Cross-Connection/Backflow

A cross connection is a temporary or permanent connection between a potable (drinking) water supply and a non-potable source. An example would be the pipes connecting the public water system to an irrigation system. Backflow is the undesirable flow of non-potable water or other substances through a cross connection back into the consumer's plumbing system or public water system. More. 

Cross Connection and Backflow Prevention Program  
Companies that provide testing for backflow prevention devices 
Backflow Test Sheet  
Cross Connection and Backflow Prevention for Commercial Users 
CCBF Regulations – VDH Waterworks Regulations, April 2011
Residential Irrigation Systems Testing Brochure