How long can water stand in a retention basin in Arizona?

Maricopa County's commercial drainage plan review requirements ask a site to retain the volume of the 100-year, 2-hour storm, plus one foot of freeboard, and to include a note that the basin will drain within 36 hours, in line with the county's Drainage Policies and Standards. Then the county checks the work at the end. Before final drainage clearance, the plan notes require each retention basin deeper than 1 foot (or as the county decides) to be tested under the county's standard, with test results, boring logs and revised drain-down calculations showing the basins drain within 36 hours. Those are signed and sealed by an Arizona registered professional engineer.

Where drywells drain the basin, the county adds that drywell percolation rates must be de-rated under county standard 6.10.13, and the engineer's package must include drywell test results and proof of drywell registration. The county does not take the drain-down on faith. It wants tested numbers with a safety margin built in.

Phoenix uses the same number for upkeep. Its stormwater program page says that for dry wells, standing water should not be present after 36 hours, which prevents damage to structures and helps prevent mosquito breeding. ADEQ's drywell guidance makes water standing longer than 36 hours in a drainage area or retention basin a trigger for an inspection.

Other cities set their own drain-down rules in their drainage manuals, so if your property is outside Phoenix or unincorporated county, check the local standard. The 36-hour figure is the one to use as a working benchmark.

### Design rule versus upkeep rule

The 36 hours shows up in two different ways, and it helps to keep them apart.

• As a design rule. At approval, the engineer proves on paper and by testing that the basin will drain in 36 hours. That is the county's requirement for new construction.
• As an upkeep benchmark. After the site is built, the owner keeps the system working. Phoenix's maintenance guidance says private owners, including homeowners associations, must maintain stormwater features to perform consistent with the original design intent. If the basin used to drain in 36 hours and now takes four days, it is no longer doing what it was approved to do. In Phoenix that duty is in the city code: section 32C-103 says maintaining basins and drywells means achieving and maintaining the original design intent, backed by written inspection and cleaning records kept at least five years.

Maricopa County Environmental Services says the primary breeding habitat for mosquitoes is stagnant or shallow water that lasts at least 72 hours, or 3 days. Mosquito development varies with species and conditions; the county’s 72-hour description is a maintenance warning, not a guarantee that shorter-lived water cannot support mosquitoes. Meet the applicable drainage requirements and address persistent standing water rather than waiting for that threshold.

You do not need equipment to keep an honest record.

1. 1Note when the rain stops. Weather service data or your own log works.
2. 2Mark the high-water line. A photo of the basin at peak, with a fixed landmark such as a light pole in the frame, is enough.
3. 3Check at 24 and 36 hours. Photograph the same view.
4. 4Check again at 72 hours if any water remains.
5. 5Keep the records with your drainage and drywell files.

Two or three storms of data show whether the basin is healthy, slowly getting worse or has a problem after one event only.

### Which basins the rule covers

Surface basins are the most familiar. The plan notes also cover an underground retention system, where the county's plan notes call for professional third-party inspection of the excavation, bedding, placement, compaction, backfill and overall construction. Whatever the form, the drain-down promise is the same, and the owner inherits it.

The county also says all drainage control structures, including retention, must be in platted tracts or easements. That matters to owners because the basin is drainage land: filling it in, building on it or regrading it to add parking changes the design the county approved. Check with the county or city, and your engineer, before you touch a basin.

### What "standing water" means and what it does not

The 36-hour benchmark is about water left in the basin after a storm. It does not mean a basin has to look dry to the eye at the same moment:

• A damp, muddy basin floor with no pooled water is a different matter from a basin with a visible pond.
• A basin that is wet because a sprinkler or a leak is feeding it is not a storm drain-down problem at all. Turn the source off, then measure again.
• Very large storms can leave water in a basin longer than a small one, so record the storm size along with the drain time, and give the engineer the whole story.

### Quick answers

Is 36 hours a state law? The sources here show it as a Maricopa County plan review requirement, as Phoenix maintenance guidance and as ADEQ's inspection trigger. They do not show a single statewide statute setting it, which is why local checks matter.

Does it apply to a homeowners association basin? Phoenix's maintenance guidance names homeowners associations among the private owners who must keep stormwater features working as designed, and the county's as-built rules cover residential lots with on-lot retention basins.

Who decides if my basin passes? The engineer's tested drain-down calculations decide it at approval. After that, your own storm-by-storm records tell you whether it still holds up.

Is a wet basin a plumbing problem? Usually not. Basin design and drain-down are site drainage and civil engineering. Plumbing enters the picture when a leak or a building drain is feeding the water, or when the pipes carrying water to the basin need attention.

• Look at the basin and its inlets and outlets. Trash, silt and overgrown plants slow drainage. Maricopa County's guidance says to remove trash, overgrown vegetation and organic material that can restrict flow.
• Have the drywells checked, if the basin uses them. See how to inspect and maintain a commercial drywell.
• Bring in an engineer if it keeps happening. Why is my retention basin not draining walks through the usual causes and who fixes each.
• Do not leave it for the next storm. The county's mosquito guidance says maintained drainage facilities help prevent both flooding and mosquito outbreaks.

Treat 36 hours as your line. After a big monsoon storm, check the basin within a day and a half. If it is still holding water, fix the cause before the next storm, not after the next complaint. For the bigger picture on monsoon drainage, see why a parking lot floods after a monsoon storm, and for what a drywell does, see what a drywell is.