What is the appropriate material to cover the distribution line in a gravel system?

The appropriate material to cover the distribution line in a gravel system is coarse gravel. This choice is correct because coarse gravel allows for adequate drainage and aeration, which are crucial for the effective operation of the septic system. It provides a structure that supports the distribution lines while preventing soil from clogging the lines. The gaps between the gravel particles facilitate the passage of effluent from the distribution line into the surrounding soil, promoting proper treatment and preventing system failure.

Using soil only would impede drainage and could lead to system overload, while sand, though it might initially seem suitable, can compact over time and restrict water flow. Clay is not appropriate either, as it has low permeability and would trap effluent, leading to potential backups and system failure. Thus, coarse gravel ensures that wastewater is properly treated by allowing it to disperse evenly into the soil.