What potential issue arises from sites with level surfaces?

The correct response highlights the concern that when a septic system is installed on a level surface, there is a risk that groundwater mounds can develop beneath the system. This can occur because level sites may not allow for adequate drainage, leading to a saturation of the soil underneath the septic system. If the soil becomes overly saturated with groundwater, it can interfere with the proper functioning of the septic system. This could result in the leach field being unable to effectively treat and dispose of wastewater, creating a potential overflow or backup situation. Ultimately, an accumulation of groundwater can compromise the health of the septic system and the surrounding environment, making it crucial to assess the landscape's slope and drainage when planning septic installations.