Can a plugged drywell be cleaned or must it be replaced?

Before you pay for anything, rule out the simple stuff. Maricopa County Environmental Services says drainage facilities that are not maintained can back up in heavy storms, and tells owners to remove trash, overgrown vegetation and organic material like leaves, silt and dirt that restrict flow. A blocked inlet grate, a packed catch basin or a crushed pipe upstream can look just like a failed drywell. See what a catch basin is and how often to clean it.

Then use the timing test. ADEQ's guidance says to inspect if water stays in the drainage area or retention basin longer than 36 hours, and the City of Phoenix says standing water should not be present at dry wells after 36 hours. If the water is still there on day two or three, the system is not doing its job.

ADEQ's guidance puts sediment at the center of drywell upkeep. Silt and debris collect in the settling chamber, and the guidance calls for removing all sediment, cleaning the filters and screens and replacing absorbents once the chamber is 10 percent full in paved areas or 25 percent full in landscaped ones. A chamber choked with silt can hold water back even when the well below it is fine.

So the first step is the same as routine maintenance. Have the chamber pumped and cleaned, and dispose of the material at a facility approved to take it, as ADEQ's guidance says. Then run a test: wait for a storm or fill the system and watch how fast it drains. The steps are in how to inspect and maintain a commercial drywell.

Two cautions before the truck arrives. If there is oil sheen, a chemical smell or discolored sediment, do not clean until sampling is sorted out, because ADEQ's investigation guidance says chamber sediment samples are valid only if collected before cleanup. And if the drainage area handles fuel or hazardous materials, talk to an environmental consultant first.

If the chamber is clean and the water still stands, the drywell itself may no longer pass water into the ground at the rate it was designed for. Persistent water beyond the applicable drain-down requirement calls for an engineer’s assessment; a time observation alone does not determine whether the drywell must be replaced. Maricopa County’s published commercial drainage requirements require tested drain-down calculations and drywell percolation results. The county's plan review rules also call for a constant head percolation test on drywells that drain retention basins, which is how the drain rate is measured. ADEQ's guidance says a percolation test on the completed system is the way to determine performance when the standard penetration is not achieved or when the required design flow is high.

Deciding between refurbishing and replacing takes a test and a design. A civil engineer or a drywell contractor does that work.

A new or replacement drywell is a Class V well, so it has to be on the state inventory; see do drywells still need to be registered in Arizona. ADEQ's design guidance also sets out what a well should look like. A few examples:

• Depth and soil. A minimum of 10 continuous feet of penetration into permeable, porous soil is recommended for standard installations, and the drywell should be at least 10 feet above the water table.
• Distance. No closer than 100 feet from any water well, and no closer than 20 feet from underground storage tanks or fuel loading areas.
• Protection during construction. Inlets should be sealed with two layers of ultraviolet-protected geotextile fabric so sediment stays out until paving and landscaping are done.
• Licensing. Installers should meet the licensing requirements of the State Registrar of Contractors and the Arizona Department of Water Resources.

Ask your engineer to tell you which of these apply on your site, and whether the city or county has stricter rules.

### What drives the cost of cleaning versus replacing

Nobody can give an honest figure without seeing the site, so here is what moves the number instead.

• Cleaning. How much sediment is in the chamber, how many drywells you have, how easy the covers are to reach with a truck, and where the removed material has to be hauled and disposed of.
• Refurbishing. Whether the fix is in the chamber and inlet, or in the well below. Testing the well before and after is part of the job.
• Replacing. Drilling depth, the soil, the size of the settling chamber, engineering and testing, permits or approvals, and restoring the paving or landscaping on top.

Get more than one written scope and make sure each one says whether testing and disposal are included. Compare scopes, not just totals.

### Questions to ask before you hire anyone

1. 1Did you measure the sediment level and check the well's drain rate before proposing work?
2. 2Where will the removed sediment go, and will I get the disposal record? ADEQ recommends keeping those records.
3. 3How will you test it afterward? A constant head percolation test is the measure Maricopa County uses for drywells that drain retention basins.
4. 4Are you licensed for drywell work? ADEQ's guidance refers to State Registrar of Contractors and Arizona Department of Water Resources licensing for installers.
5. 5Is the well going to be registered or updated in myDEQ?
6. 6Does the county or city need to sign off on the change? If your site's drainage approval relied on those wells, the engineer should check.

### Patterns worth writing down

These notes help an engineer tell a drywell problem from an inlet or basin problem:

• How the water level drops: steadily, or fast at first and then slowly.
• Whether the inlets and surface are clear while the water still lingers past 36 hours.
• How quickly sediment builds back up after a cleaning.
• Whether neighboring drywells on the same site drain normally.

Keep a simple log with the storm date, how long the water took to clear and which drywells served that basin. It helps the engineer and it helps a future buyer.

### If the drywell is being abandoned

ADEQ's page says a Class V registration is valid for the life of the well, until it is plugged and abandoned. Read that closely: a drywell taken out of service is meant to be plugged, not just covered over. ADEQ publishes drywell decommissioning guidelines on its drywell page, and ADEQ's investigation guidelines say drywells are typically decommissioned as part of a clean closure when pollutants have reached them. Get the current decommissioning steps from ADEQ before anyone fills a well.

Do not replace a plugged drywell before it has been cleaned and tested. Do not leave a failed one either, because standing water for days is where mosquitoes breed. If you suspect anything other than stormwater has been going in, tell the engineer up front. ADEQ's investigation guidance treats a drywell that has received pollutants as a separate, more serious matter. For the retention side, see why is my retention basin not draining.