What is Backflow?

Drinking water flows from the source to your tap in pressurized water pipes. Normally it flows in one direction, although under certain circumstances it can flow in the opposite direction, or “backflow.” When backflow occurs, water runs backwards through the water system. The water flowing backwards could contain something that might contaminate the drinking water supply. The North Coast County Water District strives to maintain positive pressure in the system at all times, but events such as main breaks and fire fighting can contribute to backflow conditions.


What is a Cross-Connection?

A cross connection is any connection between piping that carries drinking water (also known as potable water) and the piping or fixtures that carry other types of water or substances that are not safe to drink or may impact the taste or odor of water (also known as non-potable water). Common cross connections include hot tubs, sprinkler and irrigation systems and auxiliary water systems (wells).



What Are Hose Bibb Vacuum Breakers and Where Are They Used?
Why Should You Use a Hose Bibb Vacuum Breaker?

San Mateo County Approved List Certified Backflow Prevention Testers

San Mateo County Testing of Fire Service Backflow Prevention

San Mateo County Lead-Free Requirements for Backflow Prevention

San Mateo Environmental Health Cross Connection Control Program

North Coast County Water District---Resolution #1048

San Mateo Ordinance No. 04643 Relating to Backflow Prevention

NCCWD Standard Construction Details-NC-05 Double Detector Backflow

NCCWD Standard Construction Details-NC-06 Reduced Pressure Backflow

For more information please contact Michelle Bilodeau-Cross Connection Specialist, County of San Mateo at 650.372.6204 or by email at


If you have any questions for the North Coast County Water District, please contact Bob Lange at 650.355.3462 or by email at