimension.
• To contain an assemblage of perforated pipe.
• As an improved sinkhole.
• As a chamber or vault designed to capture and infiltrate stormwater.

Examples include: sump pump, drywell, drainfield, an infiltration trench containing perforated pipe, a stormwater chamber, and temporary injection points.

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. See our other uses section below for more details.

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.

What type of wells are allowed in Washington?

The U.S. Environmental Protection Agency (EPA) groups UIC wells into six classes, depending on the type of fluid received by the well. Class 1, Class 3, and the majority of Class 4 wells are prohibited in Washington. Class 2 wells and one type of Class 4 well are allowed.  For descriptions and more information see Chapter 173-218 WAC.

Class 5 is the most common 

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.

There are different types of Class 5 wells; see document for definitions. 

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.

Read EPA's clarification on which stormwater infiltration practices/technologies we may regulate as a Class 5 UIC well.

Regulated stormwater facilities use stormwater manuals

For regulated stormwater injection wells along roads and parking areas, or used to collect roof runoff at non-industrial settings use the UIC Program guidance in the Stormwater Management Manuals. The guidance, including best management practices (BMPs), helps reduce pollution such as solids, metals, and oil from injection wells. 

• Eastern Washington Stormwater Management Manual (2024) —  Chapter 5
• Western Washington Stormwater Management Manual (2024) — Volume I-4

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.