A cross connection between potable and nonpotable water sources poses a high potential risk for contamination. Cross connections can occur through a direct piping linkage of a nonpotable water source to a potable water source. In many cases, however, the cross connection results from a backflow. Backflow can occur from either backpressure or backsiphonage. Both types of backflow are due to a pressure differential and result in the flow of water from a water system user's premises or establishment toward the potable water distribution system.

Backpressure backflow occurs when the water system user's water pressure is greater than the potable water system's pressure. When this happens, potentially contaminated water from the higher pressure in the user's pipeline will flow into the potable water system pipeline. Increased pressure on the user side of the system can result from a booster pump, an elevation differential, a water storage tank, and even from temperature differences (e.g., as in a boiler).

Backsiphonage backflow results in pressure in the distribution system falling below atmospheric pressure (i.e., 14.7 psi at sea level). Potentially contaminated water in the user's pipelines is "sucked" into the potable water system because the supply pressure is less than the atmospheric pressure. A reduce pressure or partial vacuum can occur when high water flow conditions exist. This condition can result from a water line break or a peak demand on the system.

Backflow will occur through any unprotected cross connection whenever backpressure or backsiphonage conditions exist.

Both SDWA and 64E-8 prohibit cross connections. The American Water Works Association Publishes a manual entitled Recommended Practice for Backflow Prevention and Control that may be ordered through their web site.