What is a drywell and how do commercial sites use one?

Arizona's underground injection rules define a drywell as a well, other than an improved sinkhole or a subsurface fluid distribution system, that is completed above the water table so its bottom and sides are typically dry except when it is receiving fluids. In everyday terms, it is a bored hole, often many feet deep, that lets water drain into the soil instead of running off the site.

The U.S. EPA files stormwater drainage wells under Class V injection wells, the catch-all class for wells that put non-hazardous fluids in the ground. EPA's page notes that most Class V wells are shallow disposal systems that depend on gravity to drain fluids directly into the ground. That classification is why a plain-looking parking lot drain is regulated like a well.

Most commercial drywells are built as a system, not just a hole. ADEQ's design and maintenance guidance for drywells (May 2018 revision) describes a standard setup with these pieces:

• An inlet and a settling chamber. Water enters through a grate or pipe and slows down in a chamber where sediment can drop out. The guidance sets a minimum effective settling capacity, which is the volume between the chamber floor and the overflow outlet.
• A shield and absorbent. A shielding device helps separate floating petroleum from water, and a petroleum absorbent catches what floats.
• A debris screen. It keeps paper, leaves and trash in the chamber instead of letting them travel down the well.
• The drywell bore. This is the part that reaches into permeable soil. ADEQ recommends a minimum penetration of 10 continuous feet into permeable soils for standard installations.
• A locked-down cover. The guidance calls for a 24 inch nominal cast iron grate or manhole cover, bolted in at least two places and marked in raised cast letters "stormwater only." Those letters are the tell that you have found a drywell.

ADEQ's guidance says the system must be accessible from the surface for maintenance and inspection, which is why the covers exist.

Arizona cities and Maricopa County make new commercial sites hold their own stormwater on site rather than send it down the street. Phoenix's grading code, for example, requires on-site retention of stormwater for all developments, with a waiver for isolated developments under one-half acre where no critical drainage problem would result. A drywell gives that water a place to go. Two uses are common:

• Draining a retention basin. The basin holds a big storm, and one or more drywells drain it down. Maricopa County's plan review rules say basins must drain within 36 hours. See how long water can stand in a retention basin.
• Draining a paved area directly. A parking lot can drain to a drywell through a catch basin and pipe, with no basin at all.

ADEQ's guidance gives sizing rules of thumb. For paved areas it recommends at least one standard drywell for each 6,000 cubic feet of drainage volume, and for landscaped areas one for each 15,000 cubic feet. It also recommends spacing multiple drywells at least 100 feet apart, and keeping a drywell at least 100 feet from any water well. Those are recommendations from guidance. Your site's engineer works from the city or county drainage manual. Still, they show how much thought goes into where the wells go.

Busy sites get more protection. For heavy use areas, such as truck docks, maintenance areas and public rights of way, the guidance calls for a system with pretreatment. Those areas carry more sediment and debris.

The City of Phoenix lists drywells, along with retention and detention basins, green stormwater infrastructure and pretreatment devices, as post-construction stormwater controls that owners must keep maintained.

The short list: stormwater and ordinary urban runoff. ADEQ's guidance says drywells should be installed to dispose of only stormwater and urban surface runoff, and that other discharges may trigger regulation as an underground injection well. The guidance defines urban surface runoff to include things like fire hydrant flushing, uncontaminated foundation drains, non-agricultural irrigation water, individual vehicle washing, evaporative cooler discharge and air conditioning condensate.

Arizona law does allow a few minor flows to mix with stormwater in a drywell without an aquifer protection permit. Two examples are routine building wash down without detergents, and pavement wash water when no toxic or hazardous material has spilled and no detergents were used. Anything with fuel, solvents or process fluids is a different story. EPA puts the reason plainly: owners and operators are responsible for protecting the ground water below and are not allowed to endanger underground sources of drinking water.

That is why a dumpster-pad rinse or a fuel spill is a problem. See can you hose a dumpster pad into the storm drain.

Some sites handle hazardous materials in the area that drains to the drywell. ADEQ’s older investigation guidance explains circumstances where investigation and aquifer-protection permits are needed; have ADEQ or an environmental consultant determine what applies to the actual site and discharge. A general permit covers drywells at industrial sites and at motor fuel dispensing facilities.

### Who regulates drywells now

The final federal rule transfers Arizona UIC primacy to ADEQ effective October 15, 2025, except for wells on Indian lands. ADEQ’s current inventory page directs Class V owners to register and manage wells in myDEQ, with stated exemptions. Confirm the classification and jurisdiction for the particular drywell rather than relying on an old registration number. Details on the paperwork are in do drywells still need to be registered in Arizona.

### How to tell whether your property has one

• Look for round iron covers or grates in parking lots and basins. Many are marked "stormwater only," and some sit in landscaped basin floors, not just pavement.
• Check the site's grading and drainage plan or the as-built drawings for drywell symbols.
• Ask the property's original developer or engineer for the drywell registration and percolation test records that were submitted to the county.
• Search your inspection and maintenance file. Cities like Phoenix now require a site-specific maintenance plan for post-construction stormwater controls on projects with an acre or more of disturbance, and that plan lists each control.

Drywells are easy to forget because they sit underground. A short walk-through once a year, with the site plan in hand, keeps them from becoming a surprise.

If you own or manage a commercial property built in the last few decades, know how many drywells you have, where they are, and when someone last checked them. The routine is covered in how to inspect and maintain a commercial drywell. If a drywell is not draining, start with can a plugged drywell be cleaned or must it be replaced. Design, testing and replacement are engineering and drilling work, so start with a civil engineer or a drywell contractor.