Septic tank absorption fields, drain fields and leach fields, are areas in which effluent from a septic tank is distributed into the soil through subsurface tiles or perforated pipe.
Saturated hydraulic conductivity, depth to a water table, ponding, depth to bedrock or a cemented pan, and flooding affect absorption of the effluent.
Excessive slope may cause lateral seepage and surfacing of the effluent in down slope areas. Some soils are underlain by loose sand and gravel or fractured bedrock at a depth of less than 4 feet below the distribution lines. In these soils the absorption field may not adequately filter the effluent, particularly when the system is new. As a result, the ground water may become contaminated.
Soil containing amounts of clay may be, or may become hardpan restricting the flow of effluent through the gravel field.
See Hardpan Soil