Installing a new septic drainfield

The old drainfield had failed and sewage was surfacing due to being buried too deep. In order for a drainfield to function properly it has to get oxygen in order to promote aerobic bacteria and avoid anaerobic bacteria. If the drainfield can't get oxygen due to being too deep or being covered with too dense of soil then the anaerobic bacteria can take over and secrete a black impermeable layer referred to as biomat. Once biomat is built the effluent can't drain into the soils and instead comes to the surface. In order to solve this problem the new drainfield is buried shallow (trench depth is 8 inches) and topped with a thin layer of topsoil (6 inches) to allow for plenty of oxygen to get through so that the new drainfield will have a long, healthy life.