 Backflow can be hazardous. In the EPA's "Best Practices Guide", it is stated: "Cross connections are ever present dangers that exist in most water systems and can result in serious chemical or microbiological contamination events in drinking water systems." Backflow can occur from something as simple as a garden hose left in a bucket of cleaning solution, but can also be much more serious. One example of what can happen occurred within 00 miles of Durango at a public school. A chemical used in the heating system boilers backflowed into the drinking water. Samples taken determined that the chemical was chromium in levels as high as 000 parts per million. The head chemist said that it was miraculous that no one was seriously injured or killed. They were lucky. The yellowish water was discovered by a home economics teacher before school started and proper precautions were taken.  