Dry Wells

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What is a dry well?

A dry well is defined as a bored, drilled, or driven shaft or hole whose depth is greater than its width. A dry well is designed specifically for the alleviation of flooding and the disposal of storm water.

drywell-schematic

Why do we use dry wells?

Drywells are primarily used as a means of flood control here in the arid southwest. With our intense, short duration storms it is important to have a way to get that storm water off of the streets and away from buildings where it could do damage and be a safety hazard. Drywells are meant to be safe way of quickly disposing of flood waters by allowing quick infiltration into the ground.

Are dry wells regulated?

Yes. Dry wells are regulated by the Arizona Department of Environmental Quality (ADEQ). Arizona state law requires that all new, existing, or abandoned dry wells be registered with ADEQ.

Drywells can only receive storm water runoff or discharges that are exempted by A.R.S. § 49-250(23). If other fluids have been directed to the drywell, it is subject to the aquifer protection program (APP) and/or closure requirements and may be considered an underground injection well that requires both ADEQ and USEPA permitting. Spills to the drywell may also trigger permitting, clean closure, or enforcement actions. An APP permit is required for any drywell located in an area where hazardous substances, including motor fuels, are used, stored, treated or loaded.

Two important types of dry well permits available from ADEQ include:

Dry Well Permits

Type Covers
2.01 Drywells that drain areas where hazardous substances are used, stored, loaded or treated. Facilities with potential for spills of pollutants into those drywells
2.04 Drywells that drain areas at motor fuel dispensing facilities Facilities where motor fuels Are used, stored, or loaded

drywell2For more information on Dry Wells visit ADEQ’s website OR call ADEQ's Drywell Unit at (602) 771-4385, if you have any questions regarding the registration or permitting of your dry well.

Care should be taken to eliminate common sources of pollutants to dry wells:

  • Hot water heater drainage (should go to sanitation sewer)
  • Dumping of excess chemicals, like oil, paint, cleaners, etc., on the ground or in the gutter (excess chemicals should be taken to Household Hazardous Waste Collection Event for proper disposal)
  • Irrigation runoff containing excess pesticides/herbicides/ fertilizers these products should be applied following the instructions, excess chemicals that can no longer be used should be taken to the Household Hazardous Waste Collection Event)
  • Detergent/oil from car and engine washing (engine washing should be performed at a self-serve car wash because they have an underground oil/water separator that collects the dirt and oil for proper disposal)

How can dry wells worsen environmental contamination?

drywell3Because of their design, dry wells, if not properly maintained can serve as a "straight-shot" conduit for ground water contamination. Dry wells use a filtration and sediment separation system that includes an upper settling chamber with an injection pipe and a lower rock fill section. As water drains into the dry well, the settling chamber slowly fills with water, allowing sediment to settle out and become trapped in the bottom of the chamber. The water then drains to an overflow (injection) pipe, is released to the bottom of the drywell, and slowly percolates down to the water table.

In urban areas, storm water frequently carries significant levels of petroleum products, pesticides, and other chemicals washed from streets and landscaped areas. Drywells are designed to prevent this pollution from reaching the water table but not be able to do so if not properly maintained. Storm water containing pollutants may drain into dry wells and percolate into the water table. This is one reason why it is important to make sure pollution does not enter the storm system. The storm system is composed of the streets, gutters, and retention basins just to name a few.

What other environmental problems are linked to dry wells?

A dry well that is not functioning properly can lead to improper drainage and the flooding of the area surrounding the dry well.

The resulting standing water can become a perfect breeding ground for mosquitoes. Mosquitoes can have a serious impact on the health, comfort, and economic welfare of people. Generally, a body of water that stands for more than three days will produce mosquitoes. In addition to inoperable dry wells, mosquito breeding sites can be caused by irrigated or poorly leveled yards and retention basins, bird baths, decorative ponds, improperly maintained swimming pools, and flood control drainage areas. More information on mosquitoes is available by visiting the Public Works Mosquito Control webpage.

What do I need to do to ensure proper dry well operation?

Chapter III, Article XI, Section 11.15 (Drywell Policy) of the Town's Unified Land Development Code states:

"It is the owner's responsibility to maintain drywells . . . Drywells that cease to drain a project within a 36-hour period are to be replaced with new ones where alternate methods of drainage are still not available."

  1. It is important that property owners maintain their dry wells to ensure the efficient disposal of storm water.
  2. Dry wells should be inspected and cleaned annually by a dry well services company to ensure proper operation.
  3. In addition to regular inspections, dry well performance should be monitored more frequently during the monsoon season.
  4. If drainage appears slow, or if water is standing for more than 36 hours, the system should be inspected by a dry well service company.
  5. If it is determined that the dry well needs to be replaced, the Town's Community Development Department should be consulted for information on dry well replacement requirements.

Further Questions?

  • For general information on dry wells, contact the Town's Environmental Program Section at (480) 503-6400.
  • For information on the placement or replacement of dry wells, contact the Town's Community Development Department at (480) 503-6833.
  • For regulatory information on dry wells, contact the ADEQ Dry Well Unit at (602) 771-4385.
  • Access ADEQ's website
  • For general information on the Town of Gilbert's mosquito control program, contact the Town's Wastewater Section at (480) 503-6400.
  • For general information on mosquito control, call Maricopa County Vector Control at (602) 506-0700.
  • For a copy of the Town's Mosquito Control brochure, contact Public Works at (480) 503-6400.