Septic systems failures can occur when wastewater either breaks out at the surface or seeps into the soils and travels to groundwater sources, contaminating the water and threatening public health.

In areas where both individual water wells and septic systems are used by homeowners, there is a greater danger of drinking contamination by septic system failure because of the relative proximity of the two systems. Check with the local health department to ensure a safe distance between a septic tank and a drinking water well.

In areas where soils are sandy and less dense, there is potential that a septic failure will allow household wastewater to move quickly to the source of water serving homes in the area. Some research also indicates that septic failure rates are highest in well-drained soils because of inadvertent undersizing of leach fields.

In areas with clay soils, septic failures lead to runoff of pollutants to surface waters as the clay does not allow water to easily move through soil underground.

Source: http://nirpc.org/media/4392/septicsystems_rev_032012.pdf