Today's question: What are cross-connections?
"Plain Talk" answer: A cross-connection is a connection between a drinking water pipe and a contamination source. Here's a common example: You're planning to spray weed killer on your lawn. You hook up your hose to the faucet on your house and to the sprayer containing the weed killer. If the water pressure drops at the same time you turn on the hose, the chemical in the sprayer may be sucked back into your home's plumbing system through the hose. This is called backsiphonage and would seriously contaminate the water system in your home. If your hose was connected to a fire hydrant or a public access faucet (e.g., at a campground), then the weed killer would be sucked into the public water supply.
Backsiphonage can be prevented by using an attachment on your hose called a backflow-prevention device. The simplest backflow-prevention system is an air gap, which is a physical separation of the supply pipe by at least two pipe diameters vertically above the overflow rim of the receiving vessel (the sprayer containing the weed killer in the example). A hose bibb vacuum breaker installed on the outdoor spigot will also work.
More sophisticated backflow-prevention devices are mandatory for certain industrial and commercial operations, such as dry cleaners and restaurants. Most water suppliers have cross-connection control programs, particularly in major cities.
For more information on this and many other water-related topics, check out "Plain Talk About Drinking Water" by Dr. James M. Symons.