Why are septic systems installed so close to the ground surface?
Systems should be kept shallow because the top 18 to 30 inches of the soil is more porous and will percolate water faster than the clay that usually comprises the subsoil. Evapotranspiration is best at this level. Aerobic bacterial action also increases at shallower depths. Water rises to the surface by capillary action and is drawn into the root systems of plants. Moisture evaporates from the ground surface and leaves.
