Treatment of waste in septic tanks occurs by bacterial decomposition. The resulting material is called sludge. Large portions of the population are still served by septic systems as opposed to public waste treatment facilities. Leachfields are part of the septic system for land based tanks and include an area where waste water percolates through soil as part of the treatment process. Badly constructed percolation systems may allow water to escape without proper treatment.
System failure can result in clogging and overflow to land or surface water.
High density placement of tanks , as in suburban areas, can result in regions containing very high concentrations of waste water. This water may seep to the land surface, run-off into surface water or flow directly into the water table. It may contain industrial waste chemicals. When leaking sewer lines are located deep underground below the biologically active portion of the soil, the sewage can enter groundwater directly. High pressure systems will push leaks to the soil surface where they can be easily detected by sight or odor.