Backflow is when water flows backwards into your water supply instead of out of it due to a change of pressure. If the source of the backflow is your lawn irrigation system, this can introduce chemicals from fertilizer, herbicides, and pesticides into your drinking water. If the source is your sewer or septic system, your potable water can be contaminated with wastewater. No matter where the backflow is sourced from, though, it can make your water at worst, undrinkable, and at most, dangerous to your health.