The drainfield or soil absorption field is generally a system of perforated pipes set underground in a bed of crushed rock that allows the effluent to seep slowly into the ground, undergoing further cleansing. If trenches are used there are usually 2 or more parallel trenches approximately 3 ft. wide. Gravel is placed around the pipe in the trenches and covered by fabric that prevents clogging of the pipe with dirt. Beds are used when space or soil considerations indicate their use instead of a trench system. The number and length of the trenches or the size of the bed is determined by the number of square feet of drainfield that is specified in the Health Department permit.
Some soils are better suited to septic systems than others.
Well aerated soil with good permeability is desired. Clay soil will generally have lower permeability and will require a larger drainfield than a system located in sandy soil. Before a permit is issued a site evaluation must be performed, which identifies these criteria by boring holes at several locations on the lot and examining the soil in these holes. The location of any surface water, such as a lake, and underground water supplies, such as wells, must be identified. Wells should be located at least 75 feet from any part of a septic tank system.