Many of the newer installation have a drain around the outside of the leach field area to carry off excess ground water. The concept behind this "Curtain Drain" is to remove enough natural ground water table enough to allow the liquefied sewage to leach through the necessary 36" of subsoil. These drains, often required in the township by the Board of Health, must be connected to a working drain tile system. If ground water builds up under a leach field, the dark liquefied sewage will float to the top and cause a foul odor. Sometimes referred to as a "Glazed" leach field, this is not only common with older leach fields but is also considered a dangerous failure that must be repaired.