Which of the following is a common sign of septic system failure?

Unusual odors are a common sign of septic system failure because they indicate that sewage may be escaping from the system or that the system is not functioning properly. When a septic tank is working correctly, the wastewater is treated and contained within the system. However, if there are issues such as a blockage, overflow, or a faulty drain field, untreated sewage can release foul smells. This unpleasant odor serves as an important indicator that the system may be compromised and needs inspection or repair.

In contrast, clear drinking water, nice green grass, and regular water drainage do not typically signify a problem with a septic system. Clear drinking water indicates no contamination, nice green grass can be a sign of healthy drainage fields that are absorbing water properly, and regular drainage suggests that the system is functioning as it should. These conditions do not exhibit the symptoms of failure that unusual odors do.