Plumbing cross connections, which are defined as actual or potential connections between potable, i.e. drinking water; and non-potable water supplies, i.e. sewage or other hazardous water supplies; constitute a significant public health hazard. Backflow is the situation in which a connection has occurred and hazardous water has been brought back into the drinking water supply thus contaminating the water.
You will find the more details about cross connections and back flow in the letter attached here. Also the cross connection survey form that residents need to complete periodically, per Colorado Department of Public Health and Environment.