Cross-Connection & Backflow Control Program

What Is a Cross-Connection?

A cross-connection is any connection between your drinking water and any other source of water that combines the two when a backflow condition occurs; typically resulting in contamination of the water system.

What Is a Backflow?

Backflow occurs when the water in your pipes (after the water meter, in your building/facility) goes backward (opposite its normal direction). There are two situations that can cause the water to go backward (backflow):
  • Backpressure – when the pressure in your pipes is greater than the pressure coming in.
  • Back-siphonage – when there is a reduction of pressure in the water distribution system

Safe Drinking Water Act

In 1974, the Safe Drinking Water Act (SDWA) was established to protect human health from contaminants in drinking water and to prevent the contamination of existing groundwater supplies. This act and its amendments (1986 and 1996) require many actions to protect drinking water and its sources. One of these actions is the installation and maintenance of an approved backflow prevention assembly at the water service point of connection. These devices are installed whenever a potential hazard can exist in a given system as the backflow prevention assembly prevents a cross-connection.

To protect the water system, there are two types of backflow prevention assembly devices that prevent the backflow of water. These are required for all customers (typically business/industrial) to prevent a potential hazard from entering the City’s water system:
  • External – to protect the City’s water system from cross-connection occurring on the customer’s property.
  • Internal – to protect the customer from cross-connections occurring within the system/pipes on their own system
To keep your drinking water safe, the City’s Cross-Connection specialist diligently checks the plans of each new business for compliance with cross-connection/backflow requirements. When a property is renovated, a backflow assembly may be required. Per the City’s Municipal Code, each unit must be tested annually at the expense of the owner.

The City of Modesto takes pride in the quality of drinking water provided to our customers. To ensure the highest water quality, distribution systems are tested weekly and quality is maintained by ensuring protections at the water meter/point of connection, with an approved backflow prevention device.

How Can You Help?

  • Be aware of potential cross-connections in your water supply and eliminate them.
  • Install backflow prevention assemblies where appropriate.
  • Repair and replace your backflow assembly when necessary.
  • All backflow prevention assemblies are to be tested annually at the owner’s expense. (Modesto Municipal Code 11-1.113 and California Health and Safety Code 116800-116820.) If you would like to view a list of certified testers, download our list (PDF).
The Cross-Connection specialist is available to help conduct a backflow survey within your facility to determine what type of backflow prevention assembly is best to protect your facility and your personnel. You will receive a survey report complete with recommendations. To arrange for this free service, you may contact us at 209-342-2207 or via e-mail at