What happens when a septic system fails? How can I tell?

Usually when a septic system fails, the drain field is not functioning properly. When a septic tank overflows, the effluent can pass to the drain field, clogging up the pipes. This causes sinks and toilets to back up in the house. Other signs include: slow draining toilets and drains, an odor of sewage, wet area on or near the drain field, or contaminated well water.