How do you inspect and maintain a commercial drywell?

ADEQ's guidance says inspection should happen at least annually, or whenever water remains standing on the drainage area or retention basin longer than 36 hours. It lists what to look at:

• The drainage area. Confirm chemicals are not used, handled or stored there. Look for unusual stains or pavement discoloration around the drywell, residue coating the inlet grate or in the sediment, and unusual odors in the settling chamber. Those are signs something other than stormwater is getting in.
• The settling chamber and interceptor. Check the type and amount of debris and the condition of the drainage components.
• Absorbents. Chemical absorbents that are discolored or sitting under the water surface should be replaced.
• The cover. The design guidance calls for a solid manhole cover, bolted in at least two places and marked "stormwater only," so a missing bolt or a cracked grate is a fix-now item.

ADEQ also offers an inspection checklist that owners can use for the annual visit, and recommends keeping a copy of every inspection and a record of findings and maintenance at the property.

Cleaning can happen with the annual inspection, or at minimum, per ADEQ's guidance:

• In paved areas, when sediment fills 10 percent of the effective settling capacity.
• In landscaped areas, when it fills 25 percent.
• When ownership of the property changes.
• When anything other than stormwater or urban runoff enters the interceptor or the settling chamber.

Maintenance means removing all the sediment, cleaning filters and screens, and replacing absorbents. ADEQ says the removed material should go to a landfill or facility approved to accept it, and that records of drywell cleaning and sediment disposal should be kept.

The easiest check needs no tools. After a storm, see whether the water is gone. The City of Phoenix tells owners of dry wells to check that stormwater dissipates after a storm event, and says standing water should not be present after 36 hours, which prevents damage to structures and helps prevent mosquito breeding. Maricopa County's mosquito guidance adds that stagnant or shallow water lasting at least 72 hours is the primary mosquito breeding habitat.

Phoenix has a matching list for the rest of the system: culverts, catch basins and drainage grates should be kept free of trash and debris so water can flow through them, and retention basins should be checked for slope erosion and for silt that keeps them from holding water.

### A simple annual checklist

1. 1Before monsoon season, open each cover you can safely reach and look at the chamber, or have a service company do it.
2. 2Note the sediment level against the cleanout triggers above.
3. 3Check absorbents, screens and filters, and replace what is discolored or clogged.
4. 4Walk the drainage area and look for stains, residue or odors that suggest a non-stormwater discharge.
5. 5Confirm the cover is intact, bolted and marked "stormwater only."
6. 6After the first big storm, time how long the water takes to drain. Write down the date and hours.
7. 7File everything. Inspection sheet, cleaning ticket, disposal record.

### If you find oil, fuel or chemicals

Sheen on the water, a chemical smell or a stained chamber means something other than stormwater has reached the drywell. Do not just pump it and move on. ADEQ's drywell investigation guidance (March 2014 revision) says a drywell that receives a discharge other than stormwater or the exempt flows needs an investigation. It describes sampling the sediment in the settling chamber before any cleanup, because samples are considered valid only if collected before the chamber is cleaned. For pollutants that were released, whether by accident or on purpose, the guidance says a drywell investigation and a Clean Closure Application to ADEQ are required.

So the order matters: stop the source, leave the sediment in place, and call an environmental consultant or ADEQ before a cleaning truck arrives. This guidance is older than the 2025 changeover, so confirm the current process with ADEQ.

### What to keep in the drywell file

ADEQ recommends keeping inspection copies and maintenance records at the property. In Phoenix it is more than a recommendation: City Code 32C-103 says owners must maintain post-construction stormwater controls, including retention and detention basins and drywells, to their original design intent, and must keep written records of all inspections, cleaning, repair and maintenance for at least five years and make them available to the City on request. A useful file has:

• The completed inspection checklist for each year.
• Cleaning tickets showing the date, the company and what was done.
• Disposal records showing where the removed sediment went.
• Post-storm drain-down times.
• The as-built drawings, percolation test results and registration confirmation.

That file helps when a city inspector, a buyer's lender or a new property manager asks how the drywells have been kept. It also shows an engineer how fast sediment builds on your site, which helps set a cleaning schedule that fits the property instead of relying on a guess. Keep both paper and digital copies, and store them where the next manager will find them.

### Industrial and heavy-use sites

ADEQ's guidance treats truck docks, maintenance areas and industrial operations as heavy use, and calls for pretreatment in the design. For industrial areas it also points owners to a separate Best Management Practices Plan guidance document. If your property handles fuels, solvents or other hazardous materials near a drywell, the rules get stricter and an environmental consultant should be involved. What a drywell is has more on what may and may not go in.

Owners are responsible for private stormwater features. Phoenix says features owned and operated by private entities, including homeowners associations, must be maintained to perform consistent with the original design intent, and calls that especially important in monsoon season. ADEQ's guidance says drywell installers should meet the licensing requirements of the Arizona Registrar of Contractors and the Arizona Department of Water Resources, and ADEQ lists installer and consultant contacts on its drywell page. That page is a good place to start when you need cleaning or repair.

If a drywell has stopped draining, read can a plugged drywell be cleaned or must it be replaced. If the whole basin is the problem, see why is my retention basin not draining. Wells also need to be on the state inventory; see do drywells still need to be registered in Arizona.

Put the annual check on the calendar before monsoon season, keep the checklist and cleaning tickets in one folder, and treat any puddle that outlasts 36 hours as a reason to inspect right away. New to drywells? Start with what a drywell is.