According to the NH DHHS, Division of Public Health Services, the legal definition of septic failure is: “The condition produced when a subsurface sewage or waste disposal system does not properly contain or treat sewage, or causes or threatens to cause the discharge of sewage on the ground surface or into adjacent surface or groundwaters.” (RSA 485-A-2:IV)
It is vital to the protection of ground and surface waters that wastewater effluent is appropriately treated and discharged by subsurface disposal systems.  Inadequately designed, improperly operated, and poorly-maintained systems can cause partially treated wastewater to contaminate ground or surface waters.
Obvious signs of septic system failure include sewage backups in your home, or on the surface of your yard. Difficult to detect yet equally unhealthy signs of failure include pollutants entering ground or surface waters.