Cross Connection and Backflow Prevention

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What is cross-connection?


Please note:  this list is provided to you as a convenience only and is not comprehensive.  The District does not recommend any specific contractor.  View the full list of certified backflow testers by county.

Cross-connection occurs when a contaminated substance or source of pollution is accidentally drawn into the public water supply. It usually happens when there is a rapid drop in water pressure (such as a main break) which can cause a reverse flow or suction in the water lines of your home.  If a water line (such as a garden hose) is submerged in a polluted or contaminated substance when the pressure drops, that substance can be drawn into the water supply.  When the pressure returns to normal, the substance will flow in the opposite direction.  It is possible to have pesticides and fertilizers pulled into your water lines where it may later flow out of the tap into your drinking water.  The results can be deadly.

  • Irrigation Safety and Cross-Contamination flier

To prevent cross-connections from occurring, follow these precautions:

  • Never submerge a garden hose in a bucket, sink, or anything else. 
  • Do not use a spray attachment on your garden hose without first installing a backflow prevention assembly. 
  • Keep the end of the garden hose clean and free of contaminants. 
  • Install a backflow prevention assembly on your irrigation system. To install an irrigation system, you must comply with the following requirements: 
    • Irrigation systems connected to a drinking water system must be equipped with a backflow prevention assembly that has been approved by the Washington State Department of Health; and     
    • Assemblies must be tested upon installation and annually thereafter by a state certified Backflow Assembly Tester.

Why do I need a backflow assembly?

If you have an irrigation system or fire sprinkler system, Washington State Department of Health requires you protect the connected drinking water system from the potential cross-connection. There are many types of backflow prevention assemblies. They vary in design, function, and price.


If you have questions regarding cross-connection or backflow assemblies, contact (425) 398-4419 or email